High Average Power High Brightness Electron Beam Sources
High Average Power, High Brightness Electron Beam Sources Fernando Sannibale Lawrence Berkeley National Laboratory 1 The Physics and Applications of High Brightness Beams - Electron Maui, USA, November 2009 The Physics and Applications of. Electron High Brightness Beams - Maui, 18, USA, Novem
High Power, High Brightness Electron Beam Sources F. Sannibale Outline • Why high-brightness and high-average power electron sources • The ideal high-power high-brightness electron source. • The real electron source: Issues and challenges of available technologies. • Examples of present and future sources (an incomplete list!). 2 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Why High-Brightness, High-Power Electron Sources High Power, High Brightness Electron Beam Sources F. Sannibale • Very-high beam power (~ 100 k. W), 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 FELs matching conditions on emittance and energy spread drive High these quantities down: brightness 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. 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 (~ 1 k. W beam power) and military applications (up to ~250 k. W) • In high-energy physics applications requirements in beam power are usually modest (ILC: tens of m. A) and the emittance game is played in damping rings. 3 Notable exception: ERLs used in electron cooling schemes (BNL) The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Why High-Brightness, High-Power Electron Sources High Power, High Brightness Electron Beam Sources F. Sannibale • Electron sources are nowadays playing a central role in 4 th generation X-ray light sources (FELs and ERLs operating in soft and hard X-ray frequency range) • High Brightness becomes one of the main requirement for operating such a machines as well as the capability of controlling the 6 D beam distribution. • The required beam quality for all these modes of operation is set at the injector and in particular at the electron gun. • 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_1 st. Version. pdf) • Low repetition rate sources have already brilliantly achieved the brightness performance required (LCLS, PITZ, Spring 8, …) 4 High repetition rate sources not yet! The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
4 th Generation X-Ray Light Sources High Power, High Brightness Electron Beam Sources F. Sannibale • ERLs. 100 MHz-GHz-like reprate, very high average current: hundreds of m. A, normalized emittances 10 -7 to 10 -6 m Cornell • Low reprate FELs. Few Hz to ~1 k. Hz reprate, or low reprate long trains of bunches, sub-micron normalized emittances, LCLS-SLAC < 10 m. A average currents. • High reprate FELs. MHz-class reprates, sub-micron emittances, several m. A average currents LBNL ANL • XFELO Oscillator. MHz-like reprate, tens of p. C bunches, 10 -7 m normalized emittance. • Electron sources with the required performance exists only for the low reprate 5 FEL category… The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Multiple Modes of Operation Cornell Case FELs: • higher charge (> ~ 0. 2 – 1 n. C), • low charge (tens of p. C), • short bunches/broad spectra, • longer bunches/narrower spectra, • attosecond bunches, • beam “blow-up” regime, • … 6 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale The Ideal Electron Source To achieve the goals of these high-repetition rate, high-average current applications, the electron source should allow for: • repetition rates from few tens of k. Hz up to ~ 1 GHz • charge per bunch from few tens of p. C to ~ 1 n. C, • sub 10 -7 (low charge) to 10 -6 m normalized beam emittance, • beam energy at the gun exit greater than ~ 500 ke. V (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. Injector cost is a small fraction of a 4 th generation light source cost. Minimizing costs is usually not a high priority requirement. 7 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Cathodes • Cathodes are obviously a fundamental part of electron sources. The gun performance heavily depends on cathodes • 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. • In the low charge regime (tens of p. C/bunch) the ultimate emittance performance is set by the cathode thermal emittance • Photo-cathodes (most of present injector schemes) • Thermionic cathodes can in some cases, offer low thermal emittances but require sophisticate compression schemes. (Ce. B 6 at SCSS-Spring 8, XFELO-ANL) In high-repetition rates photo-sources high quantum efficiency photocathodes (QE>~ 1 %) are required to operate with present laser technology. Other cathodes under study (photo-assisted field emission, needle arrays, 8 photo-thermionic, diamond amplifiers) The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Examples of Photo-Cathodes & Lasers High Power, High Brightness Electron Beam Sources F. Sannibale PEA Semiconductor: Cesium Telluride Cs 2 Te (used at FLASH for example) - <~ps pulse capability - relatively robust and un-reactive (operates at ~ 10 -9 Torr) - successfully tested in NC RF and SRF guns - high QE > 1% - photo-emits in the UV ~250 nm (3 rd or 4 th harm. conversion from IR) - for 1 MHz reprate, 1 n. C, ~ 10 W 1060 nm required NEA Semiconductor: Gallium Arsenide Ga. As (used at Jlab