ELI PHOTOINJECTOR PARAMETERS: PRELIMINARY ANALYSIS AND SIMULATIONS C. RONSIVALLE

TENTATIVE BEAM PARAMETERS A parameters space connecting beam parameters and photon requirements (flux and bandwidth) including laser characteristics has to be done

RMS ENERGY SPREAD VS RMS PULSE LENGTH FOR DIFFERENT RF FREQUENCIES requires  t (ps) Pulse length (ps) S-BAND C-BAND X-BAND

HIGH CHARGE WPOINT: ASTRA+GIOTTO (genetic algorithm optimization) From A. Bacci tables The message is that similar values of eta parameters could be achieved, but with very long pulses giving in any case unacceptable energy spread values, but this could be only partially true.

dots=experimentally achieved parameters 250pC,  n=0.35 mm-mrad,  E/E=0.16% 25 pC,  n=0.1 mm-mrad,  E/E=0.03%

 parameter: current SCENARIO

BEST RESULT IN S-BAND

From these data (95% emittances) pC/(mm-mrad) 2

Beam length corresponds to an energy spread~0,1% LCLS measurements

Frequency=1.3 GHz Gun gradient=60 MV/m BEST RESULT IN L-BAND

From these data (100% emittances) pC/(mm-mrad) 2 Emittance vs charge: LCLS-PITZ comparison

COMMENTS: The largest  value has been achieved operating in L-band with Egun=60 MV/m with a long bunch. High frequencies (allowing high gun fields) are needed for high brightness but are not strictly required for high  values. In addition of course they reduce the accelerator size. The optimal choice for the RF frequency for our application is the best compromise between the following requirements: 1) High  value 2) Reduced energy spread (0.1% or less) 3) Compactness if needed 4) Reliability

LLNL(computation) LCLS(experiment) PITZ(experiment)

STATUS OF TSTEP SIMULATIONS: -A model of the C-band line has been set based on the fields maps (gun (1.6 cells) +solenoid+TW sections) - A preliminary test of a solution found by ASTRA + GIOTTO has been done

HIGH charge working point at 250 pC: ASTRA+GIOTTO results

TSTEP MODEL: only first 4 sections 200 MV/m gives an output gun energy of 4.6 MeV

0.285 mm-mrad Emittance(mm-mrad) Envelope (mm)

INPUT SECTION 4 EXIT ( MeV)  x=50 µm

LONGITUDINAL PHASE SPACE A solution based on a shorter pulse able to reach the same performances of LLNL Is under study

CONCLUSIONS - It could be useful to individuate a parameters space -The analysis of the current scenario shows that the RF frequency that meets both the requirements on energy spread and eta parameter seems to be in the S-band. The improving in the eta factor given by the use of an increase of frequency is not so high and is paid with an increase of energy spread. The main reason to go to high frequencies seems to be the reduction in size. - We are investigating the possibility to use a full C-band solution -The possibility to achieve an emittance around 0.3 mm-mrad with a long flat pulse as it was obtained by ASTRA has been confirmed by TSTEP with a different optimization giving a better matching of the injected beam to the structure - For the high charge working point some effort must be done to find a compromise between length and emittance in order to obtain a solution similar to LLNL without energy spread compensation by X-band linearization