Plasma accelerators in the 1000 GHz regime; electromagnetic-beat- and beam-driven structures

Abstract

Summary form only given. A nearly light-speed electron plasma wave (EPW); i.e., an EPW with the Lorentz factor associated with its phase velocity /spl Gt/1, may become a useful particle accelerating structure for science/industry. The accelerating fields E/sub z/ can exceed the /spl sim/200 MeV/m breakdown limit in /spl sim/10 GHz copper structures by orders of magnitude. As one goes up in EPW frequency /spl nu//sub p/ (by increasing the plasma density n/sub e/), the wavelength /spl lambda//sub p/ /spl ap/c//spl nu//sub p/ becomes smaller and above 10/sup 18/ cm/sup -3/ is <35 /spl mu/m, comparable to the shortest electron bunch lengths available from conventional linacs. The transverse variations of E/sub z/ and the importance of the inherent, strong focusing fields E/sub r/ also imposes an upper limit in density when the beam to be accelerated is /spl sim/10 /spl mu/m in diameter, again typical of a linac. Many short-pulse, laser-driven plasma accelerator experiments, such as the Laser Wakefield Accelerator (LWFA), are operated above 10/sup 18/ cm/sup -3/, but often envision creating bunches not from an external linac, but by coercing (using another arm of the laser) some of the plasma electrons to leave the thermal distribution and be accelerated by the EPW. Such an approach to electron injection is necessary in these high /spl nu//sub p/ experiments since the beam that match into the EPW is much smaller in all dimensions than is achievable from a linac. This paper deals with two sets of experiments that produce and use EPWs in the 1000 GHz regime. First are the Plasma Beatwave Accelerator (PBWA) experiments at UCLA and second are the Plasma Wakefield Accelerator (PWFA) experiments carried out at the Stanford Linear Accelerator Center. In both cases, the accelerated bunch is produced by a linac and thus by measuring the EPW-induced modifications to the transverse size and beam energy, some properties of E/sub z/(r, z, t) and E/sub r/(r, z, t) can be inferred and compared to theory.