Seeding of Self-Modulation using Truncated Seed Bunches as a Path to High Gradient Acceleration

Abstract

This manuscript proposes a method to enable controlled high-gradient particle acceleration when requiring self-modulation of the drive bunch. While electron bunch seeding of self-modulation (eSSM) has been realised at a plasma electron density $n_\mathrm{pe}\cong10^{14}\mathrm{cm}^{-3}$, it has not been demonstrated at higher plasma densities due to limitations of available seed bunch properties. As experimentally shown in this manuscript, truncating available seed bunches with a relativistic ionisation front allows these limitations to be overcome. This seeding method is called truncated electron bunch seeding of self-modulation (teSSM) and experiments confirm that -- when using teSSM -- self-modulation becomes reproducible at $n_\mathrm{pe}=7\times10^{14}\mathrm{cm}^{-3}$. Additionally, the seed wakefield amplitude is also increased, which is known to be advantageous because it shortens the length needed to reach self-modulation saturation. The presented results establish teSSM as a method for achieving controlled, high-gradient particle acceleration with long drivers and available seed bunches.

Figures (5)

• Figure 1: Transverse wakefields (black solid lines) in plasma (pink shaded areas), driven by Gaussian seed pulses (blue shaded areas). Two vertical gray dashed lines mark the wakefield period $\tau_\mathrm{pe}$. Figures a and b: wakefield phase difference $\Delta\phi_{\mathrm{wake}}$ (difference between wakefield peak locations, marked by the black crosses and the two vertical gray dotted-dashed lines) from seed arrival-time jitter $\Delta\tau_{\mathrm{seed}}$ (difference between $\tau_\mathrm{seed,1}$ and $\tau_\mathrm{seed,2}$, marked by the blue crosses and the two vertical gray dotted lines) for the eSSM (a) and teSSM (b). Field amplitudes $E_{\textrm{seed}}$ (vertical red solid lines) on the same vertical scale. Figure b: solid vertical orange line is the RIF position. Bunches and wakefields propagate to the right.
• Figure 2: Schematic of the experimental setup. Proton drivers (red) propagate through plasma (pink) in the vapour source (grey). Rubidium vapour ionised by the RIF (orange). There are three configurations: SMI (a): drivers preceded by a RIF; eSSM (b): drivers preceded by seed bunches, both preceded by a RIF; teSSM (c): drivers preceded by seed bunches, which are overlapped in space and time with the RIF. Downstream the plasma and the laser beam dump, longitudinal proton bunch densities are measured using an OTR screen and two streak cameras karlrieger. One streak camera measures the time resolved drive bunch density with a time window of [1]ns ($n_\mathrm{b}(y,t)$) and the second one with a time window of [73]ps ($n_\mathrm{b}(x,t)$), where $x$ and $y$ are perpendicular transverse dimensions and $t$ is axis along the bunch. Sub-figures d and e: measurements without plasma, in vacuum; f and g: and with plasma (in configuration a). Bunches and pulses propagate to the right.
• Figure 3: Measurements of twelve summed proton bunch density profiles $n_\mathrm{b}(x,t)$, aligned in time following the same procedure as in Ref. IFR, when using SMI (a), eSSM (b), and teSSM (c). Figures a-c: on the same colour scale and identical streak camera settings. Figure d: vertical projections for SMI (a, blue dotted line), eSSM (b, orange dashed line) and teSSM (c, green solid line). Intensity normalised to the peak value. Bunches propagate to the right.
• Figure 4: Normalised amplitude resulting from the fast fourier transform (FFT) analysis as a function of the normalised period $\tau/\tau_\mathrm{pe}$ on the projections in Fig. \ref{['fig:microbunches']}d, for SMI (blue dotted line), eSSM (orange dashed line) and teSSM (green solid line). Black vertical line centres around the FFT peak and its width indicates the FFT bin width.
• Figure 5: Single measurements of $n_\mathrm{b}(y,t)$ for SMI (a), eSSM (b) and teSSM (c). Figures a-c: on the same colour scale and identical streak camera settings. Median filter numpy with size$=3$ applied to reduce measurement noise. Where [$<\pm2$]mm, coloured lines in a--c mark where $n_\mathrm{b}(y)$ profiles reach [20]% of their peak value, as in prllv. Figure d: same quantity, but displays the mean (line) and standard deviation (colorband) of the twelve measurements of Figs. \ref{['fig:microbunches']}a-c for SMI (blue dotted line), eSSM (orange dashed line) and teSSM (green solid line). Figure e shows the mean (line) and standard deviations (colorband) of the vertical projections of $n_\mathrm{b}(y)$ for the twelve measurements. Bunches propagating to the right.