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The Plasma Density Transition Trapping Experiment


The difficultly of achieving the sub-picosecond timing required to inject an externally generated electron beam into a plasma wake field accelerator (PWFA) has lead many researchers to examine self-trapping scenarios. Most of these ideas have centered around inducing conventional wave breaking in the system. While conventional wave breaking does trap large amounts of charge in the plasma wave, this charge tends to fill large areas of phase space forming a poorly defined beam. Plasma density transition trapping replaces conventional wave breaking with a single, stimulated, wave breaking event.

The Mechanism

In the plasma density transition trapping scenario a PWFA drive beam passes from a region of high plasma density into a region of lower plasma density. As the plasma wake crosses the sharp boundary between the two plasma density regions it must instantaneously increase in wavelength. The sudden growth in the plasma wavelength effectively dephases a significant volume of plasma electrons into an accelerating phase of the wake. A large portion of these electrons become trapped in a well defined beam. When this technique is scaled up to high plasma densities the trapped beams can exceed the parameters specified for the LCLS injector.

Animation of Transition Trapping Simulations (avi).

This animation shows a simulation of the plasma electrons (red dots) begin blown out by a passing high charge electron beam (not shown). When the wake passes the density transition, which is at 15 mm, the sudden change in plasma wavelength causes many plasma electrons to become trapped and form a bunch inside the wake where they are accelerated.

The Experiment

The proof-of-principle experiment is designed around a low energy 14 MeV drive beam of approximately 6 nC charge and sigmat=1.5 psec. When this beam passes through a 2x1013 cm-3 plasma that has a sharp transition to 3.6x1012 cm-3 a 100 pC, 1.2 MeV beam with only 11% total energy spread will be captured. The plasma density and drive beam parameters were chosen to make this initial transition trapping experiment as easy as possible.

The experiment will be conducted at the Fermilab/NICADD Photoinjector Laboratory Facility . The FNPL accelerator is a 18 MeV electron linac. The system consists of a normal conducting L-band RF gun with a cesium telluride photo-cathode and a 9-cell superconducting accelerating cavity. Bunches with charge in excess of 8 nC can be produced and compressed to durations of 1.6 ps rms using magnetic compression. The plasma source for this experiment arrived at FNPL from UCLA on Dec 1, 2003. The plasma is scheduled to see beam in January 2004. Click on the categories at left for more information.

Selected Papers

New PRSTAB Article - Plasma density transition trapping as a possible high-brightness electron beam source - M.C. Thompson, J.B. Rosenzweig, and H. Suk

The UCLA/NICADD Plasma Density Transition Trapping Experiment - M.C. Thompson, et. al.

Beam-Plasma Interaction Experiments at the UCLA NEPTUNE Laboratory -M.C. Thompson, et. al.