BEAM DYNAMICS SIMULATIONS OF SUB-PS ELECTRON BUNCH PRODUCED IN A PHOTO-INJECTOR

A growing number of experiments require low emittance ultra-short electron bunches in the 100 fs range (rms value) for the production of coherent light or the injection into a plasma for laser-plasma acceleration. Especially in the last case it is highly desirable to have a compact accelerator; hence a strong experimental activity is carried out to get such a beam directly from a photoinjector. We have performed beam dynamic simulations using the PARMELA code to study the performances of the alphaX photo-injector installed in the University of Strathclyde in UK. This RF gun is aimed to produce electron bunches which have 100 pC of bunch charge, 100 fs bunch length and 1 π mmmrad transverse emittance. We will show the results of systematic parametric studies as a function of charge and laser pulse duration as well as the natural evolution of the beam phase space as a function of the distance from the photocathode. INTRODUCTION Up to now the preferred way to get very short bunch is to use a magnetic chicane but this could lead to a degradation of the emittance due to the emission of coherent synchrotron radiation. Moreover it is difficult to make a compact beamline in such a scheme. Some experiments as alphaX [1] in the University of Strathclyde strive to produce X ray in the SASE [2] regime with a very short accelerator which the length is roughly 7 meters. The idea is to inject a very short electron bunch coming from a photo-injector into a laser-plasma [3] accelerating cell to bring it at 1 GeV over few millimeters instead of ≈ 30 m in classical linear accelerators. In order to get an efficient acceleration in the plasma, the specification on the bunch length must be absolutely fulfilled. So the purpose of this study is to investigate the behaviour of sub-ps electron bunches directly issued from the photo-injector using numerical simulations. PARMELA SIMULATIONS PARMELA is a well-known code used for beam dynamics in linacs which takes into account the space charge force effect. It was created by Los Alamos National Laboratory and modified at LAL by B. Mouton to include, for instance, the photo-injectors. We acknowledge the support of the European Community-New and Emerging Science and Technology Activity under the FP6 “Structuring the European Research Area” programme (project EuroLEAP, contract number 028514). AlphaX Photo-injector The alphaX photo-injector is made of 2.5 cells at 3 GHz resonant frequency. This gun designed at the Eindhoven University of Technology [4] exhibits two noticeable features. First, the shape of irises is elliptical because, according to the RF simulations, the surface electrical field is reduced by a factor 2 with respect to the more usual cylindrical shape. So one hopes to operate the gun at gradient higher than 100 MV/m without electrical breakdowns. Secondly the RF power is sent to the gun via a coaxial “doorknob” antenna in a coupler at the output of the gun. In this way the gun keeps a perfect cylindrical symmetry in order to avoid possible degradation of the emittance rising in non-symmetric coupling. Simulations in PARMELA are performed with the electrical field of alphaX calculated with the 2D RF code SUPERFISH. Results are shown in table 1 for 6000 macro-particles used in the simulation. Table 1: Results of PARMELA simulations at the output of the gun, input parameters are (rms value): Q = 100 pC, laser has a Gaussian profile, radius = 1.4 mm cut at 10 mm and pulse duration = 100 fs cut at 500 fs. The peak electrical field is 92 MV/m and the phase is chosen to optimise the energy gain, rms value. Beam radius (mm) 3.1 Bunch length (fs) 206