Emittance and Current of Electrons Trapped in a Plasma Wakefield Accelerator

Abstract

Abstract. In recent experiments plasma electrons became trapped in a plasma wakefield accelerator (PWFA). The transverse size of these trapped electrons on a downstream diagnostic yields an upper limit measurement of transverse normalized emittance divided by peak current, ε N,x /I. The lowest upper limit for ε N,x /I measured in the experiment is 1.3 · 10 −10 m/A.Keywords: Electron, Trapping, Plasma Wakefield Accelerator PACS: 52.35. Py, 52.40.Mj, 41.75.Lx In this experiment, ultrarelativistic electron drive bunches field ionized neutral vapor, creating a plasma [1] and expelling the plasma electrons from the bunch axis. The ions then pulled the plasma electrons back to the bunch axis with a time scale set by the inverse of the plasma frequency, ω p = n p e 2 /(mε 0 ), where n p , e, m, and ε 0 are the plasma density, the electron charge magnitude, the electron mass, and the permittivity of free space. The resulting plasma wake could accelerate some plasma electrons from rest to relativistic energies before they slipped out of the wake, trapping bunches of electrons. This process can produce electron bunches with small emittance and high peak current. Direct measurements of the trapped electron emittance and peak current were not possible; however, the ratio of these quantities is experimentally accessible. This paper presents a measurement of transverse emittance divided by peak current.This experiment took place in the Final Focus Test Beam facility, located at the end of the Stanford Linear Accelerator Center linac. Let the quantities x and y represent the transverse coordinates and z denotes the longitudinal coordinate. Electron drive bunches of 3 nC and 42 GeV with normalized emittances of ε N,x = 60 µm and ε N,y = 7 µm, transverse sizes of 10 µm, and longitudinal bunch lengths of tens of µm entered a heat-pipe oven [2]. This heat-pipe oven confined lithium vapors of density, n p , equal to 2.7 · 10 23 m −3 with a helium buffer gas. The lithium density was uniform in the middle of the oven, had a full width at half max (fwhm) length of 85 cm, and had Gaussian roll-offs at its up and downstream edges:, where z − z 0 denotes the distance from the edge of the uniform region and σ L = 3.97 cm. Figure 1 displays the partial pressures of the two gas species in the heat-pipe oven. Beryllium windows of thickness 50 and 75 µm were located up and downstream of

Properties of Trapped Electron Bunches in a Plasma Wakefield Accelerator

“…In the presence of helium ionization, an upper limit measurement of N,x /I t results from the determination of upper bounds for the trapped electron bunch's transverse size and divergence at the point it started to diverge freely [37]. The trapped electrons diverged freely either before or when they reached the downstream beryllium window.…”

“…Then, simulations and charge measurements from the energy spectrometer combine to show that the trapped electrons satisfied these requirements. Finally, upper bounds on the transverse size and divergence of the trapped electron bunches, at the point they started to diverge freely, yield an upper limit measurement of the transverse emittance over the peak current for these bunches [37,39].…”