Optimizing Design of SRF Electron Guns

1. Optimizing Design of SRF Electron Guns Joe Bisognano University of Wisconsin SRC

2. Starting Point:What Do Light Source Users Want? • Frontier is where physical, chemical, and biological systems can be viewed on their characteristic temporal, spatial, and energy scales—femtoseconds, nanometers, millivolts • Dynamics rather than statics (today’s 3rd generation light sources) of fundamental processes, diffractive imaging of nanoscale structures, nonlinear phenomena

3. Where is Leverage Lower energy per pulse: Signals for experiments limited by damage or space charge. Giant pulses can be overwhelming Higher rep rate: Could compensate for smaller pulses without loss of average flux. Megahertz usable since pump lasers at megahertz now Shorter pulses: Time resolutions of 0.1 ps to fs and lower are needed for studying atomic and electronic motions or relaxations Stability: Pulse to pulse variation of SASE unloved Higher average flux: 2D imaging or photon in/photon out flux starved

4. For Example: Wisconsin Free Electron Laser (WiFEL) Next Generation VUV/Soft X-ray Light Source

5. Cost Breakdown of a Soft X-ray FEL • Conventional wisdom: ~ 2.5 GeV with few cm period undulators with cost at least a good fraction of a billion dollars and probably a good bit more • Cost Breakdown • Linac : 20-25% (less w/ pulsed RT rather than CW SRF) • Injector, R&D, etc.: 5-10% • Photon Generation: 20 % (fifty/fifty undulator and beamline; clearly depends on number of beamline, say six) • Maybe scalable stuff: civil and contingency: 50% • Linac energy reduction and multiple users provides best value • That is, high rep rate at lower charge and lowest normalized emittance

6. Phased Approach to a Full Service FEL Facility

8. Electron Gun for CW WiFEL At lower charge per bunch, higher rep rate (up to 200 MHz) and lower emittance (tenths of mm-mrad) possible

9. Wisconsin SRF Electron Gun Concept

10. Inherent Quarter Wave Advantages Over Elliptical Gun Designs • Compact structure, so low frequency practical • Extremely high mechanical stability • BCS losses go as Freqency2 , so 4.2K operation possible • EPeak/ECath is less than elliptical, so Higher ECath • Bpeak / EPeakis less than elliptical, so higher quench threshold • Builds on work at BNL and NPS

11. A Brief Interlude But Deemed Too Persnickety from Fabrication Point of View

12. Blowout with Superconducting RF Electron Gun • High gradient allows operation in so-called “blow out” mode • SRF offers higher exit energy; less time for space charge to do evil • Lower frequency for temporal field flatness (quasi-DC) O.J. Luiten, et al., PRL 93, 094802-1 (2004). S.B. van der Geer, Proc of Future Light Sources 2006,

13. Ellipsoidal bunch expansion

14. Blow-out Mode Bunches Produce Uniform Charge Distribution • Less susceptible to collective effects Bunch with Initial Longitudinal Modulation “Bad” laser x vs z Z=0 Distribution in t Distribution in t, Z=13 m Bunch with Initial Transverse Modulation Z=0 “Bad” cathode Histogram in x Histogram in x, Z=13 m

15. Key Gun Parameters • Electric field at cathode – up to 45 MV/m • Peak surface magnetic field – 93 mT • Dynamic power loss into He – 39 W at 4K • Q – 2.5E9 • Frequency – 199.6 MHz Key Bunch Parameters • RMS bunch length at gun exit – 0.18 mm • Cathode spot ~1 mm for 0.85 mm-mradthermal emittance • At gun exit, dp/p ~ 2.5%, divergence – 7 mrad • Q – 200 pC • Kinetic energy – 4.0 MeV • With smaller spot, can be operated in lower charge modes with lowered emittance; also more exotic cathode materials

16. Sequence of Eventsfor Wisconsin Electron Gun • Start of three year grant in August 2010 • ~FY 2011: final design, procurements, and vault prep • ~FY 2012: fabrication and subsystem installation • ~FY 2013: final integration, commissioning and beam tests • Expect commissioning to start in April-May • Total DOE program $4.125 million

17. Wisconsin Superconducting Electron Gun

18. RF system uses Low level RF controls from JLAB upgrade • Standard EPICS interface • Existing hardware base

19. 20 kW 200 MHz RF Harris Corporation Broadcast Communications Division

20. Active tuner control Cavity compression assembly LLRF Controller Mechanical Drive

21. RF Coupler and HPA and LLRF • Power is introduced through a ceramic rf window and a tuned resonant structure. • Relatively low power, <10kW, at 1 mA of beam

