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1High Average Power, High BrightnessElectron Beam SourcesFernando Sannibale

Lawrence Berkeley National LaboratoryThe Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 200912The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009OutlineHigh Power, High BrightnessElectron Beam SourcesF. Sannibale The real electron source: Issues and challenges of available technologies. Examples of present and future sources (an incomplete list!). Why high-brightness and high-average power electron sources The ideal high-power high-brightness electron source.3The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Why High-Brightness, High-Power Electron SourcesHigh Power, High BrightnessElectron Beam SourcesF. SannibaleIn FELs matching conditions on emittance and energy spread drive these quantities down: Very-high beam power (~ 100kW), low brightness e- sources.Industrial applications, sterilization by irradiation. High to very-high beam power, higher brightness e- sources. FELs and ERL based light sources.

In high-energy physics applications requirements in beam power are usually modest (ILC: tens of mA) and the emittance game is played in damping rings. Notable exception: ERLs used in electron cooling schemes (BNL) In FELs and ERL operating at relatively long wavelengths (IR to NUV), the longer wavelength allows relaxing the normalized emittance (~ 10 mm) and hence the brightness requirements while maintaining a relatively low beam energy.Basic science (~ 1kW beam power) and military applications (up to ~250kW)At the same time, the number of electrons/bunch is pushed up and the bunch lengths are pushed down by gain requirements.In ERLs the requirements for high photon brightness translate into high brightness electron sources.Highbrightness4The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Why High-Brightness, High-Power Electron SourcesHigh Power, High BrightnessElectron Beam SourcesF. Sannibale Electron sources are nowadays playing a central role in4th generation X-ray light sources (FELs and ERLs operating in soft and hard X-ray frequency range) Not only high-brightness! A growing user request pushes towards high-repetition rate, high-average power/ current electron sources.(https://hpcrd.lbl.gov/sxls/Workshop_Report_1stVersion.pdf) Low repetition rate sources have already brilliantly achieved the brightness performance required (LCLS, PITZ, Spring8, )High repetition rate sources not yet! High Brightness becomes one of the main requirement for operating such a machines as well as the capability of controlling the 6D beam distribution. The required beam quality for all these modes of operation is set at the injector and in particular at the electron gun.

5The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 20094th GenerationX-Ray Light SourcesHigh Power, High BrightnessElectron Beam SourcesF. Sannibale ERLs. 100 MHz-GHz-like reprate,very high average current: hundreds of mA,normalized emittances 10-7 to 10-6 m XFELO Oscillator. MHz-like reprate,tens of pC bunches,10-7 m normalized emittance. Low reprate FELs. Few Hz to ~1 kHz reprate, or low reprate long trains of bunches,sub-micron normalized emittances,< 10 mA average currents. High reprate FELs. MHz-class reprates,sub-micron emittances,several mA average currents

Cornell

LCLS-SLAC

LBNL

ANL Electron sources with the required performance exists only for the low reprate FEL category6The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Multiple Modesof OperationHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

Cornell CaseFELs: higher charge (> ~ 0.2 1 nC), low charge (tens of pC), short bunches/broad spectra, longer bunches/narrower spectra, attosecond bunches, beam blow-up regime, 7To achieve the goals of these high-repetition rate, high-average current applications, the electron source should allow for: repetition rates from few tens of kHz up to ~ 1 GHz

charge per bunch from few tens of pC to ~ 1 nC,

sub 10-7 (low charge) to 10-6 m normalized beam emittance,

beam energy at the gun exit greater than ~ 500 keV (space charge),

electric field at the cathode greater than ~ 10 MV/m (space charge limit),

bunch length control from tens of fs to tens of ps for handling space charge effects, and for allowing the different modes of operation,

compatibility with magnetic fields in the cathode and gun regions (mainly for emittance compensation)

