–10.06 Milan Italy LUCX system and dark current (1) LUCX project (2) Phase Ⅰ and results (3) Phase Ⅱ and dark current Liu shengguang and LUCX group

–10.06 Milan Italy (1) Purpose of LUCX LUCX ( Laser Undulator Compton X ray source) is for generation x- ray photon by inverse Compton scattering. X-ray is for medical application ----K-edge digital subtraction angiography. In this application, 33KeV X ray photons are required. Collision angle between electron beams and the laser beams is 20 degree. The detector size is 4cm×4cm, 2m away from the collision point. Acceptance angle of the detector is 10mrad.

–10.06 Milan Italy (1) Main parameters of LUCX Electron beamLaser pulse Energy43MeV1064nm (1.17eV) Intensity2nC/bunch (5nC/bunch) 1.25*10^10electron/bunch 17µJ/pulse 9.1*10^13photon/pulse Width (FWHM)10ps Bunch number100bunches/train Beam sizeσ x =64um σ y =32um σ= 100um Repetition rate12.5Hz Expected X ray photon number on the detector : /bunch /train /sec

–10.06 Milan Italy (2) Phase Ⅰ Until the end of 2005, RF gun study had been finished and multi bunch beams had been generated.

–10.06 Milan Italy (2) Cs2Te cathode system Mover Preparation system RF gun Cathode load system Transport system Oct. 2005, we prepared a cathode and loaded it. At the beginning, QE is 3.7% offline by Xe light One years later, QE become 0.7% online by laser in gun

–10.06 Milan Italy (2) RF-gun Cavity Cu Beam Mo Cathode plug RF gun cavity (1.6cells) UV laser For LUCX gun, space charge effect is severe. In order to decrease it, field distribution be tuned into not balanced distribution. Eh/Ef =1.3

–10.06 Milan Italy (2) Chicane A chicane composed of 4 dipole magnets kicks the beam off the main beam line in order to inject laser on the cathode perpendicularly

–10.06 Milan Italy (2) Laser system 266nm 11µJ/pulse 13ps(FWHM) Gaussian Now we have installed a shaper to convert gaussian laser---> Uniform laser (transverse distribution)

–10.06 Milan Italy (2) Experimental results Because of high charge and multi bunch, we must consider beam loading effect and try to compensate it. We apply ΔT method. Injection time is 1.703µs with 2.05µs RF pulse Injection time is 0.908µs with 2.05µs RF pulse

–10.06 Milan Italy (2) Experimental results Momentum and momentum spread: 220nC/100bunch : 4.65MeV/c 1.3% bunch to bunch one bunch : 0.9% Normalized emittance: 49nC/100 bunch 15πmm.mrad 220nC/100bunch 48 π mm.mrad High charge: 300nC/100bunch has been generated

–10.06 Milan Italy (3) Phase Ⅱ

–10.06 Milan Italy (3) RF pulse compressor 3dB Coupler Klystron GUN ACC Dummy Load RRCS Phase Shifter dB Coupler Traveling wave type RF compressor 3.7µs flat ---> 1.0µs

–10.06 Milan Italy (3) Dark current Dark current comes from field emission. In a RF period, dark current occurs at phase region nearby 90 degree (red shadow). During a RF pulse, dark current occurs at the time region that blue shadow shows. If RF pulse width increase, the blue region expands lots, dark current increase much.

–10.06 Milan Italy (3) Dark current data 1 RF pulseForward powerField at cathode surface 1.0us8.3MW81.8MV/m 1.5us7.0MW83.8MV/m 2.0us6.1MW81.54MV/m If we regard the value of field gradient at cathode surface as a marker. For LUCX gun, if it is less than 80MV/m, no dark current occurs

–10.06 Milan Italy (3) Dark current data 2 Dark current is maximum at solenoid current 50A, corresponding maximum magnetic field is about 1.90KGauss 1.0µs 13.8MW

–10.06 Milan Italy (3) Dark current data pC / 735.5pC = 28

–10.06 Milan Italy Summary LUCX has generated 300nC/100bunch electrons Comparing to dark current from the RF gun two years ago, dark current level decrease greatly, gun condition improves greatly. Now the dark current is not big problem on our operation conditions. QE of the cathode decrease from 3.7% to 0.7% during one year

–10.06 Milan Italy END

–10.06 Milan Italy