Experimental Observation of Plasma Wakefield Growth Driven by the Seeded Self-Modulation of a Proton Bunch

We measure the effects of transverse wakefields driven by a relativistic proton bunch in plasma with densities of $2.1\times {10}^{14}$ and $7.7\times {10}^{14}\text{electrons}/{\mathrm{cm}}^3$. We show that these wakefields periodically defocus the proton bunch itself, consistently with the development of the seeded self-modulation process. We show that the defocusing increases both along the bunch and along the plasma by using time resolved and time-integrated measurements of the proton bunch transverse distribution. We evaluate the transverse wakefield amplitudes and show that they exceed their seed value ($<15\mathrm{MV}/\mathrm{m}$) and reach over $300\mathrm{MV}/\mathrm{m}$. All these results confirm the development of the seeded self-modulation process, a necessary condition for external injection of low energy and acceleration of electrons to multi-GeV energy levels.

Figure 1

(a) Schematic location of the imaging stations (IS1 and IS2) and of the OTR streak camera screen with respect to the plasma. The proton bunch moves from left to right. (b) Schematic drawing of the optical setup of the imaging stations.

Figure 2

Streak camera image showing the transverse distribution of the self-modulated proton bunch as a function of time. The image is obtained by summing ten individual measurements. The bunch moves down along the time axis. The timescale is set to show only 34 ps of the $\sim 73\mathrm{ps}$ image. The white line indicates the observed increase of the maximum defocusing of the protons along the bunch.

Figure 3

Time-integrated proton bunch charge distributions after the bunch propagated in rubidium vapor with a density of $7.7\times {10}^{14}\mathrm{atoms}/{\mathrm{cm}}^3$ (a) and (c) and plasma with a density of $7.7\times {10}^{14}\text{electrons}/{\mathrm{cm}}^3$ (b) and (d). Panels (a) and (b) show the core camera and panels (c) and (d) the halo camera measurements. In panels (c) and (d), the center of the proton bunch is blocked by a mask. Note the logarithmic scales. (e) Combined projections of the images of Figs. 3. The blue lines show the measurement without plasma and the red lines with plasma. Green vertical bars show the maximum radius of the proton distribution, defocused by the plasma.

Figure 4

Transverse wakefield amplitude as a function of proton bunch population for two plasma electron densities $2.1\times {10}^{14}\text{electrons}/{\mathrm{cm}}^3$ (left) and $7.7\times {10}^{14}\text{electrons}/{\mathrm{cm}}^3$ (right). The red lines show the seed wakefield amplitude. The black dots show the lowest limit of the transverse wakefield amplitude $W_{\perp ,\text{av},\min }$ assuming that the maximum defocused protons exit the wakefields after $L=10\mathrm{m}$. The blue diamonds show the best estimate from simulations $W_{\perp ,\text{av}}$ obtained with $L=1.5\mathrm{m}$ and assuming that the protons exit at 4 m.