10-9 - 10-11 Torr operation vacuum pressure (high QE photo-cathodes),

easy installation and conditioning of different kind of cathodes,

high reliability compatible with the operation of a user facility.The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 The Ideal Electron SourceHigh Power, High BrightnessElectron Beam SourcesF. SannibaleInjector cost is a small fraction of a 4th generation light source cost. Minimizing costs is usually not a high priority requirement.78In high-repetition rates photo-sources high quantum efficiency photo-cathodes (QE>~ 1 %) are required to operate with present laser technology.The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009CathodesHigh Power, High BrightnessElectron Beam SourcesF. Sannibale Thermionic cathodes can in some cases, offer low thermal emittances but require sophisticate compression schemes. (CeB6 at SCSS-Spring 8, XFELO-ANL) Photo-cathodes (most of present injector schemes)Other cathodes under study (photo-assisted field emission, needle arrays,photo-thermionic, diamond amplifiers) The ideal cathode should allow for high brightness (have a low thermal/intrinsic normalized emittance, low energy spread, high current density) full control of the bunch distribution, and long lifetimes. Cathodes are obviously a fundamental part of electron sources.The gun performance heavily depends on cathodes In the low charge regime (tens of pC/bunch) the ultimate emittance performance is set by the cathode thermal emittance8

9PEA Semiconductor: Alkali Antimonides eg. SbNa2KCs, CsK2Sb, - 1%- requires green/blue light (eg. 2nd harm. Nd:YVO4 = 532nm)- for nC, 1 MHz reprate, ~ 1 W of IR required PEA Semiconductor: Cesium Telluride Cs2Te (used at FLASH for example)- 1%- photo-emits in the UV ~250 nm (3rd or 4th harm. conversion from IR)- for 1 MHz reprate, 1 nC, ~ 10 W 1060nm requiredThe Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Examples of Photo-Cathodes & LasersHigh Power, High BrightnessElectron Beam SourcesF. SannibaleNEA Semiconductor: Gallium Arsenide GaAs (used at Jlab for example) - tens of ps pulse capability - reactive; requires UHV ~1 GHz) NC RF guns

DC guns

The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Available ElectronGun TechnologiesHigh Power, High BrightnessElectron Beam SourcesF. SannibaleLow freq. (~ 10 MV/m) 12 Potential for relatively high gradients (several tens of MV/m)Pros: Move technology from R&D to mature phaseChallenges:

Rossendorf CW operation Evaluate and experimentally verify cathode compatibility issues(Promising results with Cs2Te at Rossendorf, DC-SRF Peking approach) Develop schemes compatible with emittance compensation (cohabitation with magnetic fields, HOM schemes, ). Brookhaven National Laboratory April 17, 2009 Excellent vacuum performance.The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Super-ConductingRF GunsHigh Power, High BrightnessElectron Beam SourcesF. Sannibale13 High gradients ~50 to ~140 MV/m Mature technology. Compatible with most photocathodes Full compatibility with magnetic fields. Pros: High power density on the RF structure (~ 100 W/cm2) limits theachievable repetition rate at high gradient to ~ 10 kHz (LUX).Challenges: Proved high-brightness performance. (LCLS and PITZ)Brookhaven National Laboratory April 17, 2009 Relative small volume and small apertures can limit the vacuum performance.The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Normal ConductingL and S Band RF GunsHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

LCLS

PITZ14 Can operate in CW mode Based on mature RF and mechanical technology. Compatible with most photo-cathodes Full compatibility with magnetic fields. Pros: Gradient and energy increase limited by heat load in the structureChallenges: Potential for excellent vacuum performance.Brookhaven National Laboratory April 17, 2009The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Normal ConductingLow Frequency RF GunsHigh Power, High BrightnessElectron Beam SourcesF. Sannibale Beam Dynamics similar to DC but with higher gradients and energies CW high brightness performance still to be proved

