Refraction of a Particle Beam at a Plasma-Gas Interface Introduction A recent experiment at the Stanford Linear Accelerator Center (SLAC) was designed to investigate the refraction and reflection of a beam of 30 GeV electrons off of a boundary between dilute plasma and a gas. Although refraction of light at an interface is as common as looking through a glass of water, the similar refraction of particle beams has not been previously demonstrated. The results are remarkable in that the beam is intense enough to bore through a mm thick sheet of steel, but bounces off of a dilute layer of plasma that is one million times less dense than air! This is a result of collective effects that enhance the refraction in the plasma. The work is in preparation for submission to Nature. Who: A collaboration of researchers from USC, UCLA and SLAC led by T. Katsouleas, C. Joshi and R. Siemann. When: June, 2000. Theory Figure 1. Schematic of refraction mechanism:. a) side view and b) front view of beam and plasma illustrating how asymmetric blowout creates a net deflection force. Experimental Results Images of the electron beam showing refraction of a portion of the beam: a) Cerenkov image without the plasma (i.e., laser off). b) Cerenkov image with the laser on at an angle (phi) of 1mrad to the beam. Cross-hairs show the undeflected beam location. This shows a characteristic splitting of the beam into two in qualitative agreement with the simulation shown below it. c) PIC simulation of electron beam, side view with plasma shown (blue). The inward motion of the plasma electrons is visible as a depression in the blue plasma surface behind the beam. d) PIC simulation, head on view corresponding to (b). The code used was OSIRIS. The simulation parameters were chosen to be similar to be similar to the experiment; namely a 30 GeV beam with 2e10 electrons in a bi-Gaussian distribution (spot size sr=70mm, bunch length sz=0.63 mm) incident on a plasma of density 2e14 cm-3. In the simulation, the beam propagated through approximately 80 cm of plasma before encountering a sharp plasma/vacuum boundary at an incident angle of 0.9 mrad in the x-z plane. The model consisted of 2e7 particles on a 160x120x88 grid representing a 2.9mmx2.2mmx6.5mm section of plasma (x,y,z) moving with the beam (z-direction). Experimentally measured electron beam deflection (theata) as a function of angle between laser and e-beam (i.e., angle of incidence (phi)). Solid curve is the impulse model. 3-D particle-in-cell simulation of an electron beam refracting as it emerges from a plasma (blue) Movie Download Quicktime Movie (2.83MB)