Optimization of the ELBE Injector for the Production of Electron Beams with Minimal Transverse Emittance



Fig. 1: Design of the ELBE accelerator; the figure shows the injector, the first cavity and that part of the beam transport system that leads the beam to the radiation physics cave.

For radiation physics and radiation biology experiments an electron beam with a normalized transverse RMS emittance of less than 1 p mm mrad and an average current of 100 mA is required. This requirement can be met by cutting down the 1 mA average current delivered by the cw thermionic RF gun [1, 2] with two diaphragms in the injector beam path, thus reducing the initial 1.2 p mm mrad transverse emittance to less than 0.5 p mm mrad before the entrance of the accelerator cavities.



Fig. 2: Envelope and transverse emittance of a 100 mA electron beam in the ELBE accelerator; double lines mark the positions of solenoid magnets, single lines mark the positions of the two bunchers, diaphragms are shown in blue and the accelerator cavities are underlaid green.

The proposed design was developed using the PARMELA [3] code. Figure 2 shows the computed beam properties within the ELBE accelerator. Solenoid fields and RF phases of bunchers and cavities have been adapted to the smaller space charge of the reduced electron current and optimized to achieve a minimum possible transverse emittance of the beam at the accelerator exit. With these settings an 18 MeV electron beam with a normalized transverse RMS emittance of 0.75 p mm mrad and an energy spread of less than 100 keV can be produced.


[1]  F. Gabriel ed.; FZD Internal Design Report (1995)
[2]   D. Janssen, P. von Stein; Nuclear and Hadron Physics and Project ELBE, Annual Report (1996)
[3]   L.M. Young; Los Alamos National Laboratory (1996) LA-UR-96-183