LBNL15The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Gun - 4th GenerationLight Source MatchingHigh Power, High BrightnessElectron Beam SourcesF. SannibaleERL Up to hundreds of MHz reprate

DC gun, SC RF Gun, Low freq. NC RF Gun>~ 1 GHz reprateDC gun, SC RF Gun, FEL Reprate < ~ 10kHzHigh freq. NC RF Gun,pulsed DC gun Up to hundreds of MHz reprateDC gun, SC RF Gun, Low freq. NC RF GunXFELO Few MHz reprate Low freq. NC RF Gun with, DC guns16 Cathodes (cathode test facilities capable of accepting all kind of cathodes, vacuum performance, load-lock, ).The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Required R&DHigh Power, High BrightnessElectron Beam SourcesF. Sannibale The performance of an electron source is never fully characterized and demonstrated until the source is integrated in an injector Pursue development of various electron source schemes Important to built R&D injector facilities that allow testing and optimization of: Emittance compensation and beam manipulation techniques, emittance exchange, velocity bunching, Beam diagnostics (especially when considering high repetition rate very low charge and very short bunches

17The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009The Road to HanaHigh Power, High BrightnessElectron Beam SourcesF. Sannibale A long and difficult way to go A lot of it has been already done

, but potentially very rewarding!

18Courtesy of C. Hernandez-Garcia

19Courtesy of C. Hernandez-Garcia

20Courtesy of C. Hernandez-Garcia

21Courtesy of C. Hernandez-Garcia22The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Cornell DC GunHigh Power, High BrightnessElectron Beam SourcesF. SannibaleCourtesy of I. Bazarov Present operation limited to ~ 250kV to limit field emission and minimize probability of field punctuation of the ceramic (750kV initial design).A new ceramic with bulk resistivity is being installed. Same ceramic material was used in Daresbury to get to over ~450kV. The present gun was in beam operation for a number of years allowing for a rich experimental program. For ensuring continuity of such program, the present and funded plan is to build a second DC gun (~500kV) as an R&D effort separated from the beam running.

23Courtesy of Boris Militsyn

ALICE photocathode gun. Performance so far 24Courtesy of Boris Militsyn

25 Based on mature technology. Compatible with most photocathodes Full compatibility with magnetic fields. Pros: Modulator technology limits maximum repetition rate (60 Hz presently, can it go to kHz?).Challenges: Proved high brightness performance. (SCSS)Brookhaven National Laboratory April 17, 2009 Significant injector system complexity when used with thermionic cathodes (adiabatic compression requires chopper and multiple RF frequencies) The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Pulsed DC GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

SCSSThe pulsed nature relaxes many DC gun issues

26 0.6 mm sliced norm. emittance, at ~0.3 nC ~300 X compression factor at the injector exit, 2 ms, 1 A at the gun, 500 kV, 5 cm gap, ~ 10 MV/m 60 Hz reprateT. Shintake et al., PRST-AB 12, 070701 (2009)The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Spring 8Pulsed DC GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

27Courtesy of Thorsten Kamps

28Cs2Te cathodes at 77 K, cavity at 2K, QE ~ 10-3 (poor vacuum transfer chamber)The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009BESSY-DESY-FZD-MBISC RF GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

Gradient limitedby damaged cavity1.3 GHz TESLA-like cells.J. Teichert et al., FEL08, Gyeongju, Korea p.467BNL Low Frequency RF Gun

Courtesy of Ilan Ben-ZviRG gun for electron cooling of RHIC at low energy.Investigate the potential of SRF guns at low frequency.Also motivated by BNL/C-AD work on low frequency SRF cavities, e.g. 56 MHz beta=1 QWR resonator for RHIC storage.MotivationStatusThe niobium has been procured and fabrication of the forming and machining dies is complete. Fabrication of the niobium cavity components is complete. The niobium to stainless-steel flanges have been successfully brazed and leak checked. Preparations are being made concurrently for electron beam (EB) welding of the niobium cavity components. The cathode beam tube and inner and outer conductors have been EB welded and future necessary weld fixtures are being designed and fabricated.The design of the stainless-steel helium vessel is complete, and the nitrogen and Mu metal shields are currently being designed. BNL Cryogenics/Pressure Safety issues are currently being implemented.