for example) - tens of ps pulse capability - reactive; requires UHV <~ 10 -10 Torr pressure - high QE (typ. 10%) - Photo-emits already in the NIR, - low temperature source due to phonon scattering - for n. C, 1 MHz, ~50 m. W of IR required - operated only in DC guns at the moment - Allow for polarized electrons FLASH INFN-LASA PEA Semiconductor: Alkali Antimonides eg. Sb. Na 2 KCs, Cs. K 2 Sb, … - <~ps pulse capability (studied at BOING, INFN-LASA, BNL, Daresbury, LBNL, …. ) - reactive; requires ~ 10 -10 Torr pressure - high QE > 1% - requires green/blue light (eg. 2 nd harm. Nd: YVO 4 = 532 nm) - for n. C, 1 MHz reprate, ~ 1 W of IR required 9 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Available Electron Gun Technologies DC guns Low freq. (<~ 700 MHz) NC RF guns SC RF guns 10 High freq. (> ~1 GHz) NC RF guns The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale DC Guns Pros: • DC operation • DC guns reliably operated at 350 k. V (JLAB) for many years, ongoing effort to increase the final energy (Cornell, Daresbury, Jlab, …). • Extensive simulations (Cornell, …) “demonstrated” the capability of sub-micron emittances at ~ 1 n. C, if a sufficient beam energy is achieved • Full compatibility with magnetic fields. • Excellent vacuum performance • Compatible with most photo-cathodes. (The only one operating Ga. As cathodes) 350 k. V DC gun Challenges: • Higher energies require further R&D and significant technology improvement. • In particular, improvement of the high voltage JLab breakdown ceramic design and fabrication. • Minimizing field emission for higher gradients (>~ 10 MV/m) • Developing and test new gun geometries (inverted geometry, SLAC, JLab) 11 Very interesting results from a “pulsed” DC gun at Spring-8. The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Super-Conducting RF Guns Pros: • Potential for relatively high gradients (several tens of MV/m) Rossendorf • CW operation • Excellent vacuum performance. Challenges: • Move technology from R&D to mature phase • Evaluate and experimentally verify cathode compatibility issues (Promising results with Cs 2 Te at Rossendorf, DC-SRF Peking approach) • Develop schemes compatible with emittance compensation (“cohabitation” with magnetic fields, HOM schemes, …). 12 The Physics and Applications of High Brightness Electron –Beams Maui, USA, November 18, 2009 Brookhaven National Laboratory April -17, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Normal Conducting L and S Band RF Guns Pros: LCLS • High gradients ~50 to ~140 MV/m • “Mature” technology. • Full compatibility with magnetic fields. • Compatible with most photocathodes PITZ • Proved high-brightness performance. (LCLS and PITZ) Challenges: • High power density on the RF structure (~ 100 W/cm 2) limits the achievable repetition rate at high gradient to ~ 10 k. Hz (LUX). • Relative small volume and small apertures can limit the vacuum performance. 13 The Physics and Applications of High Brightness Electron –Beams Maui, USA, November 18, 2009 Brookhaven National Laboratory April -17, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Normal Conducting Low Frequency RF Guns Pros: • Can operate in CW mode • Beam Dynamics similar to DC but with higher gradients and energies • Based on mature RF and mechanical technology. • Full compatibility with magnetic fields. • Compatible with most photo-cathodes LBNL • Potential for excellent vacuum performance. Challenges: • Gradient and energy increase limited by heat load in the structure • CW high brightness performance still to be proved 14 The Physics and Applications of High Brightness Electron –Beams Maui, USA, November 18, 2009 Brookhaven National Laboratory April -17, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale ERL Gun - 4 th Generation Light Source Matching Up to hundreds of MHz reprate DC gun, SC RF Gun, Low freq. NC RF Gun >~ 1 GHz reprate DC gun, SC RF Gun, Reprate < ~ 10 k. Hz FEL Up to hundreds of MHz reprate XFELO Few MHz reprate High freq. NC RF Gun, pulsed DC gun, SC RF Gun, Low freq. NC RF Gun with, DC guns 15 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Required R&D • Pursue development of various electron source schemes • The performance of an electron source is never fully characterized and demonstrated until the source is integrated in an injector • Important to built R&D injector facilities that allow testing and optimization of: • Emittance compensation and beam manipulation techniques, emittance exchange, velocity bunching, … • Cathodes (cathode test facilities capable of accepting all kind of cathodes, vacuum performance, load-lock, …). • Beam diagnostics (especially when considering high repetition rate very low charge and very short bunches 16 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale The Road to Hana • A long and difficult way to go, but potentially very rewarding! • A lot of it has been already done … 17 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Courtesy of C. Hernandez-Garcia 18
Courtesy of C. Hernandez-Garcia 19
Courtesy of C. Hernandez-Garcia 20
Courtesy of C. Hernandez-Garcia 21