22. Particle Free Cathode Holder and Transfer Arm • Transfer mechanism and cathode holder specifically designed (and tested) to be particle free in operation • Support structure needs to be accurate from 10 to 20 microns in every axis and linear direction. The cathode adjustment support is fixed to the vacuum vessel • The cathode stem is designed to allow nitrogen to flow through a channel forcing it near the exchangeable stalk insert

23. Cavity Filter Design Details • Cavity provides rf short circuit and thermal gap between the warm cathode holder and the srfcavity • The small gap region acts to minimize the radial field across the cathode holder face • Bellows in filter allows final alignment and tuning of filter • Copper plated SS acts as to manage RF heating X position, mm Z position, cm

24. Ar:O Processing of SRF Cavity • Need to clean cavity after receipt from Niowave, but too large for conventional HPR facilities with He vessel attached • New technique demonstrated at SNS and JLAB using plasma processing • Uses RF driven Ar:O plasma to “ash” surface contaminants • Plasma process monitored spectroscopically

25. Plasma Glow

26. Spectrum Intensity vs Wavelength in Nanaometers • Argon dominates spectrum; makes seeing contaminants hard. • Use techniques from semiconductor industry for etching SiO using rf plasmas; • Look at 483 and 520 nm lines over time. All major lines are Argon CO lines

27. How semiconductor processing determines the oxide is ‘done’1 Note amplitude of emission line drops to half initial value at completion. 1. John G. Shabushnig, Paul R. Demko and Richard Savage, Proceedings of Mat. Res. Soc., Vol 38, Materials Research Society, 1985

29. High Temp Superconducting Solenoid and Compensating Quad • Magnet can be closer to the cavity; Closer the focusing field is to cathode, the better the emittance compensation • Field specified to minimize emittance dilution from quad and dipole terms • Downstream superposed skew and normal quad magnets to remove particle rotation caused by quad terms in solenoid reduces final transverse emittance

30. Synchrotron and Materials Physicists For Cathode Research Integrated into Program Schematic view of the corrugated film geometry and the wave interference or propogation patterns. The inset shows the Fraunhofer single-slit diffraction pattern as a function of Dkx. • EXAMPLE: • Bi thin film in the rombohedral phase. The surface state ~0.4 eV below the Fermi edge (blue spot) only has +2° emission angle. • Potential for prompt emitter with very low thermal emittance G. Bian, T. Miller, and T.-C. Chiang, Phys. Rev. B 80, 245407 (2009)

31. Spectra-physics Tsunami (oscillator) + Spitfire (amplifier) system • Pulse duration: 100 femtoseconds • Repetition rate: 1 kHz – 1 Hz • Pulse energy • Up to 4 mJ per pulse at the fundamental (800 nm) • ~ 1 mJ per pulse at the second harmonic (400 nm) • ~ 300 microjoule per pulse at the third harmonic (266 nm) • Average power: 4 W

33. Current Scope • Demonstrate single bunch beam dynamics and operation of SRF gun • Low repetition rate (kilohertz) drive laser • Cu Cathode Used for Initial Operation • Little chance of cavity contamination from evaporated cathode material • Cathode will not degrade over time like semiconductor • No cathode preparation chamber needed

34. Overall layout of SRF gun facililty

35. 3D engineering drawing of Wisconsin electron gun hardware

36. Wisconsin SRF Electron Gun Preparations for final e-beam weld Bake at JLab to prevent Q-disease

37. Frequency Map • Map which starts with a cold cavity at the correct frequency and moves back through the series of production steps producing an expected resonant frequency at each step • Goal is to understand any deviations from the calculated frequency map and apply that knowledge to next generation FEA to Evaluate Stress and Deformation

38. Tests at Niowave successful

39. Preliminary Tests Successful • Initial cryogenic test at Niowave successful • Low field Q of 3 109 • Gradients of about 7 MV/m obtained, limited by test configuration • Demonstrated potential to reach design Q and design gradient (40 MV/m) after final processing at Wisconsin • Cavity installed in helium vessel and delivered to SRC • Cold shock test carried out • Plasma processing • Integration under way

40. Titanium Helium Vessel with Niobium Cavity Inside

41. Cold Shock Test

42. Cryostat Configuration of quarterwave cavity superconducting RF electron gun. Nitrogen Shield Magnetic Shield

48. Phase II Proposal • 3 years more years • Key thrusts • Detailed measurements as function of key parameters, establishing technology reach • Helium refrigerator for extensive testing program • High repetition rate laser for high average current operation (5-40 MHz, milliamp average current) • High QE photocathodes and exotic photocathode material

49. AcknowledgmentWisconsin FEL Team