Brookhaven National Laboratory 6152015 1 IEB Workshop 2015 Outline Overview SRF gun technology Overview high current SRF gun projects SRF guns Photocathodes Recent progress of BNL 704MHz SRF gun ID: 760685
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Slide1
High current SRF gun
Erdong WangBrookhaven National Laboratory
6/15/2015
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IEB Workshop 2015
Slide2Outline
Overview SRF gun technologyOverview high current SRF gun projectsSRF gunsPhotocathodesRecent progress of BNL 704MHz SRF gunChallenges and considerationsSummary
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Slide3Why SRF photocathode gun?
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To get high average current , the gun must operate in CW mode.Candidate: DC gun, VHF NCRF gun, VHF~L band SCRF gun
DC gun:
Mature
Cathode
gradient<7MV/mGun voltage:550kVCornell, Jlab, KEK, JAEA,IHEP
NCRF gun:
Commissioning
Cathode Gradient:
20MV/mGun voltage:1MVLBNL, SLAC,SHLS
SCRF gun:
Commissioning
CW at high frequencyCathode gradient:20~35MV/mGun voltage:2~9.4MVBNL, HZD,HZB, NPS, WiFEL
High gradient :High bunch charge Small emittanceLarge gap voltage:Small emittance
Slide4Overview : SRF gun technology
Advantages:CW mode of operation: High rate of acceleration; high average beam current; Very stable(phase, amplitude) system reduce time jitter.High gradient on cathode: high bunch charge ; small emittance.SRF cavity and operation have matured.Elliptic cavity(low-β cell + 0~3 cell high-β) geometries based SRF gun has demonstrated and under developing for high current operation.Quarter wave resonator SRF gun is under development and got beam on NPS gun.The integration of a high QE semiconductor photocathode and the SRF gun is still a challenging topic.
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Slide5Overview SRF gun :Elliptic cavity+NC_cathode
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BNL /AES 704MHzHZD 3.5 CellHZB Berlin ProGun structureBunch charge(nC)5/1.4/0.71/0.0770.077Ave. Current(mA)50/3000.5/14Gun freq.(MHz)703.7513001300Peak gradient(MV/m)2030/22<10photocathodeCsK2SbCs3Te,MgCsK2SbN_Emittance(mm-mrad)5/2.3/1.42.5/11Kinetic energy(MeV)29.4/43.5Current statusBeam commissioningBeam commissioning ,upgradeVertical tested
Mature cavity shape; high kinetic
energy; high average current
Slide6Overview SRF gun :QWR cavity+NC_cathode
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BNL 112MHz(CeC)NPS (FEL)Wi FEL (FEL)Gun structureBunch charge(nC)510.2Ave. Current(mA)5011Gun freq.(MHz)112500200Peak gradient(MV/m)19.72545photocathodeCsK2SbCsK2SbCs3Te, CuN_Emittance(mm-mrad)340.9Kinetic energy(MeV)21.24.58Current statusRF conditioningBeam commissioningBeam commissioning
4K operation; high gradient; long bunch; constant field(small energy spread)
Slide7Overview SRF gun :Other type
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PKU DC-SC 3.5 cellHZB HoBiCatGun structureBunch charge(nC)0.10.015Ave. Current(mA)1~50.0045Gun freq.(MHz)13001300Peak gradient(MV/m)4.510photocathodeCs3TePb/NbN_Emittance(mm-mrad)1.21Kinetic energy(MeV)53.5Current statusCommissioning finished, upgradeupgrade
Prevent cathode contamination;
Full superconducting
Slide8Photocathode
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To generate high average current beam, high QE(>1%) photocathode is required.
Wavelength(nm)
QE
Thermal
emittance
(mm-
mrad
/mm)
Lifetime
K
2
Cs(Na)Sb
532
10%~1%
0.4
Long
GaAs
532/780
~10%/1%
0.44
Short
Cs
3
Te
266
16%~1%
0.5
Long
Diamond amplifier
Electron
beam
Gain >200 times
?/ Energy spread
σ
=0.12eV
Long
Diamond
/Si field
emitter
Field emitter
NA
?
Very
long
Plasmonic
800
nonlinear
1.4
Very
long
Pb
250
0.0001%
?
Very
long
Slide9704MHz gun and beamline
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SRF gun: Pulse to CW 2MV
Laser: LUMERA: Nd: YVO4, single pulse to 9.38 MHz, up to 6 W at the cathode surface.
RF: adjustable pulse length
Raw signal
Integrated signal
ICT window
7
μ
S
Laser pulse
(controlled by
pockels
cell)
Slide10Choke joint cathode stalk upgrade
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Re-designed choke
structure to eliminate
multipacting
. Allow CW operation
up to
2MV.
Change the cathode substrate to
Ta to have better QE for high current, high bunch charge operation.Better cooling design to dump the heat dissipation on the stalk.
Double choke with groove
Slide11Photocathode for BNL SRF gun
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CsK
2
Sb @
532nm ,10%
Slide12Photocathode preparation and transferring
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Slide13Conditioning result and beam commission
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550pC bunch charge
Dark current
Photo current
Courtesy by Wencan Xu
The
QE of CsK
2
Sb was 4.1% at initial prepared and 3.8% before inserting into the gun.
The
gun was then conditioned and
sent beam to
a Faraday
cup. Pulsed
operations yielded bunches with up to 0.55nC
per-bunch
.
The gun was operated in pulsed mode at 0.85MV
kinetic
energy. The
ICT and Faraday cup measurements
confirmed
QE=1% at low
current.
A
fter
running for a couple of
days including condition and high bunch charge test,
there was no observed degradation of the QE
.
Slide14Challenges and considerations
SRF gun merging high QE photocathode, superconducting RF, high repetition rate laser, beam diagnostic. All complex.Semiconductor cathode material contamination the gun/stalk induce the multipacting.Bias the cathodeShielding the RF fieldProtect the multipacting zone in cathode preparationHigh bunch charge and high average current operation with long lifetime cathodeGood vacuum in gun operationHigh input power couplerBeam halo controlGood beam qualityHigh peak gradient on the cathodeUniform ,smooth and low thermal emittance photocathodeLaser shaping on both transverse and longitudinalLaser and gun synchronization precisely. CW mode Dark current eliminatingKick the dark currentReduce the field emitter areaStable SRF operationCryogenic operationCoupler kick and HOM damping
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Slide15Summary
SRF photoinjectors are promising to provide high bunch charge, high average current and small emittance beam.Significant progress on SRF gun researches in last a few years. For example: BNL 704MHz gun generated 550pC per bunch and no observed degradation of the QE in two days gun condition and operation.ELBE SRF gun I generated beam for FEL operation and PKU DC-SC gun delivered beam for THz and UED test.Demonstration of high average current operation and high bunch charge operation are under going.Beginning of a new adventure, look forward to having more exciting results soon.
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Slide16Acknowledgements
Thanks to BNL ERL and CeC project team.I. Ben-Zvi; T. Rao; S. Belomestnykh; D. Kayran; B. Sheehy; H. Xie; T. Xin; W. Xu; L. Hammons; R. Kellerman; C. Liaw; V. Litvinenko; G. McIntyre; T. Miller; T. Seda; R. Than; D. Weiss; B. Xiao; J. Skaritka; I. Pinayev and many others.
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Thanks for you attention!
Slide18Backup
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Slide196/15/2015
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From J. Teichert IPAC 2014 slides