Superconducting RFHigh-Brightness Injectors R&D at BNL and

Stony Brook University

Ilan Ben-Zvi

Accelerator R&D Division

Collider-Accelerator Department

Brookhaven National Laboratory

and Center for Accelerator Science and Education

Stony Brook University

BES Photon Workshop October 2009

• “…the quest for ultimate performance leaves the LINAC-driven light sources as sources for the fourth generation, to replace and complement today’s storage rings.”

• The LINAC-driven light sources include FELs and ERLs.

• The workshop further concludes that MHz repetition rate is needed.

BES Photon Workshop• “This points to the ideal tool kit for assembling an X-

ray source in the future:– A superconducting linear accelerator;

– Various electron guns for injection, with different repetition rates, bunch charges, and

– bunch compression schemes;

– One or more return arcs equipped with a series of sophisticated undulators for

– multiple experiments; and

– Several fast kickers to extract single bunches into long FEL undulators.”

BES Photon Workshop

• “In addition to the demonstration of energy recovery on a quantitative scale, the development of an X-ray ERL, as well as the realization of FEL designs with megahertz pulse rates, is also hindered by the lack of technical developments as far as gun performance is concerned. Today’s guns cannot yet deliver the bunch charges, emittances, and repetition rates required for the full ERL or FEL designs outlined above.”

National AcademiesScientific Assessment of

High-Power Free-Electron Laser Technology

… in the committee’s opinion, the two “tall poles” in the free-electron-laser development “tent” are these:

· An ampere-class cathode-injector combination.

· …

4a. Drive-laser-switched photocathodes are the likely electron source for megawatt-class

free-electron lasers. Photocathodes have been used in accelerator applications for over

two decades; however, they have not reached the level of performance in terms of

quantum efficiency and robustness that will likely be required for a reliable megawatt class free-electron laser.

My Conclusions and Program• A high-brightness, ampere-class-current

electron gun is essential for FLS programs.• It is generally agreed that SRF guns are best

suited for the task, but that a lot of R&D needs to be done.

• BNL and Stony Brook University have a comprehensive program of SRF injector development, involving 3 types of SRF guns and 3 different photocathode technologies.

Why SRF gun?

• High electric field on the cathode for initial small emittance at high charge

• Large total voltage to reduce emittance dilution when bunch is most vulnerable

• Reduce size of RF supply, increase efficiency

• SRF guns are already in operation and are the choice of many new facilities.

Power supply for a 1 MW klystron(as needed for 10 MeV at 100 mA)

ARDD and CASE

Accelerator R&D Division at BNL / C-AD• Establishment of SRF infrastructure• Polarized electron guns

– SRF polarized electron gun– DC polarized electron gun

• eRHIC R&D: Energy Recovery Linac– Electron guns (cavities, photocathodes,

laser)– ERL accelerating cavities at 704 MHz– The 500 mA R&D ERL

• Low frequency SRF cavities (56 and 28 MHz)• Electron cooling

• LARP (Crab cavity, 704 MHz 5-cell cavity)

Center for Accelerator Science and EducationStony Brook University:•Partner in ARDD programs•Education in accelerator science•Use BNL accelerator resources

BCP facility (above)Vertical testing (right)

More about CASE / ARDD program• The SRF guns are at 112 MHz, 704 MHz and 1.3 GHz.• The photocathode technologies are K2SbCs, GaAs(Cs)

and diamond amplifiers.• A 0.5 ampere, 20 MeV Energy Recovery Linac (ERL) is

in an advanced stage of construction. The ERL will be injected by the 704 MHz superconducting laser-photocathode RF gun, using a load-lock high quantum efficiency photocathode. The ERL employs a unique very-high current accelerating cavity and a novel emittance preserving injection system.

Our first SRF gun: 1.3 GHzFoundation: ASE SBIR

Started 1999

Cathode isolationValve

Cathode installationassembly

Beam lineisolation valve

Top cover withfacilities feedthru

Cavityassembly

InternalHelium dewar

Adjustable supports

Magnetic and thermalshielding

HOM Load

Powercoupler

500 mA 2 MeV SRF gun with AES

500 mA 2 MeV SRF electron-gun

Tuner and cathode clamp

R&D ERL under construction.Aim: 0.5 amp CW

Magnets and stands

The BNL prototype ERL is the major experimental research and development effort towards RHIC II, the electron cooling project for RHIC. The objective is to reduce the risk and costs of the RHIC II project, as well as developing and demonstrating the electron beam parameters required for electron cooling. The prototype will also serve as a test bed for studying issues relevant to very high current ERLs. All quadrupole and dipole magnets are of the warm bore variety. All magnets are to be accurately CNC machined and will be installed on similarly machined bases. A portion of the ring will be mounted on a movable gantry with a total stroke of plus/minus 5cm.

