Use of localized self-induced plasmas to focus high energy electron beams

We have observed the focusing of high energy electron beams by self-induced gaseous plasmas. This work is being done in the Final Focus Test Beam line (FFTB) at the Stanford Linear Accelerator Center (SLAC). The level of impact ionization of the neutral nitrogen gas target by the 29 GeV electron beam is adequate to produce clear focusing action. A 40% reduction in the size of each transverse beam dimension is observed for plasma lengths of only 3 mm. Experimental data is integrated over the electron bunch length of 700 microns in which we have 1.5/spl times/10/sup 10/ particles (per bunch). The incident transverse beam size of 7 microns (x dimension) by 3 microns (y dimension) corresponds to an incident beam density of about 7/spl times/10/sup 16/ cm/sup -3/. Normalized transverse beam emittances are 5/spl times/10/sup -5/ and 6/spl times/10/sup -6/ m.rad. For x and y directions respectively. Neutral molecular nitrogen is injected into the vacuum beamline with a pulsed solenoid valve. Gas target densities up to 4/spl times/10/sup 18/ cm/sup -3/ were used. Beam sizes are determined by a scanning technique. With this method a fine carbon wire is placed on the beam centroid. The electron beam is then controllably dithered about the wire position while monitoring the generated bremsstrahlung signal downstream. Because the beam operates at a 10 Hz rate while the gas is introduced at only 0.5 Hz each scan provides transverse profile data with and without the plasma lens. These results describe a small plasma element with a focusing strength which exceeds that of conventional magnets by about two orders of magnitude.