The Ucla Pegasus Plane-wave Transformer Photoinjector*

A photoinjector based on a multi-cell plane wave transformer accelerating structure has been commissioned at the UCLA Department of Physics’ PEGASUS Laboratory. Design and construction of the novel structure have been previously reported [J. Rosenzweig, et al. PAC Proceedings 1997], and recent operation with a thermionic cathode is being presented at this conference [P. Frigola, et al. these proceedings]. This paper describes the planned operation of the PWT gun as a photoinjector, including design and construction details of the drive laser. Progress to date and future plans are discussed. THE PHOTOINJECTOR STRUCTURE The PEGASUS Photoinjector is a novel standing-wave S-band structure based on the Plane-Wave Transformer (PWT) design [1]. The injector consists of a replaceable cathode, an initial half-cell, and ten full cells, and a final half-cell for a total length of 60 cm [2]. Each cell is, in fact, a volume separated by disks of 4.2 cm diameter in a 12 cm diameter tank (see Figure 1). The RF structure features strong (0.3) cell-cell coupling to prevent mode overlap. Figure 1: A cross section of the PEGASUS PWT Photoinjector showing the 11 discs forming the cells inside the tank and the solenoid surrounding the cathode region. The peak field-gradient is designed to be 60 MV/m, and the nominal beam-energy is 17 MeV. The structure has a fill time of 2-3 μs, a shunt impedance of approximately 50 M/m, and a QL of roughly 6000. Due to the compact and simple design of the gun, a simple solenoid can be used for emittance compensation. Simulations indicate that the design specifications of Table 1 should be readily achievable [3]. The interchangeable cathode design allows for a variety of cathode materials to be tested including the planned use of copper, magnesium, heated LaB6, and conventional thermionic emitters. Status The status of the PEGASUS laboratory is reviewed elsewhere [4]. To date, the PWT linac structure has been conditioned at high power (20 MW) [5], and dark-current emission has been measured with a peak energy of 15 MeV over the 4 μs RF pulse [6]. An effort to retrofit a thermionic cathode has been underway [7] in order to deliver beam for testing and preliminary measurements until a drive laser can be procured. Table 1: Photoinjector design beam specification. Beam Parameter Value