Dynamics of a 28.5 GeV electron or positron beam in a meter-long plasma

Summary form only given, as follows. A plasma wakefield acceleration (PWFA) experiment is presently being conducted at the Stanford linear Accelerator Center (SLAC). In this experiment 28.5 GeV, 2 ps long electron (e/sup -/) or positron (e/sup +/) bunches with /spl ap/2/spl times/10/sup 10/ particles are sent in a 1.4 m long plasma with a density in the 0-2/spl times/10/sup 14/ cm/sup -3/ range. The plasma is obtained by single-photon ionization of a lithium vapor. The focusing of the bunches is observed by imaging on a CCD camera the optical transition radiation emitted by the bunches when traversing thin titanium foils placed /spl ap/1 m upstream and downstream from the plasma. The time resolved dynamics is studied by imaging onto a streak camera the light emitted by the beam when traversing a thin piece of aerogel located /spl ap/25 m downstream from the plasma, after a dispersive magnet. Quadrupoles located between the plasma and the aerogel image the particle beam at the plasma exit onto the aerogel. At the aerogel location, the beam image in the dispersive (vertical) plane is dominated by the beam energy, whereas in the perpendicular, non-dispersive plane it is dominated by the beam spot size. The incoming bunches have a correlated energy spread, and time integrated images of the beam after dispersion in energy also give an insight into the beam transverse dynamics. The beam density is larger than the plasma density, and the experiment can access the non-linear or blow-out regime of the PWFA. The plasma acts as a thick focusing element. Both e/sup -/ and e/sup +/ bunches are focused by the plasma. The focusing dynamics is observed with single bunches. The bunch particles work on the plasma electron to expel them from (e/sup -/), or attract them into (e/sup +/) the bunch volume, and energy loss of the particles in the bunch core is observed. Some of the energy is transferred back from the plasma electrons to the particles in the back of the bunch, and plasma a- celeration is observed with e/sup -/ bunches. The experimental set up as well as experimental results will be presented.