WO2004088708A2

WO2004088708A2 - System and method for generating an electorn beam

Info

Publication number: WO2004088708A2
Application number: PCT/NL2004/000224
Authority: WO
Inventors: Sergey Wasiliewich Mitko
Original assignee: Nederlands Centrum Voor Laser Research B.V.
Priority date: 2003-04-04
Filing date: 2004-04-05
Publication date: 2004-10-14
Also published as: WO2004088708A3; NL1023111C2

Abstract

A system for generating an electron beam, comprising: a dielectric body; a device for generating an electric field; a source of electrons; and a control unit for controlling the device, wherein the control unit operatively controls such that in a first phase of a cycle, electrons end up from the source on the dielectric surface, and in a second phase of the cycle, electrons leave the dielectric body.

Description

Title: System and method for generating an electron beam.

The invention relates to a system for generating an electron beam. The invention further relates to a method for generating an electron beam.

Such a system is known per se. Such known systems often comprise a cathode and an anode. Upon applying a sufficiently large potential difference between the cathode and the anode, electrons are drawn from the cathode, or drawn near the cathode from, for instance, a metal, and accelerated in the direction of the anode. Such a system is often provided with a filament with a tip of, for instance, tungsten or lanthanum boride. The filament is typically surrounded by the cathode which is of cap -shaped design and has a recess situated adjacent the tip. In common situations, the anode is plate-shaped and provided with a recess which is situated opposite the recess in the cathode and opposite the tip of the filament. Upon electron emission from the tip, the electrons find their way through the recesses which are aligned with the tip. The electrons are accelerated by the potential difference between the anode and the cathode. A disadvantage of such a system is that the electrons often cannot be drawn from the tip until after the tip has been heated. Such heating is necessary to give the electrons sufficient kinetic energy to enable their escape from the tip. To be able to facilitate the escape of the electrons, no foreign atoms or molecules should be present at positions on the surface of the tip. To prevent any settlement of foreign atoms and/or molecules on the surface of the tip as much as possible, the tip is often held in a relatively high vacuum. Despite this high vacuum, it still happens that foreign atoms and/or molecules settle on the surface of the tip. These foreign atoms and/or molecules can be removed by giving an enormous voltage pulse to the tip. In this way, the tip is, as it were, stripped of the foreign atoms and/or molecules. Unfortunately, the high voltage pulse often leads to minute damages of the tip. The shape of the tip, specifically the shape of a damaged tip, leads to^*an inhomogeneous electron beam, both in energetic and in geometric respect. The necessary vacuuming of the tip moreover requires the presence of a number of vacuum pumps and relatively expensive materials for sustaining the vacuum. The necessary vacuuming also entails some loss of time upon replacement of an unduly damaged tip before the electron source can be active again.

The object of the invention is to meet at least one of the above- mentioned problems.

This object of the invention is realized by the system according to the invention, comprising: a dielectric body; a device for generating an electric field; a source of electrons; and a control unit for controlling the device, the control unit in use controlling such that

• in a first phase of a cycle, electrons from the source end up on the dielectric surface, and

• in a second phase of the cycle, electrons leave the dielectric -surface.

In the first phase of the cycle, the electric field is directed away from the surface of the dielectric body, and the electrons move opposite to the direction of the electric field lines, to the dielectric body. The strength of the , field is such that the electrons from the source of electrons follow the field lines.

In the second phase of the cycle, the electric field is directed towards the surface of the dielectric body and the electrons move opposite to the direction of the electric field lines, away from that surface. For the electrons to come off the dielectric surface in the second phase of the cycle, an electronic field directed towards the surface is needed, whose strength can be minimal. What this means is that the surface itself need not sustain any damage as a result of the strength of that electric field. This prolongs the life of the body with the dielectric surface. For a particular embodiment, it holds that the device is further provided with a first and a second electrode and a voltage source for applying a potential difference between the first and the second electrode, the first electrode being disposed against the dielectric body, such that the dielectric body is situated substantially between the first and the second electrode, the control unit being arranged to: apply in the first phase a potential difference between the first electrode and the second electrode, such that the first electrode is positive with respect to the second electrode. As, in use, in the first phase of the cycle, the first electrode is positive with respect to the second electrode, electrons will move in the direction of the first electrode, and at least a number of these electrons will end up on the dielectric body. These electrons may for instance have been drawn from the second electrode. The second electrode then functions as source of electrons. It is also possible that a gas situated between the first electrode and the second electrode discharges as a source of electrons and that the electrons thereby released end up. on the dielectric body of the first electrode. This is because the electrons follow a direction opposite to the direction of the electric field lines which are then directed from the dielectric body to the second electrode. This is because in the first phase of the cycle the second electrode is negative with respect to the first electrode. According to one theory, the amount of electrons that can end up on the dielectric body is directly proportional to the potential difference between the first electrode and the second electrode. The maximum amount of charge which can be adsorbed onto the dielectric body, according to the theory, is given by: εA Uch Q = wherein Q represents the amount of electric charge,

