Optimizing Design of SRF Electron Guns

Joe Bisognano University of Wisconsin SRC

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 3 structures, nonlinear phenomena rd generation light sources) of fundamental processes, diffractive imaging of nanoscale

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

For Example: Wisconsin Free Electron Laser (WiFEL)

Next Generation VUV/Soft X-ray Light Source

4

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

Phased Approach to a Full Service FEL Facility

7

Electron Gun for CW WiFEL

Gun repetition frequency I peak at a soft X-ray undulator D E /E at a soft X-ray undulator Normalized e Transverse Bunch length at undulator, rms Charge/bunch I average 5 MHz or higher 1000 Amps < few 10 -4 <1 mm-mrad 70 fsec (seed jitter concerns) 200 pC 1 mA At lower charge per bunch, higher rep rate (up to 200 MHz) and lower emittance (tenths of mm-mrad) possible

Wisconsin SRF Electron Gun Concept

• • • • •

Inherent Quarter Wave Advantages Over Elliptical Gun Designs

Compact structure, so low frequency practical Extremely high mechanical stability BCS losses go as Freqency 2 , so 4.2K operation possible E Peak B peak /E Cath / E Peak is less than elliptical, so Higher E Cath is less than elliptical, so higher quench threshold E Peak /E Cath B peak / E Cath , mT / MV/m

UW Gun

1.31

1.57

BNL QWR

2.63

1.92

FZD Gun

2.7

5.76

• Builds on work at BNL and NPS

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

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

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

• • • • • 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-mrad thermal emittance • At gun exit, d p/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 15

Sequence of Events for 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

Wisconsin Superconducting Electron Gun

• 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

• Active tuner control Cavity compression assembly LLRF Controller 350 300 250 200 150 -10000 100 50 0 0 Measured Delta Freq vs Force Calculated 10000 20000

Delta Freq., Hz.

30000 40000 Mechanical Drive

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 21

• Particle Free Cathode Holder and Transfer A rm • • • 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

• Cavity Filter Design Details • Cavity provides rf short circuit and thermal gap between the warm cathode holder and the srf cavity • 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 Z position, cm

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

Plasma Glow 25

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

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

500 300 900 700

CO Line Strength Before and After Plasma Processing

147mT -10.7db

147mT -14.3db

148mT -14.3db

148mT -10.7 db Initial intensity of CO emission lines at two levels of Rf power Final Intensity of CO emission lines after Plasma processing 100 -100 500 505 510 515 520 525

Wavelength, nm

530 535 540 545 550

• • • • 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 2,10E-06 2,05E-06 2,00E-06 Effect of Downstream Correction Quad Rotation Angle on Emittance

Nominal emittance with no quad error term is 1.687e-6 Nominal emittance with no correction term is 2.04e-6

1,95E-06 1,90E-06 1,85E-06 1,80E-06 1,75E-06 1,70E-06 0 150 mm Solenoid, using -7e-3 T/m for quad component. No Dipole moment.

Compensationg quad is at 0.6 m downstream of cathode and 150mm length.

10 20 40 50 60

• Synchrotron and Materials Physicists For Cathode Research Integrated into Program 17.6

17.2

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 D k x .

• •

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

4 0 deg -4 16.0

G. Bian, T. Miller, and T.-C. Chiang, Phys. Rev. B 80, 245407 (2009)

15.6

4 0 deg -4

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

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 33

Overall layout of SRF gun facililty

3D engineering drawing of Wisconsin electron gun hardware

Wisconsin SRF Electron Gun Preparations for final e-beam weld 36 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

State Freq, MHz D Freq, MHz

-

Volume, in^3

6269.213

Nominal, 4 K 199.58953

Remove 1600 lb preload on tuner 199.65256

0.06303

6267.753

Warmed to 273 K Skin depth vs temp at 200 MHz 199.3704

-0.28216 6294.653

199.3185945 -0.05180 6295.853

Remove vacuum load 199.2485945

Change in permitivity, fvac/fair -0.07

6300.243

199.1947645 -0.05383 6300.243

Undo BCP etch Final weld shrinkage, 0.7 mm 199.3688075 0.174042 6282.793

199.280

-0.088

6294.87

D volume, in^3

-1.46

26.9

1.2

4.39

0 -17.45

12.08

TABLE 1. Steps from cavity blank to final frequency

38 Tests at Niowave successful

39 Preliminary Tests Successful • • • • • Initial cryogenic test at Niowave successful – – – Low field Q of 3 10 9 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

Cold Shock Test

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

Magnetic Shield

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

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Acknowledgment

Wisconsin FEL Team