Ultrarelativistic-Positron-Beam Transport through Meter-Scale Plasmas

We report on the first study of the dynamic transverse forces imparted to an ultrarelativistic positron beam by a long plasma in the underdense regime. Focusing of the 28.5 GeV beam is observed from time-resolved beam profiles after the 1.4 m plasma. The strength of the imparted force varies along the $\sim 12\mathrm{p}\mathrm{s}$ full length of the bunch as well as with plasma density. Computer simulations substantiate the longitudinal aberration seen in the data and reveal mechanisms for emittance degradation.

Figure 1

Time-integrated measurements of the positron beam spot size in the $x$ direction vs $n_e$ from the two profile monitors downstream of the plasma exit. The symbols at zero density are the mean no-plasma spot sizes at the Cherenkov radiator (△) and OTR (▽) for 50 pulses. The bars indicate the error of the mean.

Figure 2

Time-resolved measurements of the positron bunch at the Cherenkov plane for various plasma densities. (a) Measured bunch intensity $A(\zeta )$ (right arrows) and Gaussian fit (solid line). Temporal variation ${\sigma }_x(\zeta /c)$ of the $x$ spot size for zero density (circles) and for $n_e=4.1\times {10}^9{\mathrm{c}\mathrm{m}}^{-3}$ (diamonds). (b) Temporal variation of the normalized $x$ spot size $\overline{{\sigma }_x}(\zeta /c)$ for several densities (see key). Some of the data points for $\zeta /c=-5$ and $+6\mathrm{p}\mathrm{s}$ are close to the noise floor which tends to artificially broaden the slice sizes. (c) Variation with density of $\overline{{\sigma }_x}$ for slices at $-4$ and $+4\mathrm{p}\mathrm{s}$, labeled head and tail, respectively.

Figure 3

Results from PIC simulations. The solid arrow in each graph indicates the propagation direction of the bunch. (a) Two-dimensional slice $\eta (x,y=0,\zeta )$ ($\propto$ charge density) at the plasma entrance. The overlay shows the 61%, 22%, and 1% elliptical contours of the beam density. (b) Lineouts of $\eta (x,y=0,\zeta )$ for values of $x/{\sigma }_x$ of 1, 0.5, 0.25, and 0.125 (moving from the dotted line to the solid line, respectively). (c) Beam intensity plots in the $x$-$z$ plane for $n_e$ values of (i) 0, (ii) $2.0\times {10}^{11}{\mathrm{c}\mathrm{m}}^{-3}$, (iii) $8.0\times {10}^{11}{\mathrm{c}\mathrm{m}}^{-3}$, and (iv) $2.0\times {10}^{12}{\mathrm{c}\mathrm{m}}^{-3}$ (see text for details). The solid and dashed contours are at 0.13 and 0.013 of the maximum intensity within all the plots, respectively.