4πh ε represents the dielectric constant of the dielectric body, A is the surface area of the dielectric body, U_ch is the positive potential of the first electrode with respect to the grounded second electrode and h represents the thickness of the dielectric body when the body is in the shape of a layer. In other words, when the dielectric body is manufactured from a material having a high dielectric constant, the dielectric body has a large surface, the positive potential of the first electrode with respect to a grounded second electrode is high, and the dielectric body is relatively thin, a great multiplicity of electrons can end up on the dielectric cover layer. When the dielectric cover layer has adsorbed the highest possible amount of electrons, the electron flow to the first electrode will stop. In a manner of speaking, an equilibrium has been achieved between the potential difference applied across the first and the second electrode, and the amount of charge which has ended up on the dielectric body.

According to a particular embodiment, it holds that the control unit is furthermore arranged to cause the potential difference between the first and the second electrode to decrease in the second phase.

In the second phase of the cycle, the potential difference between the first and the second electrode is lowered, when in the first phase an amount of electrons has ended up on the dielectric body. In the second phase, in excess of a certain potential' ifference, a "surplus" of electrons will be present on the dielectric body. Electric field lines will now orient themselves from the second electrode in the direction of the surface of the dielectric body. This is because the dielectric body is then negative with respect to the second electrode. In other words, the surplus of electrons will leave the dielectric body and follow a direction which is opposite to the direction of the electric field lines. The dielectric body in that case constitutes a generator of a burst of electrons. An advantage of such a generator is that the electrons do not need to be drawn from the dielectric body or from the first electrode. It is not necessary to heat the dielectric body or the first electrode. Nόr is it necessary to sustain the dielectric body in a vacuum. When the electrons leave the dielectric body in the second phase, a virtually homogeneous electron beam is formed. The dielectric body will sustain virtually no damages and therefore the life of the dielectric body can be long. A particular embodiment is characterized in that the control unit is arranged to cause the potential difference to decrease to zero in the second phase. In this case, all electrons adsorbed on the dielectric body in the first phase will leave the dielectric body. What is then involved is an electron beam having a high current density.

In a particular embodiment, it holds further that the control unit is arranged to apply in a third phase of the cycle a potential difference between the first and the second electrode such that, as a result, in use the electrons move acceleratedly in the direction of the second electrode. In this case, it is also possible to obtain a homogeneous electron beam with electrons which possess a high kinetic energy. It is thus possible to tune the kinetic energy of the electrons to the desired use of the electron beam.

Furthermore, it holds in particular that the device is arranged to impose,, in the first and in the second phase, on the first electrode a potential which is positive with respect to the potential of the second electrode.

This provides the advantage that switching is not necessary. In particular, it holds then that in the first phase and in the second phase the first electrode is connected with the same positive pole of the voltage source. In this case, it is wholly unnecessary to include the first electrode in a circuit that is connected with the control unit.

What can hold, furthermore, is that the control unit is arranged to impose on the first electrode a first potential and to impose on the second electrode a second potential, while in the first phase the second potential is negative with respect to the first potential and, further, in the second phase the second potential is positive with respect to the first potential.

In such a system, the control unit only needs to reverse the polarity of the second electrode, which is a relatively simple operation.

