Bunch Length Measurement on CANDELA Photo-Injector

CANDELA photo-injector is made of a 2-cell S-band RF gun, using a dispenser cathode illuminated by a Ti:sapphire laser. This electron source provides a single bunch (at 12.5 Hz), with a charge of 1 nC and an energy of 2 MeV. This paper presents the measurement of the bunch length which is done 1.8 m downstream of the gun exit. The measurement system includes a 0.3 mm thick sapphire plate used to produce Cerenkov radiation, a 27 m long optical beamline and a streak camera. Bunch lengths of less than 10 ps were measured. These measurements are the first experimental proof of the fast response of dispenser photocathodes. Introduction Many applications (including high energy linear colliders, free electron lasers, and X-ray radiation sources) need electron sources that can produce intense, bright and short electron pulses. The photo-injector being very attractive with these respects, it is studied in many laboratories around the world [1]. The CANDELA photo-injector is part of this worldwide effort, and has its own specific features. It is made of two decoupled 3 GHz cells [2, 3, 4] and uses a dispenser photocathode [5]. The Ti:sapphire laser system [6] used to illuminate this photocathode is able to produce subpicosecond pulses. To date, CANDELA that was first operated at the end of 1993 [7], is the only S-band photoinjector to use such a short laser. Basic experimental results, such as quantum efficiency were already reported in [8]. This paper is therefore concentrating on new experimental results concerning bunch length measurement. After presenting the experimental set-up, results are given for several conditions (laser spot size and charge). Experimental setup The gun RF cavity characteristics are given in reference [9], the cathode performance in reference [8] and the laser system in reference [6]. In order to analyze the beam properties, several diagnostics systems are located along the beamline as shown in figure 1. Bunch charge is measured with Faraday cups and wall current monitors. The latter have the advantages to be nondestructive and to respond only to the photo-emitted current. The Faraday cup followed by an integrator gives an indication of the total current (photo-current plus dark current). Two ceramic fluorescent screens and CCD cameras allow to visualize the beam. A commercial software designed for laser profile analysis [10] provides the information on beam profile. Pepper-pot Emittance Measurement System