C. Joshi

Update of Proton Driven Plasma Wakefield Acceleration

In this paper, the update of proton driven plasma wakefield acceleration (PDPWA) is given. After a brief introduction to the scheme of PDPWA, a future demonstration experiment is discussed. The particle‐in‐cell simulation results based on the realistic proton beams from the CERN Super Proton Synchrotron (SPS) are presented, followed by a simulation study of proton bunch compression.

Test of the electron hose instability in the E157 experiment

The E157 experiment is designed to demonstrate high gradient plasma wake field acceleration over a significant length. It has been suggested that the electron hose instability of the drive beam will degrade the performance of this experiment because the hosing tail electrons will not fully sample the highest acceleration field. In this paper a parasitic experiment designed to test the extent of the hosing instability is described. In particular, we discuss how the initial beam conditions are determined so that the extent to which any transverse perturbations grow due to hosing can be determined.

Progress toward E-157: a 1 GeV plasma wakefield accelerator

A plasma based wakefield acceleration (PWFA) experiment, scheduled to run this summer, will accelerate parts of a 28.5 GeV bunch from the SLAC linac by up to 1 GeV over a length of 1 meter. A single 28.5 GeV bunch will both induce the wakefields in the one meter long plasma and witness the resulting acceleration fields. The experiment will explore and further develop the techniques that are needed to apply high-gradient PWFA to large scale accelerators. This paper summarizes the goals of the first round of experiments as well as the status of the individual components: construction and diagnosis of the homogeneous lithium oven plasma source and associated ionization laser, commissioning of the electron beam, simulated performance of the electron beam energy measurement, and first PIC simulations of the full meter long experiment.

The Plasma Wakefield Accelerator

In the Plasma Wakefield Accelerator (PWFA) scheme a short bunch of high current (density nb), but low voltage electron beam is shot through a dense plasma. For a properly shaped bunch the background plasma electrons are adiabatically displaced from their initial positions and are subsequently released after the bunch has passed, resulting in a plasma wakefield. A trailing low current bunch can be accelerated by the plasma wakefield to energies on the order of.-1-11ln y0 mc2 when the driving bunch is appropriately shaped. The plasma thus acts like a transformer since it increases voltage at the expense of current. This scheme is thus in the general class of wakefield transformers, except that transformer ratios greater than 2 can be obtained even for axial co-propagating beams. In this paper we win describe recent work at UCLA on the PWFA.

Positron Source from Betatron X-Rays Emitted in a Plasma Wiggler

In the E-167 plasma wakefield accelerator (PWFA) experiments in the Final Focus Test Beam (FFTB) at the Stanford Linear Accelerator Center (SLAC), an ultra-short, 28.5 GeV electron beam field ionizes a neutral column of Lithium vapor. In the underdense regime, all plasma electrons are expelled creating an ion column. The beam electrons undergo multiple betatron oscillations leading to a large flux of broadband synchrotron radiation. With a plasma density of 3 × 1017cm-3, the effective focusing gradient is near 9 MT/m with critical photon energies exceeding 50 MeV for on-axis radiation. A positron source is the initial application being explored for these X-rays, as photo-production of positrons eliminates many of the thermal stress and shock wave issues associated with traditional Bremsstrahlung sources. Photo-production of positrons has been well-studied; however, the brightness of plasma X-ray sources provides certain advantages. In this paper, we present results of the simulated radiation spectra for the E-167 experiments, and compute the expected positron yield.

A Dc To Optical Frequency Converter Based On Plasma Ionization

The authors describe an idea for a new class of high power radiation source. Rather than create electromagnetic fields, the present scheme frequency upshifts an existing static field ({omega} = 0, k = k{sub 0}) by temporally varying the dielectric properties of a medium (i.e., ionizing a gas). An array of alternating capacitors is charged to a large voltage to produce a static electric field of the form E {approximately} (E{sub 0} sin k{sub 0} z) {cflx y}. When the area between the capacitors is filled with a low density working gas and ionized by a short-pulse laser, a phased discharge current generates a radiation pulse following behind the ionizing laser. The frequency of the radiation generated will be shown to scale as {omega} = {omega}{sub p}{sup 2}/2k{sub o}c where {omega}{sub p}{sup 2} = 4{pi}n{sub o}e{sup 2}/m and n{sub o} is the gas/plasma density. The output frequency, pulse duration, bandwidth, arbitrary chirp, etc. can be easily controlled by varying the gas pressure and/or capacitor spacing. Surprisingly, the larger is the spacing between capacitors, the higher is the output frequency. Theory, 2-D PIC simulations and plans for proof-of-principle experiments in the microwave to infrared regime will be presented.

High transformer ratio drive beams for wakefield accelerator studies

For wakefield based acceleration schemes, use of an asymmetric (or linearly ramped) drive bunch current profile has been predicted to enhance the transformer ratio and generate large accelerating wakes. We discuss plans and initial results for producing such bunches using the 20 to 23 GeV electron beam at the FACET facility at SLAC National Accelerator Laboratory and sending them through plasmas and dielectric tubes to generate transformer ratios greater than 2 (the limit for symmetric bunches). The scheme proposed utilizes the final FACET chicane compressor and transverse collimation to shape the longitudinal phase space of the beam.

Plasma Dark Current in Self-Ionized Plasma Wake Field Accelerators

Evidence of particle trapping has been observed in a beam driven Plasma Wake Field Accelerator (PWFA) experiment, E164X, conducted at the Stanford Linear Accelerator Center by a collaboration which includes USC, UCLA and SLAC. Such trapping produces plasma dark current when the wakefield amplitude is above a threshold value and may place a limit on the maximum acceleration gradient in a PWFA. Trapping and dark current are enhanced when in an ionizing plasma, that is self-ionized by the beam. Here we present experimental results.

Plasma Light diagnostic for PWFA at SLAC

Summary form only given. A highly relativistic electron beam passes through an oven filled with the particular gas used in the experiment creating a plasma and a large amplitude wake field which causes the beam to lose and gain energy. The energy dumped into the plasma is dissipated through recombination and thermalization. Intensity of the plasma light is proportional to the wakefield amplitude. As the only non-beam diagnostic, study of plasma light can be used to characterize the plasma beam interaction to get the highest acceleration gradient. Moreover the unique spectrum of the gas can be used to as a reliable tool to measure the density vial the theory of Stark Broadening as an alternative to the other plasma density diagnostic tools which may not be available at the higher densities. Application of Plasma Light diagnostic to the past and future plasma experiments will be presented.

Plasma wakefield acceleration experiments with 28.5 GeV electron and positron beams

Summary form only given, as follows. Large gradient accelerators are necessary to reach the very high energies required at the collision point of future electron/positron colliders In the plasma wakefield accelerator (PWFA), a short electron or positron bunch drives a large amplitude plasma wave or wake. The transverse component of the wake leads to focusing of the particle bunch, while longitudinal components of the wake lead to energy loss and energy gain by particles. The PWFA is an energy transformer in which the energy is transferred from the particles in the core of the bunch in a single bunch scheme, or from a driver bunch in a two bunch scheme, to the particles in the back of the same bunch, or to a trailing witness bunch In the experiments described here, the 28.5 GeV electron or positron beam of the Stanford Linear Accelerator Center Final Focus Test Beam line is sent in a long lithium plasma. The bunch charge density is density is larger than the plasma density and the plasma wake is driven in the non-linear regime. In the case of an electron bunch, the bunch space charge field expels all the plasma electrons from the beam volume. The pure plasma ion column left behind the bunch head acts as an aberration-free plasma lens on the bunch core.