A particular embodiment is further characterized in that on a side of the second electrode remote from the first electrode, a third electrode is arranged, the device being further arranged to impose at least in the second phase such a potential on the third electrode that the electrons released from the dielectric body are accelerated in the direction of the third electrode. This provides the advantage that the electrons, once released from the dielectric body, can obtain a high velocity and hence the beam can become a high energy electron beam.

What holds in particular here is that the control unit is arranged in the first phase to impose such a potential on the second electrode that the electric field between the first and the second electrode is directed from the first electrode to the second electrode.

In such an embodiment, it is possible that the third electrode is connected, unchanged, during the first and the second phase with a positive pole of a high- voltage source. It is possible that in such an embodiment the control unit only needs to control the potential of the second electrode.

Preferably, it holds furthermore that between the first and the second electrode, a conductor is included as an electron source, which conductor is provided with recesses for allowing, at least in the second phase, passage through the recesses of electrons that move from the first electrode in the direction of the second electrode. It is possible here that the conductor is connected via a relatively high electrical resistance with an electron source.

What can hold here is that the resistance is electrically connected to earth, which in that case functions as electron source.

In such an embodiment, the conductor will often take a potential which is between the potential of the first and the potential of the second electrode.

In particular, it holds that the control unit is arranged to perform a cycle frequently in succession. In this case, it is possible that a virtually continuous flow of electrons can be obtained as a beam of electrons. Preferably, it holds that the first body is of plate -shaped design. This is beneficial to the homogeneity of the electron beam and moreover enlarges the adsorbing capacity of the dielectric body for adsorbing electrons in the first phase. Also, it preferably holds that the second electrode is provided with at least one ring. This means that the electrons which leave the dielectric body in the second phase through the opening in the ring of the second electrode can travel for further use of the electron beam.

It is also possible that the second electrode comprises a grid. This promotes the homogeneity of the electron beam.

Preferably, it holds that the dielectric body is manufactured from a ceramic or a polymer. These materials normally have a high dielectric constant.

As stated, the invention also relates to a method for generating an electron beam, the method at least comprising generating an electric field, characterized in that the method further comprises a cycle of which a first phase at least comprises:

• generating the electric field such that electrons end up on a dielectric body; and of which a second phase at least comprises:

• generating the electric field such that electrons leave the dielectric body.

The invention is presently elucidated with reference to a drawing, in which: Fig. 1 schematically shows a first embodiment of a system according to the invention in use in a first phase of the cycle;

Fig. 2 shows the system according to Fig. 1 in use in a second phase of the cycle;

Fig. 3 schematically shows a second embodiment of a system according to the invention in use in a first phase of the cycle; and Fig. 4 shows the system according to Fig. 3 in use in a second phase of the cycle.

Fig. 1 shows a first embodiment of a system 1 according to the invention. The system comprises a dielectric body which is here designed as a dielectric cover layer. The system 1 further comprises a device for generating an electric field. In this example, the device comprises a first and a second electrode and a voltage source 5 for creating a potential difference between the first and the second electrode. It holds here that the first electrode lies against the dielectric body such that the dielectric body is situated substantially between the first and the second electrode. In the examples shown, the dielectric body has been applied as a dielectric cover layer to the first electrode 3. Furthermore, the system comprises a control unit B for controlling the device. The control unit B operatively controls such that in a first phase of a cycle, as shown in Fig. 1, electrons end up from the source onto the dielectric surface. Furthermore, the control unit B operatively controls such that in a second phase of the cycle electrons leave the dielectric surface. In this example, the second electrode can additionally serve as source of electrons.

