Optimizing Design of SRF Electron Guns

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

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

Where is Leverage

4

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

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

5

Phased Approach to a Full Service FEL Facility

7

Electron Gun for CW WiFEL

Gun repetition frequency 5 MHz or higher

I peak at a soft X-ray undulator 1000 Amps

DE /E at a soft X-ray undulator < few 10-4

Normalized eTransverse <1 mm-mrad

Bunch length at undulator, rms 70 fsec (seed jitter concerns)

Charge/bunch 200 pC

I average 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 Freqency2 , so 4.2K operation possible• EPeak /ECath is less than elliptical, so Higher ECath • Bpeak / EPeak is less than elliptical, so higher quench threshold

• Builds on work at BNL and NPS

UW Gun BNL QWR FZD Gun

EPeak /ECath 1.31 2.63 2.7

Bpeak / ECath , mT/MV/m 1.57 1.92 5.76

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 Distribution

• Less susceptible to collective effects

Bunch with Initial Longitudinal Modulation

Bunch with Initial Transverse Modulation

Z=0

Histogram in x, Z=13 m

x vs z Z=0

“Bad” cathode

“Bad” laser

Distribution in t, Z=13 mDistribution in t

Histogram in x

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

• RMS bunch length at gun exit – 0.18 mm• Cathode spot ~1 mm for 0.85 mm-mrad thermal 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

Key Bunch Parameters

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

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

-5000 0 5000 10000 15000 20000 25000 30000 35000 400000

50

100

150

200

250

300

350

Measured Delta Freq vs Force

Calculated

Delta Freq., Hz.

Disp

lace

men

t, m

icron

s

LLRF Controller Mechanical Drive

Cavity compression assembly

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 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

• 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

X p

osi

tion

, m

m

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

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.

CO lines

All major lines are Argon

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

500 505 510 515 520 525 530 535 540 545 550

-100

100

300

500

700

900

CO Line Strength Before and After Plasma Processing

147mT -10.7db

147mT -14.3db

148mT -14.3db

148mT -10.7 db

Wavelength, nm

Inte

nsity

, Arb

itrar

y co

unts

Initial intensity of CO emission lines at two levels of Rf power

Final Intensity of CO emission lines after Plasma processing

• 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

0 10 20 30 40 50 601.50E-06

1.60E-06

1.70E-06

1.80E-06

1.90E-06

2.00E-06

2.10E-06Effect of Downstream Correction Quad Rotation

Angle on EmittanceNominal emittance with no quad error term is

1.687e-6Nominal emittance with no correction term is

2.04e-6

Quad angle of rotation, deg

Nor

mal

ized

emitt

ance

, mm

-mr

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.Field in compensating quad is fixed at -7e-3 T/m.Emittance is measured at the first emittance minimum after the solenoid, ~2.82 m.

• Synchrotron and Materials Physicists For Cathode Research Integrated into Program

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)

-404

deg

17.6

17.2

16.8

16.4

16.0

15.6

eV

-404

deg

17.6

17.2

16.8

16.4

16.0

15.6

eV

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.

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

36

Preparations for final e-beam weld

Bake at JLab to prevent Q-disease

Wisconsin SRF Electron Gun

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

37

FEA to Evaluate Stress and Deformation

StateFreq, MHz

D Freq, MHz

Volume, in^3

D volume, in^3

Nominal, 4 K 199.58953- 6269.213

Remove 1600 lb preload on tuner 199.65256 0.06303 6267.753 -1.46

Warmed to 273 K 199.3704 -0.28216 6294.653 26.9

Skin depth vs temp at 200 MHz 199.3185945 -0.05180 6295.853 1.2

Remove vacuum load 199.2485945 -0.07 6300.243 4.39

Change in permitivity, fvac/fair 199.1947645 -0.05383 6300.243 0

Undo BCP etch 199.3688075 0.174042 6282.793 -17.45Final weld shrinkage, 0.7 mm 199.280 -0.088 6294.87 12.08

TABLE 1. Steps from cavity blank to final frequency

38

Tests at Niowave successful

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

39

40

Titanium Helium Vessel with Niobium Cavity Inside

Cold Shock Test

Cryostat

Configuration of quarterwave cavity superconducting RF electron gun.

Magnetic Shield

Nitrogen 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

Acknowledgment

Wisconsin FEL Team

49