Courtesy of Ilan Ben-Zvi

31A SRF 200 MHz Cavity Design for the WIFEL, the Wisconsin FELCourtesy of Robert Legg

The WIFEL accelerator is required to supply each of the six FEL end stations simultaneously at up to a 1 MHz repetitionCs2Te cathode, beam blow up regime 30 fs ~0.9 mm hemispherical transverse profile, 37 MV/m at cathode, 200 MHz SRF cavity, 5MeV final energy32

Beam Dynamics Simulations of Injector using Blow Out Bunches

GunCryomoduleCourtesy of Robert Legg200 pC

33The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009Peking DC-SRF GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

Brings the cathode out of the cryogenic environment1.5 cell already in operation3.5 cell under fabrication

THz/IR ERL FELJiankui Hao, et al., SRF2009, p 205, Berlin, Germany34Derived by the BNL-SLAC-UCLA design (S-Band).Great care in minimizing dipolar and quadrupolar field components. Courtesy of Dave DowellThe Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009SLAC NCS-Band RF GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

0.5 microns emittance at 250 pC0.14 microns emittance at 20 pC Up to date best performanceIn operation351.3 GHz CopperCourtesy of Frank StephanThe Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009PITZ NC RF L-band GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

In operation36Courtesy of D. Nguyen and B. CarstenThe Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009LANL/AESNC CW 700 MHz GunHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

700 MHz CW normal-conducting gun.

Hundreds of kW dissipated in the glidcop structure.

Part of a 100 mA injector for ~ 100kW IR FEL

RF conditioning successfully completed.

First beam tests in spring summer 2010

37 Based on mature and reliable normal-conducting RF and mechanical technologies.The Berkeley normal-conducting scheme satisfies all the LBNL FEL requirements simultaneously. Frequency 187 MHzOperation modeCWGap voltage750 kVField at the cathode19.47 MV/mQ030887Shunt impedance6.5 MWRF Power87.5 kWStored energy2.3 JPeak surface field24.1 MV/mPeak wall power density25.0 W/cm2Accelerating gap4 cmDiameter69.4 cmTotal length 35.0 cm 187 MHz compatible with both 1.3 and 1.5 GHz super-conducting linac technologies.K. Baptiste, et al, NIM A 599, 9 (2009)J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006

At the VHF frequency, the cavity structure is large enough to withstand the heat load and operate in CW mode at the required gradients. Also, the long lRF allows for large apertures and thus for high vacuum conductivity.The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 The LBNLCW NC VHF gunHigh Power, High BrightnessElectron Beam SourcesF. SannibaleIn fabrication

38 The vacuum system has been designed to achieve an operational pressure down into the low 10-11 Torr range. NEGs pumps are used (very effective with H2O, O2, CO, ). This arrangement will allow testing a variety of cathodes including "delicate" semiconductor cathodes.The nominal laser illumination configuration for the cathode is quasi-perpendicular with laser entrance in the beam exit pipe.An additional 30 deg laser entrance port has been added to allow testing of more exotic cathodes (surface plasma wave cathodes, ...) Cathode area designed to operate with a vacuum load-lock mechanism (based on the FLASH, FNAL, INFN design) for an easy in-vacuum replacement or reconditioning of photocathodes. An ion pump accounts for noble gasses and hydrocarbons. The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009A Cathode Test FacilityHigh Power, High BrightnessElectron Beam SourcesF. Sannibale

39Courtesy of Kwan-Je Kim

40Courtesy of Kwan-Je Kim41Mahalo!The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009High Power, High BrightnessElectron Beam SourcesF. Sannibale