In this example, the first electrode 3 is of plate-shaped design. The second electrode 4 can also be made of substantially plate-shaped design. In this example, the second electrode 4 comprises a grid which is schematically shown in cross section. For instance, the second electrode can comprise electrically conductive mesh. The first electrode 3 in this embodiment is arranged parallel to the second electrode 4. The first electrode 3 is further provided with a dielectric cover layer 6, which comprises, for instance, a ceramic or a polymer. In this embodiment, the second electrode 4 is grounded. The first electrode 3 and the second electrode 4 are connected in parallel with a positive pole of the voltage source 5. The control unit B is at least provided with a switch S which is included between the second electrode 4 and the voltage source 5. With such a system 1 according to the invention, it is possible to generate a burst of electrons. To this end, in a first phase of a cycle, a potential difference is created between the first electrode 3 and the second electrode 4, such that the first electrode 3 is positive with respect to the second electrode 4. In the exemplary embodiment shown, this means that the control unit B keeps the switch S in an open position. In that case, the potential of the second electrode 4 is equal to zero; the potential of the first electrode 3 is equal to the positive pole of the voltage source 5; and the potential difference is therefore equal to the potential U_Ch of the positive pole of the voltage source 5. The potential difference between the first electrode 3 and the second electrode 4 is such that electrons end up on the dielectric cover layer 6 of the first electrode 3. It is possible that electrons have been drawn from electrode 4 which, in the exemplary embodiment, is grounded. In that case, the second electrode 4 functions as a source of electrons. It is also possible that a gas present between the first electrode 3 and the second electrode 4 discharges, which results in the release of electrons and positive ions. ^"The gas then functions as a source of electrons. In each case, the electrons thereby released will move under the influence of the electric field E present, in the direction of the dielectric layer 6 on the first electrode -3. Once having arrived on the dielectric cover layer 6, the electrons will be held onto the dielectric cover layer 6 due to the presence of the electric field E.

A second phase of the cycle comprises at least decreasing the potential difference between the first electrode 3 and the second electrode 4, so that the electrons leave that dielectric cover layer 6. This is shown schematically in Fig. 2.

In this exemplary embodiment, the separation between the first phase of the cycle and the second phase of the cycle is determined by the position of the switch S, which entails a very simple and very robust control unit. Through the use of just one switch S, the transition can be a clear-cut transition and comprise a single step. According to this example, for the transition from the first phase to the second phase of the cycle, the control unit B closes the switch S. At the time of closing of the switch S, the potentials of the first electrode 3 and the second electrode 4 are equal to each other in a single step, since both the first electrode 3 and the second electrode 4 are then grounded. It could also be stated that in the second phase of the cycle the first electrode is connected with the second electrode and that consequently the potential difference between the first and the second electrode in the second phase is equal to zero. It is then not necessary that the first electrode and the second electrode are connected to earth in the second phase. In the second phase, in any case, due to the presence of the electrons on the dielectric cover layer 6, an electric field E arises which is directed perpendicularly to the surface of the dielectric cover layer;. The electrons will leave the dielectric cover layer in a direction which is opposite to the direction of the electric field E. In a device according to the embodiment shown, the electrons will leave the dielectric cover layer in the direction of the second electrode. The electrons will therefore travel away from the dielectric cover layer 6 in the direction of the second electrode 4. In this example, the electrons are not attracted by the second electrode 4 since the second electrode 4 is grounded. The electrons will move through the grid and thus form an electron beam. It is now possible to enter the first phase of a next cycle again very fast by simply closing switch S in a single. step using the control unit B. Upon switch S being closed again, the potential of the first electrode becomes U_Ch again, and the direction of the electric field changes round again. Thus, the cycles can be performed in rapid succession by rapidly switching the switch S. In the exemplary embodiment shown, a resistance R is included to ensure that the voltage source 5 is not short-circuited in the second phase. This resistance can also be virtually equal to zero. In that case, it is advisable to provide the voltage source with a short-circuit protection. In the exemplary embodiment shown, the control unit is arranged to cause the potential difference between the first electrode 3 and the second electrode 4 to decrease to zero in the second phase. However, it is also possible that the potential difference merely becomes less than the potential 5 difference that was applied in the first phase of the cycle. In that case, for instance the second electrode 4 is not grounded in the second phase and in the switch S a resistor is included which causes a voltage drop. It will be clear that the control unit in such a situation requires a somewhat more complex design. It is also possible that the control unit B is arranged to

10 cause the potential difference to decrease by steps in the second phase. In such an embodiment, for instance, a resistance included in the switch S is for instance controllable.

It is further possible that the control unit is arranged to apply in a third phase of the cycle such a potential difference between the first

15 electrode 3 and the second electrode 4 that in use the electrons thereby move acceleratedly in the direction of the second electrode 4. In such an embodiment, the second electrode 4 in the third phase should be positive with respect to the first electrode 3. It is also possible, of course, to have the potential difference decrease, to below zero directly in the second phase.

