Multi-GeV Energy Gain in a Plasma-Wakefield Accelerator

Abstract

3module. A 28.5 GeV electron beam with 1:8 10 10 electrons is compressed to 20 m longitudinally and focused to a transverse spot size of 10 m at the entrance of a 10 cm long column of lithium vapor with density 2:8 10 17 atoms=cm 3 . The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.Plasmas have extraordinary potential for advancing the energy frontier in high-energy physics due to the large focusing and accelerating fields that are generated.Beam-plasma interactions have demonstrated focusing gradients of MT=m [1] while laser plasma interactions have demonstrated GeV=cm accelerating gradients [2 -7] over distances of a few mm. Beam-driven plasmawakefield accelerators (PWFA) have recently demonstrated acceleration and focusing of both electrons [8,9] and positrons [10,11] in meter scale plasmas.The experiment described in this Letter uses an ultrarelativistic electron bunch to simultaneously create a plasma in lithium vapor and drive a large amplitude plasma wave. When the electron bunch enters the lithium vapor, the electric field of the leading portion of the bunch ionizes the valence electron of each lithium atom in its vicinity leaving fully ionized neutral plasma for the remainder of the bunch [12,13]. The plasma electrons are then expelled from the beam volume and return one-half plasma period later. The returning plasma electrons form density concentrations on axis behind the bunch leading to a large accelerating field for the particles in the back of the bunch.In linear plasma theory [14] the wakefield amplitude increases as N= 2 z , provided the plasma density is increased such that k p z 2 p where N is the number of electrons in the bunch, z is the bunch length, and k p ! p =c is the inverse of the plasma collisionless skin depth. The nonlinear or blowout regime is reached when the electron bunch density n b N= 2 3=2 z 2 r is greater than the plasma density n p and the beam radius satisfies r c=! p . In the blowout regime, for bunch lengths on the order of the plasma wavelength, the plasma electrons are expelled from the beam volume to a radius r c 2 N= 2 3=2 z n p q leaving behind a pure ion column. This experiment is in a regime in which the electron bunch radius, bunch length, ion channel radius, and plasma wavelength are all on the same order. Although the experiments described here are on the edge of the blowout regime, numerical simulations indicate the N= 2 z increase in plasma-wakefield amplitude can still be realized [15]. Verification of the dramatic increase in accelerating gradient predicted for short bunches is a critical milestone for the application of plasma-wakefield accelerators to future high-energy accelerators and colliders.A single 28.5 GeV bunch of 1:8 10 10 electrons from the Stanford Linear Accelerator Center (SLAC) linac enters the Final Focus Test B...

Properties of Trapped Electron Bunches in a Plasma Wakefield Accelerator

“…Earlier experiments demonstrated the basic focusing properties of the ion bubble [56] and the scale of the fields in a PWFA [26]. The most recent set of experiments investigated these properties with a plasma column that was long enough to drive the minimum energy of the drive bunches to near zero (meter scale).…”

“…Recent experiments demonstrated the extraordinary focusing and accelerating properties of electron bunches in a PWFA [56,26,9] (see Ch. 5).…”

Energy Measurements of Trapped Electrons from a Plasma Wakefield Accelerator

“…The characteristic length scales of the system are the drive beam dimensions and the plasma wavelength. The drive beam was focused to a transverse spot size of 10 µm and compressed longitudinally to a minimum of about 12 µm at the entrance of the plasma [2]. The plasma wavelength was on the order of 64 µm.…”

“…Upstream of the plasma, coherent transition radiation (CTR) was collected from the drive beam [2]. This was used to monitor the amplitude of the accelerating field in the following way.…”

“…A neutral lithium vapor with a density of 2.7·10 23 m -3 was confined by a helium buffer gas in a heat-pipe oven [1]. An ultra-relativistic electron drive beam with 1.8·10 10 electrons, focused to a transverse spot size of 10 µm, and compressed longitudinally to a minimum of about 12 µm was sent through the neutral lithium vapor [2]. The electrons in the front of the bunch both field ionized the lithium vapor and drove out the plasma electrons [3].…”