CERN Accelerating science

Experimental layout of the AWAKE experiment showing key components. Dipole magnets and optical mirrors are used to align the proton and laser beams upstream of the vapor cell. Downstream of the cell, the proton beam passes through an OTR foil and optical light is generated in the process. The light is transported to a streak camera for temporal imaging. The inset illustrates a uniform, Gaussian proton bunch entering the plasma which is micro-bunched by the time it exits the plasma.

a) Streak camera image of a modulated proton beam in a 200 ps-long time window. The vertical axis is the time axis, with $t=0$ corresponding to the center of the proton bunch. The horizontal axis is the transverse spatial coordinate with $x=0$ located at the beam centroid. The ROI is denoted by dashed blue lines. The solid black line shows the temporal projection of the image data in the region of interest. b) The absolute value of the DFT of the projection shown in a) after application of the analysis procedure described in Section~\ref{sec:projection}. A peak in the spectrum is identified at $42.09\pm 0.6$ GHz using the peak-prominence algorithm and is denoted by the red dot, where the uncertainty of 0.6 GHz reflects the bin spacing of the frequency axis. The prominence is denoted by the black arrow. c) Histogram of the peak frequencies identified by the analysis described in Section~\ref{sec:column}. The entries to the histogram are weighted by their prominence and the histogram is fitted by a Gaussian centered at $41.57\pm0.07$ GHz, where the error on the centroid is extracted from the fit.

Streak camera images of the modulated proton beam for sample time delays $\Delta t$ after ionization by the laser pulse with the high-power setting. The frequency of the modulation is observed to decrease with time after ionization, indicating a decay in the plasma density.

Fourier power spectra for the 135 mJ laser energy setting. Each subplot contains approximately 10 stacked Fourier spectra. The subplots are labeled by time after ionization, with 0 s at the top and 80 $\mu$s at the bottom. For time delays $\Delta t \geq 5$ $\mu$s, a second peak appears in the spectra at twice the frequency of the main peak.

Plasma density versus time for the high, medium, and low energy laser settings, plotted on a log-log scale. The errors are shown multiplied by a factor of ten so that they are visible in the plot. The measured vapor density denoted by the black dashed line. The data points corresponding to $\Delta t = 0$ s are assigned an offset of 1 ps so that they are visible in the plot.

a) Ionization fraction for the 135 mJ laser pulse setting with a Gaussian transverse pulse shape based on the laser propagation model. The average radius of the ionized plasma column is 2.2 mm. b) Energy of electrons freed by ionization. The minimum energy after ionization is 0.59 eV, while the median energy is 0.62 eV.

Plasma density versus log time compared to the on-axis density curves from the model. Error bars on the data are multiplied by a factor of 10 for visibility.