20. The control unit may be arranged to perform the cycle frequently in succession. Thus, a substantially continuous electron beam built up from multiple bursts of electrons can be generated. The control unit can be driven with a high-voltage electric pulser, known per se, which is equipped, for instance, with a device for resonating charges or rotary spark gaps,

25 thyratrons, semiconducting opening switches, etc. It is also possible to perform the generation of the electron bursts in an environment with an ionizable gas. The ionizable gas can upon discharge in the first phase release electrons which can end up on the dielectric cover layer.6. In this case, the ionizable gas constitutes the electron source. It has been found

30 possible in a helium gas at a pressure of 30 millibar to generate an electron beam having a current density of about 8 amperes per cm², with a maximum energy of 15 kV when using a dielectric cover layer having a surface area of 60 cm². The cycles were here performed with a frequency of 200 Hz.

It is not inconceivable for the electron bursts to be generated also at an atmospheric pressure. The energy of the bursts of electrons can vary from a few kV to hundreds of kV's. The skilled person can adjust the relevant parameters such that the electrons have a predetermined energy. With routine experiments, the skilled person can determine how the current density depends, for instance, on the frequency with which the cycles are performed. The skilled person can freely determine for instance the dimensions of the first electrode 3 and the second electrode 4. It is also possible to tailor the distance d between the first electrode 3 and the second electrode 4 as desired. Also the optimum thickness h of the cover layer can be determined using routine experiments. The control means can be so arranged that the frequency with which the cycles are carried out is predetermined, is settable, or depends, for instance, on the flow of electrons directed in the first phase to the dielectric cover layer 6. What is not precluded, for instance, is that a detector, for instance on the basis of this flow of electrons, determines whether the maximum amount of electrons has ended up on the dielectric cover layer 6. Upon reaching this maximum amount, there is no point having the first phase continue longer. The control means can then proceed to carry out the second phase of the cycle, or wait for a further inspection. The second electrode 4 can comprise a grid, but may also comprise a single ring. However, the second electrode can also comprise an electrically conductive plate with perforations. Although the second electrode is preferably provided with openings through which the electrons released from the first electrode can travel, it may also be useful, in particular embodiments, for the plate to be closed, that is, free of such openings. Naturally, it is also possible to comprise a plurality of mutually linked rings. Furthermore, it is also possible that the first electrode comprises substantially a cylinder and the second electrode comprises a cylinder arranged substantially coaxially around the first electrode. It will be clear that any arbitrarily chosen position and orientation of the first electrode and the second electrode with respect to each other is possible within the framework of the invention. Furthermore, it holds that both the first electrode and the second electrode can be designed to have an arbitrarily chosen form. Below follows an example of a parameter setting for the operating condition of the device according to the invention.

The parameters can be set such that U_ch/h < Ebr, wherein U_ch in this case is the maximum potential difference between the first and the second electrode; d is the distance between the first and the second electrode; h is the thickness of the dielectric cover layer, expressed in cm; and Ebr is the dielectric strength of the material from which the dielectric cover layer is manufactured. When the method is carried out in an environment with He gas, it preferably holds that:

- hp /£< 0.2;

- h / d < l; and - pd > 0.4. wherein p is the gas pressure, expressed in Torr; wherein S is the dielectric constant of the material of which the dielectric cover layer is manufactured and wherein h and d are each expressed in cm.

For such a parameter setting, it has been found that

T ~(£AL/h)ⁱ'² when

U_ch< 800 * (£7p⁴AL), wherein J is the maximum current density of the generated electron beam, expressed in Ampere/cm²; A is the surface area of the cover layer, expressed in cm²; L is the induction accompanying the discharge of the first electrode, expressed in μH; and T is the duration of an electron current pulse, expressed in ns. Here, it holds again that h is the thickness of the dielectric cover layer and d is the distance between the first and the second electrode.

It has appeared that within the validity range of the above formula for J, with h=0.5 cm, S =2000, A = 60 cm², L= 0.2 μH and p = 10 mbar He, the current density is 60 A/cm². If U_ch is less than indicated here, the current density J will increase while the pulse duration T will decrease with respect to the above-mentioned value of J and T.

