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High Average Power, High Brightness Electron Beam Sources

The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009. High Average Power, High Brightness Electron Beam Sources. Fernando Sannibale Lawrence Berkeley National Laboratory.

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High Average Power, High Brightness Electron Beam Sources

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  1. The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 High Average Power, High Brightness Electron Beam Sources Fernando Sannibale Lawrence Berkeley National Laboratory The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009

  2. Outline High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • 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!).

  3. Why High-Brightness, High-Power Electron Sources High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • 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 FELs matching conditions on emittance and energy spread drive these quantities down: High 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 (~ 1kW beam power) and military applications (up to ~250kW) • 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)

  4. Why High-Brightness, High-Power Electron Sources High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • Electron sources are nowadays playing a central role in • 4th 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 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. • 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!

  5. 4th Generation X-Ray Light Sources High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • ERLs. 100 MHz-GHz-like reprate, • very high average current: hundreds of mA, • normalized emittances 10-7 to 10-6 m Cornell • Low reprate FELs. Few Hz to ~1 kHz reprate, or low reprate long trains of bunches, • sub-micron normalized emittances, • < 10 mA average currents. LCLS-SLAC • High reprate FELs. MHz-class reprates, • sub-micron emittances, • several mA average currents LBNL • XFELO Oscillator. MHz-like reprate, • tens of pC bunches, • 10-7 m normalized emittance. • Electron sources with the required performance exists only for the low reprate FEL category… ANL

  6. Multiple Modes of Operation High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Cornell Case • FELs: • 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, • …

  7. The Ideal Electron Source High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 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 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 - 10-11Torr 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 4th generation light source cost. Minimizing costs is usually not a high priority requirement.

  8. Cathodes High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • 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 pC/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. • (CeB6 at SCSS-Spring 8, XFELO-ANL) In high-repetition rates photo-sources high quantum efficiency photo-cathodes (QE>~ 1 %) are required to operate with present laser technology. Other cathodes under study (photo-assisted field emission, needle arrays, photo-thermionic, diamond amplifiers)

  9. Examples of Photo-Cathodes & Lasers High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 PEA Semiconductor: Cesium Telluride Cs2Te (used at FLASH for example) - <~ps pulse capability - relatively robust and un-reactive (operates at ~ 10-9Torr) - successfully tested in NC RF and SRF guns - high QE > 1% - photo-emits in the UV ~250 nm (3rd or 4th harm. conversion from IR) - for 1 MHz reprate, 1 nC, ~ 10 W 1060nm required FLASH INFN-LASA NEA Semiconductor: Gallium Arsenide GaAs(used at Jlab for example) - tens of ps pulse capability - reactive; requires UHV <~ 10-10Torr pressure - high QE (typ. 10%) - Photo-emits already in the NIR, - low temperature source due to phonon scattering - for nC, 1 MHz, ~50 mW of IR required - operated only in DC guns at the moment - Allow for polarized electrons PEA Semiconductor: Alkali Antimonideseg. SbNa2KCs, CsK2Sb, … - <~ps pulse capability (studied at BOING, INFN-LASA, BNL, Daresbury, LBNL, ….) - reactive; requires ~ 10-10Torr pressure - high QE > 1% - requires green/blue light (eg. 2nd harm. Nd:YVO4 = 532nm) - for nC, 1 MHz reprate, ~ 1 W of IR required

  10. Available Electron Gun Technologies High Power, High Brightness Electron Beam Sources F. Sannibale DC guns The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Low freq. (<~ 700 MHz) NC RF guns High freq.(> ~1 GHz) NC RF guns SC RF guns

  11. DC Guns High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Pros: • DC operation • DC guns reliably operated at 350 kV (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 nC, if a sufficient beam energy is achieved • Full compatibility with magnetic fields. • Excellent vacuum performance • Compatible with most photo-cathodes. • (The only one operating GaAs cathodes) 350 kV DC gun Challenges: • Higher energies require further R&D and significant technology improvement. • In particular, improvement of the high voltage breakdown ceramic design and fabrication. JLab • Minimizing field emission for higher gradients (>~ 10 MV/m) • Developing and test new gun geometries (inverted geometry, SLAC, JLab) • Very interesting results from a “pulsed” DC gun at Spring-8.