High Power, High Brightness Electron Beam Sources F. Sannibale Cornell DC Gun • Present operation limited to ~ 250 k. V to limit field emission and minimize probability of field punctuation of the ceramic (750 k. V initial design). Courtesy of I. Bazarov A new ceramic with bulk resistivity is being installed. Same ceramic material was used in Daresbury to get to over ~450 k. V. 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 (~500 k. V) as an R&D effort separated from the beam running. 22 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Courtesy of Boris Militsyn 23
ALICE photocathode gun. Performance so far … Courtesy of Boris Militsyn 24
High Power, High Brightness Electron Beam Sources F. Sannibale Pulsed DC Gun Pros: SCSS • Based on mature technology. • The pulsed nature relaxes many DC gun issues • Full compatibility with magnetic fields. • Compatible with most photocathodes • Proved high brightness performance. (SCSS) Challenges: • Modulator technology limits maximum repetition rate (60 Hz presently, can it go to k. Hz? ). • Significant injector system complexity when used with thermionic cathodes (“adiabatic” compression requires chopper and multiple 25 RF frequencies) The Physics and Applications of High Brightness Electron –Beams Maui, USA, November 18, 2009 Brookhaven National Laboratory April -17, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale Spring 8 Pulsed DC Gun • 0. 6 mm sliced norm. emittance, at ~0. 3 n. C • ~300 X compression factor at the injector exit, 2 ms, 1 A at the gun, • 500 k. V, 5 cm gap, ~ 10 MV/m • 60 Hz reprate T. Shintake et al. , PRST-AB 12, 070701 (2009) 26 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Courtesy of Thorsten Kamps 27
High Power, High Brightness Electron Beam Sources F. Sannibale BESSY-DESY-FZD-MBI SC RF Gun Cs 2 Te cathodes at 77 K, cavity at 2 K, QE ~ 10 -3 (poor vacuum transfer chamber) Gradient limited by damaged cavity J. Teichert et al. , FEL 08, Gyeongju, Korea p. 467 1. 3 GHz TESLA-like cells. 28 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
BNL Low Frequency RF Gun Motivation • RG 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. Courtesy of Ilan Ben-Zvi
Status • The 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
A SRF 200 MHz Cavity Design for the WIFEL, the Wisconsin FEL The WIFEL accelerator is required to supply each of the six FEL end stations simultaneously at up to a 1 MHz repetition • Cs 2 Te cathode, beam blow up regime 30 fs ~0. 9 mm hemispherical transverse profile, 37 MV/m at cathode, 200 MHz SRF cavity, 5 Me. V final energy Courtesy of Robert Legg 31
Beam Dynamics Simulations of Injector using Blow Out Bunches 200 p. C Gun Cryomodule Courtesy of Robert Legg 32
High Power, High Brightness Electron Beam Sources F. Sannibale Peking DC-SRF Gun Brings the cathode out of the cryogenic environment 1. 5 cell already in operation 3. 5 cell under fabrication THz/IR ERL FEL Jiankui Hao, et al. , SRF 2009, p 205, Berlin, Germany 33 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale SLAC NC S-Band RF Gun Derived by the BNL-SLAC-UCLA design (S-Band). Great care in minimizing dipolar and quadrupolar field components. In operation Up to date best performance Courtesy of Dave Dowell 0. 5 microns emittance at 250 p. C 0. 14 microns emittance at 20 p. C 34 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale PITZ NC RF L-band Gun In operation Courtesy of Frank Stephan 1. 3 GHz Copper 35 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale LANL/AES NC CW 700 MHz Gun 700 MHz CW normalconducting gun. Hundreds of k. W dissipated in the glidcop structure. Part of a 100 m. A injector for ~ 100 k. W IR FEL RF conditioning successfully completed. First beam tests in spring summer 2010 Courtesy of D. Nguyen and B. Carsten 36 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale The LBNL CW NC VHF gun The Berkeley normal-conducting scheme satisfies all the LBNL FEL requirements simultaneously. In fabrication J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006 K. Baptiste, et al, NIM A 599, 9 (2009) Frequency 187 MHz Operation mode CW Gap voltage 750 k. V Field at the cathode 19. 47 MV/m Q 0 30887 Shunt impedance 6. 5 MW RF Power 87. 5 k. W Stored energy 2. 3 J Peak surface field 24. 1 MV/m Peak wall power density 25. 0 W/cm 2 Accelerating gap 4 cm Diameter 69. 4 cm Total length 35. 0 cm • 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 l. RF allows for large apertures and thus for high vacuum conductivity. • Based on mature and reliable normal-conducting RF and mechanical technologies. 37 • 187 MHz compatible with both 1. 3 and 1. 5 GHz super-conducting linac technologies. The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
High Power, High Brightness Electron Beam Sources F. Sannibale A Cathode Test Facility • 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 H 2 O, O 2, CO, …). This arrangement will allow testing a variety of cathodes including "delicate" semiconductor cathodes. • An ion pump accounts for noble gasses and hydrocarbons. • 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. 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, . . . ) 38 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
Courtesy of Kwan-Je Kim 39
Courtesy of Kwan-Je Kim 40
High Power, High Brightness Electron Beam Sources F. Sannibale Mahalo! 41 The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009
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