Fig. 3 shows an alternative embodiment of a system for generating an electron beam according to the invention. In this case, too, the system

I comprises a dielectric body; a device for generating an electric field, a source of electrons; and a control unit for controlling the device. In tins embodiment top, the devicα.comprises a.first.and a second electrode and a voltage source for applying a potential difference between the first and the second electrode. The first electrode lies against the dielectric body such that the dielectric body is situated substantially between the first and the second electrode. n this embodiment too, it holds that the dielectric body is applied as a dielectric cover layer onto the first electrode. The control unit is arranged to: apply in the first phase a potential difference between the first electrode and the second electrode, such that the first electrode is positive with respect to the second electrode. The source of electrons in this embodiment comprises a conductor included between the first and the second electrodes 3, 4. This conductor is further provided with recesses for allowing, at least in the second phase, passage through the recess of electrons moving from the first electrode in the direction of the second electrode. In this example, the conductor is of grid-like design, so that the recesses are inherently present. This conductor 7 is connected via a relatively high resistance with an electron source. In the example shown, he conductor 7 is connected to earth via the relatively high electric

The control unit operatively controls the device, such that: in a first phase of a cycle, electrons from the source 7 end up on the dielectric surface 6, and in a second phase of the cycle, electrons leave the dielectric surface 6. To this end, in the first phase, the control unit B, using the voltage source (not shown), creates such a potential difference between the first and the second electrode that the first electrode is positive with respect to the second electrode. In the system according to the example as shown in Figs. 3 and 4, the electrode 3 is connected with a positive pole 5 of a voltage source (not shown), both in the first and in the second phase. In this example, the system is actually arranged such that both in the first phase and in the second phase a potential in an absolute sense of a magnitude of U_Ch is imposed on the first electrode 3. The control unit B is further arranged to impose in the first phase an absolute negative potential U_sw on the second electrode 4. The control unit B is further arranged such that in the second phase, with respect to the potential imposed on the first electrode, a positive potential can be imposed on the second electrode. Accordingly, the control unit is^' arranged to impose on the first electrode a first potential and to impose on the second electrode a second potential. Accordingly, in the first phase the second potential is negative with respect to the first potential and, furthejr, in the second phase the second potential is positive with respect to the first potential. In the example shown, this means that the control unit is so arranged that in the second phase the second potential is positive with respect to the first potential.

The system as shown in Figs. 3 and 4 further comprises a third electrode disposed on a side of the second electrode remote from the first electrode. The system is arranged to impose at least in the second phase such a potential on the third electrode that the electrons released from the dielectric cover layer are accelerated in the direction of the third electrode.

In the example shown, this potential is imposed on the third electrode both in the first and in the second phase. In that case, the control unit B is arranged to impose in the first phase such a potential on the second electrode that the electric field between the first and the second electrode in the first phase is directed from the first electrode to the second electrode.

The operation of this embodiment is as follows. In the first phase, the control unit B controls such that the second electrode 4 is connected with the aid of switch SR to a negative pole U_s (neg) of a voltage source not shown. The then prevailing potential difference between the second electrode 4 and the first electrode 3 has as a result that an electric field arises whose field lines are directed as indicated with arrow E. From the source of electrons, in this case designed as conductor 7, the electrons move as represented with the arrows 2 to the dielectric surface.6. Incidentally, the \ voltage difference between the second electrode 4 and the first electrode 3 is such that the electric field lines between the conductor 7 and the dielectric surface 6 are directed from the dielectric surface to the conductor 7. In other words, a relatively high potential applied to the third electrode has no ^■ influence on the direction of the field lines adjacent the dielectric surface that is desired in the first phase.

