Commissioning and Measurements of the Neptune Photo-injector

The photo-injector for the Neptune Advanced Accelerator Laboratory is introduced. Its component parts, including the radio frequency gun, photo-cathode drive laser system, booster linac, RF system, chicane compressor, beam diagnostics, and control system are described. The injector is designed to produce high brightness, short pulse electron beams. Measurements of the photo-injector beams including quantum efficiency, emittance, pulse length, and pulse compression are presented. THE NEPTUNE PHOTO-INJECTOR The Neptune Advanced Accelerator Laboratory consists of two main components, the RF photo-injector and the high power, short pulse, two frequency Mars CO2 laser. [1] The main goal of the lab is to accelerate a high quality, relativistic electron beam injected into a plasma beat wave accelerator (PBWA) to over 100 MeV, while preserving the phase space density of the injected beam. [2] The PBWA experiment can take two different forms, one where a beam of moderate charge and emittance (Q ~ 1 nC, 8n ~ 5 mm mrad) covers more than one plasma wavelength, and the second where a shorter, low charge and emittance beam (Q « 50 pC, £n < 0.1 mm mrad) is loaded into a single cycle of the PBWA. Because of the range of injected beams required by the two phases of the PBWA experiment, the Neptune photo-injector is designed to produce an emittance compensated [3,4], optimized beam over an extent of different charges. This is done with a powerful method of charge scaling recently developed for photo-injectors. [5] This flexibility in the beam parameters the photo-injector can produce makes feasible many advanced accelerator experiments in the Neptune lab. These include freeelectron laser (PEL) microbunching for injection into the PBWA, underdense plasma focusing [6-8], plasma wake-field acceleration (PWFA) [6,9], and inverse PEL acceleration [10]. In addition to these advanced acceleration experiments, studies of the high brightness beams themselves are underway at Neptune. These include the role of space-charge in emittance measurements, beam compressibility, and the process of emittance dilution in bends. CP569, Advanced Accelerator Concepts: Ninth Workshop, edited by P. L. Colestock and S. Kelley © 2001 American Institute of Physics 0-7354-0005-9/017$ 18.00 487 In the remainder of this section we describe the various component parts of the photo-injector and where applicable, their current performance. The components of the photo-injector include the RF gun, photo-cathode drive laser system, booster linac, rf system, chicane compressor, beam diagnostics, and control system. Figure 1 shows the layout of the photo-injector beamline. Screen 3 (Before Linac) •Screen 2 (Cathod Mlrrorjj 1.625 ell gun I Screen 5 (Before Chicane) 8 (Ernittancs Screen) FIGURE 1. The Neptune Photo-injector beamline. Accelerator Sections The photo-injector has a split accelerator design consisting of a photo-cathode gun, a drift space, and a booster linac. The gun is a 1.625 cell 7i-mode standing wave cavity produced by a BNL-SLAC-UCLA collaboration. [11] The gun has been conditioned up to an input power of 6.5 MW, which corresponds to an on-axis peak field of 100 MV/m. The current operating power in the gun is somewhat lower than this (due to limited total output power of the rf system) and the nominal peak accelerating field is 85 MV/m. The original cathode of this gun was simply the OFHC copper backplane of the half-cell. More recently however, this backplane was replaced with one including a 1 cm diameter, 1 mm thick disk of single crystal copper (Cuioo). [12] The properties of the two cathodes will be discussed further in the measurements section below. The booster linac is a 7 and 2/2 cell 7C-mode standing wave structure. The linac design is that of a plane-wave transformer (PWT) which benefits from strong cell-to-cell coupling and large mode separation. [13] The linac has been conditioned up to 13 MW of input power and runs with a nominal peak accelerating field of 50 MV/m. RF System Low level RF starts with the 38.08 MHz output signal of the mode-locked laser oscillator (which is the first component of the photo-cathode drive laser). This signal is frequency multiplied by 75 to produce S-band RF. After passing though a phase