Field Emission and Channeling Radiation for High-Spectral-Brilliance X-ray Sources

Transcription

1 Field Emission and Channeling Radiation for High-Spectral-Brilliance X-ray Sources Charles Brau, Bo Choi, William Gabella, Anthony Hmelo, John Kozub, Borislav Ivanov, Jonathan Jarvis, Marcus Mendenhall Vanderbilt University Ben Blomberg, Daniel Mihalcea, Philippe Piot Northern Illinois University and FermiLab John Lewellen Consultant 12/13/2012 Vanderbilt/NIU UESDM workshop 1

2 Electron sources span nine orders of magnitude Field emission approaches the quantum limit Brightness (A/m 2 -str) 1.E+19 1.E+17 1.E+15 1.E+13 1.E+11 1.E+09 1.E+07 (atom on tip) Nanotubes Metallic tips Needle RF photoinjectors Schottky emitter photoemission Photo-field emission Thermionic emission Quantum limit (cw) (pulsed) DC photo emission Storage rings 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 Current (A) 12/13/2012 Vanderbilt/NIU UESDM workshop 2

3 Field emission from atoms adsorbed on CNTs approaches the quantum limit Emittance of beam from adsorbed Field emission from atom is near the Heisenberg limit adsorbed atoms forms intense spots Emittance is close to Heisenberg limit Background gas atoms are weakly bound, unstable, and desorb at high current Emission is too unstable to measure energy spectrum Same thing seen on diamond pyramids Current spike due to adsorbate emission 12/13/2012 Vanderbilt/NIU UESDM workshop 3

4 Channeling radiation from tightly focused electrons produces brilliant, hard X-rays MeV electrons in crystals produce channeling radiation Theory and experiments are well established Hard x-ray emission possible from a diamond chip 70-keV photons from 35-MeV electrons Requires modest rf linac High spectral brilliance requires exquisite electron beam emittance ph/s/ m 2 /0.1%BW Requires 200-nA average current 3-nm normalized emittance 40-nm focal spot on diamond These parameters have never been explored in an rf linac Use new type cathode Explore emittance growth during acceleration Theory/simulation Experiment 12/13/2012 Vanderbilt/NIU UESDM workshop 4

5 Plan A for low emittance beam is a gated diamond field emitter operating in an RF gun Gate the cathode with dc, first-harmonic, and thirdharmonic bias <100 V to switch on Advantages: Simple gun and rf power Emission decoupled from cavity field Plan B :Ungated cathode Simple, rugged 15 A DC demonstrated Emission timing by third harmonic in cavity Current ( A) Current RF field Time (ps) 12/13/2012 Vanderbilt/NIU UESDM workshop 5

6 Simulations show that very small emittance is preserved during acceleration to 40 MeV Simulation describes a gated cathode in an RF gun followed by a LINAC Transverse emittance ~4 nm is preserved during acceleration Longitudinal emittance increases due to the long bunch (distortions) Transverse emittance evolution for different fractions of the beam (25 fc) 100% 95% 90% 80% gun CAV1 CAV2 12/13/2012 Vanderbilt/NIU UESDM workshop 6

7 Ungated diamond field-emitter arrays are fabricated in VINSE facility (Vanderbilt) Base 2 to 30 m Tip radius ~ 6 nm Pitch 3 to 200 m Array 1 to >10 6 tips Rugged Thermal oxida on of Si Oxide pa erning Anisotropic etching Tip mold sharpening oxida on diamond deposi on diamond brazing Si mold & oxide removal Wet 1100 C 60 C Wet 1100 C MPCVD 1.3 kw (~ 4 um thick) MPCVD 0.7 kw (1 um thick) TiCuSil braze on Mo ~800 C C BOE oxide etch Cleaning process 12/13/2012 Vanderbilt/NIU UESDM workshop 7

8 In volcano process, gate electrode is applied over SiO 2 and window formed by FIB milling Thickness and material of gate electrode Window size controls gate spacing 400 nm Mo 1.0 um 300 nm Cu 400 nm Cu 0.5 um 12/13/2012 Vanderbilt/NIU UESDM workshop 8

9 Emittance of individual gated field-emission cathodes is dominated by spherical aberration Phase space of electrons at anode 6.00E E-24 px (kg*m/s) 4.00E E E E E E+ 1.00E- 2.00E- 3.00E- 4.00E- 5.00E- 6.00E- 7.00E- 8.00E- 9.00E x (mm) Phase space of beam is curved >>thermal spread Measure with TEM grid 5-kV DC experiment I-V parameters of gate 12/13/2012 Vanderbilt/NIU UESDM workshop 9

10 For RF gun, cathode holder provides coaxial RF feed to drive gate of field-emitter cathode 12/13/2012 Vanderbilt/NIU UESDM workshop 10

11 RF tests of gated cathode will be carried out in electron guns at Niowave and Fermilab Small test stand is available at Niowave 700 MHz 5 MV/m Currently testing ungated field emitters After X-ray tests, the A0 injector will be available at Fermilab 1.3 GHz 40 MV/m Will test ungated emitters soon 12/13/2012 Vanderbilt/NIU UESDM workshop 11

12 HBESL facility at Fermilab will be used for first X-ray experiments and cathode tests A0 is the injector for the HBESL facility L-band (1.3 GHz) 2.6-cell, 4.5 MeV 35 MV/m 400- s macropulses 1 Hz repetition rate Standard configuration Photocathode 2-ps pulses at 1 MHz 1 nc/pulse 12/13/2012 Vanderbilt/NIU UESDM workshop 12

13 After X-ray experiments, we will test gated cathode in A0 and measure 4-nm emittance Used TEM grid 125 m pitch Reconstruction gives 30-nm emittance We need order of magnitude better [P.Musumeci, PRST-AB (2012)] 12/13/2012 Vanderbilt/NIU UESDM workshop 13

14 ASTA will be used at 40 MeV to generate 80-keV channeling X-rays with high brilliance X-ray experiment will go here Old HBESL cavity (CAV1) being refurbished Expected to be ready for first beam early in CY2013 Second cavity (CAV2) is in place 12/13/2012 Vanderbilt/NIU UESDM workshop 14

15 X-ray microscopy techniques can be used to measure X-ray source size and coherence Point-projection microscopy Resolution = source size (diagnostic) Ideally suited to channeling source [Takahashi, PRB (2000)] Ptychography Can be used to characterize X-ray wavefront 12/13/2012 Vanderbilt/NIU UESDM workshop 15

16 Summary Field emission produces exquisitely bright electron beams 2-nm emittance, near quantum degeneracy Small emittance (4-nm) is preserved during acceleration Photolithography is used to fabricate field emitters Gated and ungated varieties 40-MeV electrons produce 80-keV channeling X-rays 40-nm focal spot produces high spectral brilliance Cathodes will be tested at Niowave and Fermilab Small emittance and spot size will be challenging to measure X-ray experiments will begin in /13/2012 Vanderbilt/NIU UESDM workshop 16

17 That s all, folks!