A Polarized Electron PWT Photoinjector for the ILC

In order to provide maximum physics benefits, the International Linear Collider (ILC) must have a highly polarized electron beam with a low emittance. High electron spin -polarization (>85%) is attainable with a modern GaAs photocathode illuminated by a circularly polarized laser. Low emittance is achievable with an rf photoinjector, and can be further improved with a round-to-flat beam transformation. A low-emittance rf gun may simplify or eliminate the complicated and expensive electron damping ring downstream. DULY Research has been developing an rf photoelectron injector called the Plane Wave Transformer (PWT) which may be particularly suitable as a polarized electron injector for the ILC. A unique feature of the PWT is that it has a much larger vacuum conductance than conventional rf guns, which makes it possible to easily pump the structure to a vacuum level better than 10 -11 Torr at the photocathode. In contrast with other normal-conducting L-band gun designs which require operatio n at a high peak field (=35MV/m) in order to achieve a low normalized transverse emittance, an L-band PWT gun can achieve a low emittance at a lower operating peak field ( ̃20MV/m). The low peak field in the PWT gun is beneficial for the survivability of GaAs photocathode because electron backstreaming is greatly mitigated.