In the second phase, the control unit B controls the device such that the second electrode obtains a positive potential higher with respect to the first electrode. To that end, the switch SR is switched to a pole having a different potential. In the example drawn, this potential is U_sw (pos). In this case, the electric field lines will orient themselves perpendicularly to the dielectric surface 6. As a result, the electrons present on the dielectric surface will leave the dielectric surface in a direction which is opposite to the direction of the electric field lines, all as set out in the description of the first embodiment. The potential of the conductor 7, because of the high resistance R in the connection of the conductor 7 to earth, will take a potential that is between the potential of the first electrode 3 and the second electrode 4. In the second phase, therefore, a rising potential in the direction of the third electrode is involved. The electrons released from the dielectric surface will be accelerated in the direction of the third electrode. It will be clear that the distance dl between the first electrode and the conductor 7, the distance d2 between the conductor 7 and the second electrode, and the distance d3 between the second electrode 4 and the third electrode 9 can be chosen by the skilled person to be such that an optimum operation of the system is obtained. With regard to, for instance, the shape of electrodes, the same remarks as made in the discussion of the first embodiment hold for this embodiment. In both embodiments, it is possible that a side of the dielectric body facing the second electrode is made of slightly concave design. This promotes beam formation. For this embodiment too, it holds that the cycle can be repeated frequently with, for instance, a frequency as indicated in the discussion of Figs. 1 and 2.

For this embodiment too, it holds that the cycles can be performed in an environment with ionizable gas, such as for instance He. It is also possible to place and operate the system in an environment with an atmospheric pressure. This embodiment too can be used for the purpose of generating X-ray radiation or generating a plasma.

Such extensions and variants are each understood to belong to the invention.

Claims

1. A system for generating an electron beam, comprising: a dielectric body; a device for generating an electric field; a source of electrons; and a control unit for controlling the device, wherein the control unit operatively controls such that o in a first phase of a cycle, electrons end up from the source on the dielectric surface, and • in a second phase of the cycle, electrons leave the dielectric surface.

2. A system according to claim 1, characterized in that the device is further provided with a first and a second electrode and a voltage source for applying a potential difference between the first and the second electrode, the first electrode being disposed against the dielectric body, such that the dielectric body is situated substantially between the first and the second electrode, the control unit being arranged to: apply in the first phase a potential difference between the first electrode and the second electrode, such that the first electrode is positive with respect to the second electrode.

3. A system according to claim 2, characterized in that the control unit is further arranged to cause the potential difference between the first and the second electrode to decrease in the second phase.

4. A system according to claim 3, characterized in that the control unit is arranged to cause the potential difference to decrease to zero in the second phase.

5. A system according to claim 3 or 4, characterized in that the control unit is arranged to connect the first electrode and the second electrode with each other in the second phase.

6. A system according to any one of claims 3-5, characterized in that the control unit is arranged to connect the first electrode and the second electrode to earth in the second phase.

7. A system according to any one of the preceding claims 3-6, characterized in that the control unit is provided with a switch for causing the potential difference to decrease in the second phase in one step.

8. A system according to claim 7, characterized in that the second electrode is grounded, the first electrode and the second electrode are connected in parallel to a positive pole of the voltage source, and the switch is included between the second electrode and the voltage source.

9. A system according to claim 3, characterized in that the control unit is arranged to cause the potential difference to decrease to below zero in the second phase.

10. A system according to any one of claims 3-9, characterized in that the control unit is arranged to apply in a third phase of the cycle such a potential

I difference between the first and the second electrode that in use the electrons thereby move acceleratedly in the direction of the second electrode.

!

11. A system according to any one of claims 3-10, characterize; d in that the second electrode further functions as electron source.

12. A system according to. claim 2, characterized in that between the first and the second electrode a conductor is included as electron source, which conductor is provided with recesses for allowing, at least in the second phase, passage through the recesses of electrons moving from the first electrode in the direction of the second electrode.

13. A system according to claim 12, characterized in that the conductor is connected via a relatively high electrical resistance with an electron source.

14. A system according to claim 13, characterized in.that the resistance is connected to earth electrically.

15. A system according to claim 2, characterized in that the device is arranged in the first and in the second phase to impose on the first electrode a potential which is positive with respect to the potential of the second electrode.

16. A system according to claim 15, characterized in that in the first phase and in the second phase the first electrode is connected with the same positive pole of the voltage source.

17. A system according to claim 2 or any one of claims 12-16, characterized in that the control unit is arranged to impose on the first electrode a first potential and to impose on the second electrode a second potential, while in the first phase the second potential is negative with respect to the first potential and, further, in the second phase the second potential is positive with respect to the first potential.

18. A system according to claim 16 and 17, characterized in that the control unit is arranged such that in the second phase the second potential is positive with respect to the first potential.