  12. Super-Conducting RF Guns High Power, High Brightness Electron Beam Sources F. Sannibale Rossendorf The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Pros: • Potential for relatively high • gradients (several tens of MV/m) • CW operation • Excellent vacuum performance. Challenges: • Move technology from R&D to mature phase • 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

  13. Normal Conducting L and S Band RF Guns High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Pros: • High gradients ~50 to ~140 MV/m LCLS • “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/cm2) limits the • achievable repetition rate at high gradient to ~ 10 kHz (LUX). • Relative small volume and small apertures can limit the vacuum performance. Brookhaven National Laboratory – April 17, 2009

  14. Normal Conducting Low Frequency RF Guns High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 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. LBNL • Compatible with most photo-cathodes • 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 Brookhaven National Laboratory – April 17, 2009

  15. Gun - 4th Generation Light Source Matching High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 DC gun, SC RF Gun, Low freq. NC RF Gun Up to hundreds of MHz reprate ERL >~ 1 GHz reprate DC gun, SC RF Gun, High freq. NC RF Gun, pulsed DC gun Reprate < ~ 10kHz FEL DC gun, SC RF Gun, Low freq. NC RF Gun Up to hundreds of MHz reprate Low freq. NC RF Gun with, DC guns XFELO Few MHz reprate

  16. Required R&D High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • 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

  17. The Road to Hana High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • A long and difficult way to go , but potentially very rewarding! • A lot of it has been already done …

  18. Courtesy of C. Hernandez-Garcia

  19. Courtesy of C. Hernandez-Garcia

  20. Courtesy of C. Hernandez-Garcia

  21. Courtesy of C. Hernandez-Garcia

  22. Cornell DC Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • Present operation limited to ~ 250kV to limit field emission and minimize probability of field punctuation of the ceramic (750kV 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 ~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.

  23. Courtesy of Boris Militsyn

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

  25. Pulsed DC Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 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 kHz?). • Significant injector system complexity when used with thermionic cathodes (“adiabatic” compression requires chopper and multiple RF frequencies) Brookhaven National Laboratory – April 17, 2009

  26. Spring 8 Pulsed DC Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • 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 reprate T. Shintake et al., PRST-AB12, 070701 (2009)

  27. Courtesy of Thorsten Kamps

  28. BESSY-DESY-FZD-MBI SC RF Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Cs2Te cathodes at 77 K, cavity at 2K, QE ~ 10-3 (poor vacuum transfer chamber) Gradient limited by damaged cavity J. Teichert et al., FEL08, Gyeongju, Korea p.467 1.3 GHz TESLA-like cells.

  29. BNL Low Frequency RF Gun • 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. Motivation Courtesy of Ilan Ben-Zvi

  30. 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

  31. 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 • Cs2Te cathode, beam blow up regime 30 fs ~0.9 mm hemispherical transverse profile, 37 MV/m at cathode, 200 MHz SRF cavity, 5MeV final energy Courtesy of Robert Legg

  32. Beam Dynamics Simulations of Injector using Blow Out Bunches 200 pC Gun Cryomodule Courtesy of Robert Legg

  33. Peking DC-SRF Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 1.5 cell already in operation 3.5 cell under fabrication Brings the cathode out of the cryogenic environment THz/IR ERL FEL Jiankui Hao, et al., SRF2009, p 205, Berlin, Germany

  34. SLAC NC S-Band RF Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Derived by the BNL-SLAC-UCLA design (S-Band). Great care in minimizing dipolar and quadrupolar field components. In operation 0.5 microns emittance at 250 pC 0.14 microns emittance at 20 pC Up to date best performance Courtesy of Dave Dowell

  35. PITZ NC RF L-band Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 In operation Courtesy of Frank Stephan 1.3 GHz Copper

  36. LANL/AES NC CW 700 MHz Gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 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 Courtesy of D. Nguyen and B. Carsten

  37. The LBNL CW NC VHF gun High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 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) • 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 lRFallows for large apertures and thus for high vacuum conductivity. • Based on mature and reliable normal-conducting RF and mechanical technologies. • 187 MHz compatible with both 1.3 and 1.5 GHz super-conducting linac technologies.

  38. A Cathode Test Facility High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 • The vacuum system has been designed to achieve an operational pressure down into the low 10-11Torr range. • NEGs pumps are used (very effective with H2O, O2, 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, ...)

  39. Courtesy of Kwan-Je Kim

  40. Courtesy of Kwan-Je Kim

  41. High Power, High Brightness Electron Beam Sources F. Sannibale The Physics and Applications of High Brightness Electron Beams - Maui, USA, November 18, 2009 Mahalo!

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