Z-bend injection

10º

-10º

-20º

20º

1.5 CELL GUN 1st 5 CELLS CAVITY

48 cm

e-, 4.7 MeV

490 cm

e-, 18 MeV

e-, 30 MeV54.3 MeV

SRF Gun MERGER

0

5

10

15

20

25

30

35

40

45

0 200 400 600 800

Z, cm

Norm

ilized

emitta

nce,

mm*m

rad

verticalhorizontal

RF

1 MW CW klystron92kV at 17A380 gpm of water

ERL Beam Design ParametersR&D ERL design BNL ERL projects requirementsHigh Current

High charge PoP CeC Test *) Pre-cooling @ 40GeV

MEeRHIC eRHIC10/20

Charge per bunch, nC 0.7 5 5 5 14 (9x1.56) 5 18/3.5

Energy maximum/injection, MeV 20/2.5 20/3.0 21/3 21/3 21/3 4000/5 10000/520000/5

Rms Normalized emittances mm*mrad

1.4/1.4 4.8/5.3 5 5 3 7-73 77

R.m.s. Energy spread, δE/E 3.5x10-3 1x10-2 1.5x 10-3

1.5x 10-3

8 10-4 2x10-3 1x10-3

R.m.s. Bunch length, ps 18 31 30 30 30 6.7 30

Bunch rep-rate, MHz 700 9.383 0.078 9.383 9.383 9.383 14.1

Gun/dumped ave. current, mA 500 50 0.4 50 130 50 260/50

Linac average current, mA 500 50 0.4 0.4/50 130 300 2600/500

Injected/ejected beam power, MW 1.0 0.150 0.0012 0.15 0.390 0.250 1.3/0.250

Numbers of passes 1 1 1 1 1 3 5

The Design Principles of the BNL/AES Cavity

• All monopole and dipole HOMs propagate into the beam pipes, no trapped modes.

• Fundamental mode is evanescent in beam pipes.

• Large apertures, low frequency reduces HOM power and provides strong cell-to-cell coupling.

• Short cavity-to-cavity transitions can be made with a variety of HOM probes.

• The cavity is stiff (actually too stiff) and stable.

704 MHz eRHIC 5-cell Cavity

2K main line

Inner magnetic shield

Cavity assembly

4Ó RF shieldedgate valve

2K fill line

He vessel

Vacuum vessel

Fundamental PowerCoupler assembly

HOM ferriteassembly

Outer magnetic shield

Thermal shield

Tuner location Space framesupport structure

Vacuum vessel

The BNL High-Current R&D• Aimed at pushing the limits for beam current: 0.5

amperes

• Testing of novel components and techniques:

– Superconducting electron gun

– High quantum efficiency photocathodes

– Diamond amplifier

– Z-bend ERL beam merging

– High-current SRF cavity at 703.75 MHz

– Diagnostics and more.

• Working with industry (AES, Niowave)

112 MHz gun - Niowave

• Low frequency SRF gun, Niowave SBIR

• Interchangeable cathode

• Expect about 2 MeV

• Load-lock to be developed

Simulations of gun properties

Gun status in pictures

The cryomodule is scheduled to be complete in August 2010.

Injector testing

• Extensive diagnostic beam line

• Tests of QE, charge, emittance, halo, pulse length nearly ready to be made with the SRF guns

Photocathode techniques

• Load-lock system

• Multi-alkaline cathode - Presentation by Triveni Rao

• Cesiated gallium arsenide for polarized electrons – Presentation by Triveni Rao

• Diamond amplifiers R&D for two orders of magnitude gain in quantum efficiency –Presentation by John Smedley

Cathode load-lock

Load-lock 3rd Gen multi-alkaline System

See presentation by Triveni Rao

We also do GaAs(Cs) photocathodes

See presentation by Triveni Rao

AdvantagesSecondary current can be >300x primary

current

Lower laser power

Higher average currents

Diamond acts as vacuum barrier

Protects cathode from cavity vacuum and ion bombardment

Protects cavity from cathode (prevents Cs migration)

Should improve cathode lifetime

e- thermalize to near conduction-band minimum minimizes thermal emittance

Diamond Amplifier Concept

See presentation by John Smedley

ConclusionsI hope that I have demonstrated:

• We work on high-current (ampere class) components of SRF injectors, photocathodes and ERLs.

• We have several SRF guns and several high QE photocathode R&D projects.

• We are in an advanced state of construction of a 500 mA R&D ERL.

• SRF guns and high QE photocathodes are R&D areas that may be high risk but offer high value and could have significant impact on future light source designs.