19. A system according to any one of claims 2-18, characterized in that on a side of the second electrode remote from the first electrode, a third electrode is arranged, the device being further arranged, at least in the second phase, to impose such a potential on the third electrode that the electrons released from the dielectric surface are accelerated in the direction of the third electrode.

20. A system according to claim 17 and 19, characterized in that the control unit is arranged in the first phase to impose on the second electrode such a : potential that the electric field between the first and the second electrode is . directed from the first electrode to the second electrode.

21. A system according to any one of the preceding claims, characterized in that the control unit is arranged to perform the cycle frequently in succession.

22. A system according to any one of claims 1-21, characterized in that the body is of substantially plate-shaped design.

23. A system according to any one of claims 2-22, characterized in that a side of the dielectric body facing the second electrode is of slightly concave design.

24. A system according to any one of claims 3-23, characterized in that the second electrode is of substantially plate-shaped design.

25. A system according to any one of claims 2-24, characterized in that the first electrode is disposed parallel to the second electrode.

26. A system according to any one of claims 2-21, characterized in that the first electrode comprises substantially a cylinder, and the second electrode comprises a cylinder arranged substantially coaxially around the first electrode.

27. A system according to any one of claims 2-26, characterized in that the second electrode is provided with at least one ring.

28. A system according to any one of claims 2-27, characterized in that the second electrode comprises a grid.

29. A system according to any one of claims 2-28, characterized in that the second electrode comprises electrically conductive mesh.

30. A system according to any one of claims 3-28, characterized in that the second electrode is manufactured from an electrically conductive plate with perforations.

31. A system according to any one of the preceding claims, characterized in that the dielectric body is manufactured from a ceramic material.

32. ; A system according to any one of the preceding claims, characterized in that the dielectric body is manufactured from a polymer.

33. A method for generating an electron beam, wherein the method comprises at least generating an electric field, characterized in that the method further comprises a cycle of which a first phase at least comprises:

34. A method according to claim 33, characterized in that the body comprises a first electrode, and the dielectric surface is applied as a dielectric cover layer to the first electrode, wherein near the first electrode a second electrode is situated.

35. A method according to claim 34, characterized in that the first phase at least comprises: generating a potential difference across the first electrode and the second electrode, such that the first electrode is positive with respect to the second electrode.

36. A method according to claim 34, characterized in that the second phase at least comprises causing the potential difference between the first and the second electrode to decrease, so that electrons leave the dielectric body.

37. A method according to claim 36, characterized in that the second phase of the method comprises:

• causing the potential difference to decrease to zero.

38. A method according to claim 36 or 37, characterized in that the second phase of the method comprises:

• connecting the first and the second electrode with each other.

39. A method' according to any one of claims 33-38, characterized in that the second phase of the method comprises:

• connecting the first and the second electrode to earth.

40. A method according to any one of claims 33-39, characterized in that the second phase of the method comprises:

• causing the potential difference, with the aid of a switch, to decrease in one step.

41. A method according to any one of claims 33-40, characterized in that the second phase of the method comprises:

• connecting the first and the second electrode to earth.

42. A method according to claim 33, characterized in that the second phase of the method comprises: o causing the potential difference to decrease to below zero.

43.. A method according to any one of claims 33-42, characterized in that the method in a third phase of the cycle comprises: o applying a potential difference between the first and the second electrode, such that, in use, the electrons thereby move acceleratedly in the direction of the second electrode.

44. A method according to claim 33, characterized in that the method further comprises in the first phase imposing on the first electrode a potential that is positive with respect to the potential of the second electrode and in the second phase imposing on the second electrode a potential that is positive with respect to the potential of the first electrode.

45. A method according to any one of claims 33-44, characterized in that the method comprises:

• performing the cycle frequently in succession.

46. A method according to any one of claims 33-45, characterized in that the method is performed in an environment with an ionizable gas.

47. A method according to any one of claims 33-46, characterized in that the gas comprises Helium.

48. A method according to any one of claims 33-47, characterized in that the method is carried out at an atmospheric pressure.

49. Use of a system according to any one of claims 1-32, for the purpose of generating X-ray radiation.

50. Use of a system according to any one of claims 1-32 for the purpose of generating a plasma.