Abstract
Plasma production via field ionization occurs when an incoming electron beam is sufficiently dense that the electric field associated with the beam ionizes the neutral vapor. Experiments at the Stanford Linear Accelerator Center (SLAC) explore the threshold conditions necessary to induce field ionization in a neutral lithium (Li) vapor. By independently varying the bunch length, transverse spot size or number of electrons per bunch, the radial component of the electric field is controlled to be above or below the threshold for field ionization. A self-ionized plasma is an essential step for the viability of plasma-based accelerators for future high-energy experiments. Based on the experimental results, the incoming beam ionizes the neutral Li vapor when its peak electric field is approximately 5{approx}GV/m and higher. This electric field translates into a peak charge density of approximately 3 x 10{sup 16} {approx} cm{sup -3}. The experimental conditions are approximated and simulated in a 2-D particle-in-cell code, OOPIC. The code and the data correspond well in terms of the correct threshold conditions and the dependence on the critical beam parameters. In addition to the ionization threshold, the field ionization effects are characterized by the beam's energy loss through the Li vapor column due tomore » the plasma wake field production. The peak and average energy loss as a result of wake production and beam propagation through the plasma is compared with simulation results from OOPIC. The simulation code accurately predicts the peak energy loss of the beam, but approximations in the code produce differences between the average energy loss measured and the loss calculated by the simulation.« less
Highlights
The successful demonstration of a beam-driven plasma wake field accelerator (PWFA) over macroscopic distances is a critical milestone in the progression of plasmas from laboratories to future high-energy accelerators and colliders, where a combination of high density and long length will be required
When a high-density, ultrashort bunch enters a region filled with a neutral vapor or gas, the electric field associated with the beam can ionize the valence electron of each neutral atom in its vicinity leaving a fully ionized plasma for the remainder of the bunch [3]
The rate of field ionization is related to the local electric field associated with the incoming bunch
Summary
The successful demonstration of a beam-driven plasma wake field accelerator (PWFA) over macroscopic distances is a critical milestone in the progression of plasmas from laboratories to future high-energy accelerators and colliders, where a combination of high density and long length will be required. In a paper recently published on experiments performed at the Stanford Linear Accelerator Center (SLAC), a beam-driven PWFA showed accelerating gradients of greater than 30 GeV=m over a 10 cm-length plasma, which were achievable due to the incoming beam’s ability to simultaneously ionize a neutral Li vapor and drive a large-amplitude wake to accelerate the tail particles [1]. These experiments were performed in a nonlinear, relativistic regime and, based on simulations, the accelerating gradient of the system in this regime increases as the bunch length decreases; ultrashort bunches are preferred [2]. For the correct combination of bunch length and vapor density, the beam’s electric field can ionize a neutral vapor thereby generating its own plasma and the resulting space-charge field drives a high-amplitude wake to accelerate the beam’s tail particles
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