1. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Dark current measurements at the ELBE SRF gun Rong Xiang, Jochen Teichert, Pengnan Lu, Andre Arnold, Petr Murcek, Hannes Vennekate Helmholtz Zentrum Dresden Rossendorf Unwanted Beam Workshop 2012

2. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 2 OUTLINE 1.Motivation 2.Dark current measurement and analysis 3.Assumption of dark current in the new cavity 4.Conclusion

3. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 3 photobeam darkcurrent 1.Motivation Dark current produces beam loss that increases radiation level and activation causes damages to accelerator components produces additional background for users increases rf power consuming and heat load for cavity cathode lifetime …… SRF gun field emission

4. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 4 40% at cathode DARK CURRENT – Cavity field design value: field profile on axis surface electric field 1.Motivation SRF gun design medium I ave : 1 - 2 mA (< 10 mA) high rep-rate: 500 kHz, 13 MHz low and high bunch charge: 80 pC - 1 nC low transverse emittance: 1 - 3 mm mrad high energy: ≤ 9 MeV, 3½ cells (stand alone) highly compatible with ELBE cryomodule (LLRF, high power RF, RF couplers, etc.) LN 2 -cooled, exchangeable semiconductor photocathode

5. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 5 6.5 MV/m r=0.5mm 20 MV/m 19.6 MV/m gun operation modeCWpulsed RF acceleration gradient6.0 MV/m8 MV/m electron kinetic energy3 MeV4 MeV peak field on axis 16.5 MV/m21.5 MV/m peak field at cathode (2.5 mm retracted) 6.5 MV/m8.4 MV/m cathode field at launch phase (10°)1.1 MV/m1.5 MV/m cathode field at 10° and -5 kV bias2.2 MV/m2.6 MV/m cathode field at 90° and -5 kV bias7.6 MV/m9.5 MV/m most possible zone of field emission: the rim of cathode hole Cavity iris half cell wall Cs 2 Te emitter layer cathode rim 1.Motivation E acc =6 MV/m

6. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 6 DC 6 kV 16.2 MV/m Photobeam & dark current dark current DV02 2. Dark current measurement and analysis

7. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 7 DARK CURRENT – Measurement Dark current in Faraday cup (~1.5 m from cathode) versus gradient for different cathodes about 20 % dark current from cathode, 80% from cavity only cathodes with Cs 2 Te layer have dark current, exception: #060410Mo, but without direct comparison 2. Dark current measurement and analysis

8. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 8 DARK CURRENT – Fowler Nordheim analysis Fowler Nordheim formula for field emission current: Fowler Nordheim plot for RF fields: (J.W. Wang and G.A. Loew, SLAC-PUB-7684 October 1997) E: surface field amplitude in V/m, ɸ: work function in eV, β: field enhancement factor. ф Nb = 4.3 eV β ≈ 400 F-N plots for the SRF gun caivty 2. Dark current measurement and analysis Sharp emitter

9. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 9 2. Dark current measurement and analysis phase space of dark current cavity+cathode @16.7 MV/m 2012-11 Phase space of photo beam 1 pC, 16.2MV/m Solenoid scan Slit scan

10. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 10 2. Dark current measurement and analysis Dark current energy spectrum with cathode #170412Mo Max. axis gradient Energy @ peak Energy spread @ peak peak/ totalCurrent @ Main peak 14.8 MV/m2.3 MeV140 keV24.6 %17 nA 15.5 MV/m2.53 MeV140 keV20.8 %17 nA 16.2 MV/m2.64 MeV146 keV16.3 %15 nA 16.7 MV/m2.87 MeV145 keV12.9 %20 nA Measured with dogleg dipole + YAG screen, normalized according to the total dark current measured from Faraday cup. DC 6kV Different energy  different position? Simulation needed.

11. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 11 2. Dark current measurement and analysis Cathode hole cathode

12. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 12 Maximum (pulsed): E acc = 8 MV/m E peak = 21.5 MV/m Maximum for operation E acc = 16 MV/m E peak = 43 MV/m DARK CURRENT – Cavities with higher gradients existing cavity at ELBE with high field emission new cavity 3. Assumption of dark current in the new cavity

13. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 13 DARK CURRENT – Cavities with higher gradients 3. Assumption of dark current in new cavity New cavity will be operated at 16 MV/m. Here we expect lower field enhancement factor β for the new cavity. For instance, the same dark current as for the old cavity @ 8 MV/m (blue curve) FN fit for 20 % of dark current emitted from cathode (ϕ = 3.5 eV for Cs 2 Te, 40% peak field) and extrapolation to 16 MV/m (red curve) gives 40 µA cathode dark current Extrapolation of Fowler Nordheim results for new cavity: [J.teichert, FLS2012]

14. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 14 For cathodes used in SRF gun: If one wants a low unwanted beam ~µA in CW accelerators with SRF guns, there will be a need for photo cathodes with low dark current DARK CURRENT – fighting against it sources 3. Assumption of dark current in new cavity photoemitter But how? proper handling to prevent dust particles and surface damage proper materials for plugs and smooth surface photo layer properties - roughness, homogeneity, thickness - work function - crystal size, boundary and structure - post-preparation treatment (protect layer, heating, …) - pre-conditioning

15. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 15 4. Conclusion 1.Track the history of SRF gun dark current 2.Measure the dark current @ various gradient, F-N fitting 3.Measure phase space of dark current 4.Measure energy spectrum 5.Estimate the dark current in new cavity with higher gradient

16. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 16 Acknowledgement We acknowledge the support of the European Community-Research Infrastructure Activity under the FP7 programme since 2009 (EuCARD, contract number 227579) as well as the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8. Thank you!