Technology Challenges for SRF Guns as ERL Sources in View of Rossendorf work

Transcription

1 Technology Challenges for SRF Guns as ERL Sources in View of Rossendorf work, Hartmut Buettig, Pavel Evtushenko, Ulf Lehnert, Peter Michel, Karsten Moeller, Petr Murcek, Christof Schneider, Rico Schurig, Friedrich Staufenbiel, Jochen Teichert, Rong Xiang, (FZR, Dresden), Juergen Stephan (IKS Dresden), Wolf-Dietrich Lehmann (IfE Dresden), Thorsten Kamps, Dirk Lipka (BESSY GmbH, Berlin), Vladimir Volkov (BINP SB RAS, Novosibirsk), Ingo Will (MBI, Berlin) 1

2 Basic Design Normal-conducting cathode inside SC cavity Successful Proof of Principle Experiment, D. Janssen et al., NIM A507(2003)314 Cavity: Niobium 3+½ cell (TESLA Geometry) Choke filter Operation: T = 1.8 K Frequency: 1.3 GHz HF power: 10 kw Electron energy: 10 MeV Average current: 1 ma Cathode: Cs 2 Te thermally insulated, LN 2 cooled Laser: 262 nm, 1W Pulse frequency: 13 MHz & < 1 MHz Bunch charge: 77 pc & 1 nc 2

3 Main Components of the SRF Photogun in Rossendorf Tuning system RF input coupler Low level rf system Power rf system Control systems Synchronisation He-pressure&level Tuning, rf system, laser Beam line devices PSS, MPS, Vacuum Diagnostic beam line View ports, current, beam shape Energy and energy width Bunch length, emittance 3½-cell cavity test benches for Critical component SRF-Gun ELBE connection beam line Cryostat cathode insert&cooling He-vessel & port LN 2 cooling & port magnetic shield, vacuum diagnostics LHe transfer line & distribution box Driver Laser Laser beam line Photocathode transfer&storage Photocathode preparation equipment 3

4 Design consideration of the gun cell L1 is mainly determinated by technological conditions (pressure, multipacting, etching) The optimal L2 value follows from the beam properties One dimensional model calculation: E(Φ) max, Ez cath *sin(φ) max Shorted by 5mm α 0 = L1 L1 L2 Ez cath *sinφ [MV/m] α 0 = α 0 = A0=L2/(λ/4) L=L1+L2 width of the gun cell L1 = α 0 *A0*λ/2π 4

5 Result of numerical optimization Numerical minimization of the beam emittance by variation of the gun cell shape with the condition, that Bs max < 115mT and Es max < 52MV/m when E acc = 25MV/m Shorted by 5.7mm L1 L1 L2 Obtained result: [mm] L1+L2= λ/4-20 = 37.7 a1=9, b1=16, R1=102.5, r1=11.4, 5

6 RRR40 and RRR300 cavity of the SRF gun 6

7 Cavity Design Parameter shortened TESLA cup 1. 3 GHz, 10 kw optimized half cell & 3 TESLA cells E z,max = 50 MV/m (T cells) = 33 MV/m (1/2 cell) E, MV/m z E c E r max E z max E z E r 10 Fields are normalized to the accelerating gradient in the TESLA cells of 25 MV/m z r max z z max z, cm (origin at the back wall plane) r = 1 mm 77 pc 0.5 mm mrad RF focusing in SC gun cavities I av = 1 ma E = 9.5 MeV D. Janssen, V.Volkov, NIM A452(2000)34 1 nc 2.5 mm mrad 7

8 Magnetic RF field inside the cavity Surface B-field [T] Axis electric RF field [MV/m] z[m] Axis magnetic RF field [T] z[m] 8

9 σ x [mm ] Designparameter including the magnetic mode ε[mm mrad] ε[mm mrad] z[m] 180 φ TE [grad] Beam parameter Field parameter Laser parameter ε x [mm mrad] B TMsurf [mt] 115 Puls length [ps] 20 σ x [mm] 3.06 B TEsurf [mt] 136 Raise time [ps] 1 ε z [kev mm] 72.4 B TM + B TE surf [mt] 144 Spot size [mm] 2.6 z[mm] 2.79 E TM,axis [MV/m] 50 Bunch charge[nc] 1 E av [MeV] 8.82 φ TM [grad] 74.6 E rms [kev] 53.9 φ TE [grad]

10 Dual tuning system gun-cell tuner TESLA-cell tuner TESLA-cell tuner gun-cell tuner Gun cell TESLA cells choke-cell setting range resolution ±0.25mm 2nm ±0.3mm 2nm load ±2250N ±2700N frequency ±137kHz ±286kHz 10

11 Liquid N 2 Cathode Cooling Test bench LN 2 reservoir cathode cooler thermal conductance measurements, cathode cathode temperature? & test of the cathode transfer system Cone in cooler - centres cathode - cathode is pressed in by spring - thermal contact of cone surface? 11

12 Test bench for the cathode cooler heater cathode cathode cooler LN 2 reservoir 12

13 Cavity with cathode tuning system Cathode input End of the cryostat LN 2 reservoir Pic up flange Titanium bridge Cavity tube 13

14 Beam tube with higher order mode and main coupler HOM coupler (TESLA design) Main coupler Max. power 10kW Design M.Champion Development of HEPL, Stanford 14

15 RF power input around the cathode Power input LN 2 cooling Bellow for adjustment of Q ext and heat isolation Quater wave choke Tube for cathode exchange cathode cavity Warm RF window Cold RF window L z Power input End of the cryostat 15

16 External quality factor and head load of a cathode-rf coupler Heat load of the cathode [W], beam power [kw] External Quality Factor Heat load of the cathode 90 Beam Power Length L of the choke cell [mm] 1E9 1E8 1E7 External quality factor Heat load of the cathode [W], beam power [kw] External quality factor Heat load of the cathode Beam power Width z of the choke cell [mm] External quality factor Field parameters for W = J Ez max (r=0) = 50MV/m, Ur max = 6.5kV Es max = 43.6MV/m, Bs max = 0.11T 16

17 Cryomodule design of the SRF gun 17

18 LN 2 cooling shield of the cryostat 18

19 Present Status and next steps Cavity: Cavity tuners: Fabrication finished Fabrication of 2 (RRR 40 & 300) cavities at ACCEL finished next steps: warm tuning in Rossendorf, BCP, HPR, tests at 2K at DESY Fabrication finished design of a test bench Cathode cooling system: Fabrication finished tests are running Cathode transfer system: Design finished, in the workshop Cathode preparation chamber: Cryomodule: Design and fabrication finished, assembling and tests Design finished, in fabrication 19