1. Is a Carbon Nanotube Field- Emission Electron Source on an Upgrade Path for HIGS? Thomas B. Clegg February 2, 2004

2. Outline HIGS Upgrade Overview Present TUNL/FEL/HIGS electron sources Future electron source needs after HIGS upgrade Field emission from carbon nanotubes (CNTs) Proposed new CNT field-emission electron source for HIGS

3. HIGS Upgrade Overview Upgrade completion in March 2006 Need x20 increase in linac beam intensity to fill the booster-injector

4. Present Linac Electron Sources RF Input Waveguide Cathode Position Adjusting Mechanism Water Cooling Lines Top View Vacuum Pumpout Cathode Surface 500 keV Beam Out Laser Beam Port RF Cavity Electron Source Optimization of a thermionic microwave electron gun C.B. McKee and John M..J. Madey, Nucl. Instr. & Meth. A304 (1991) 386. Energy Φ EFEF

5. Electron Source and Accelerating Cavity Mark III RF Cavity Optical Window for Laser Beam RF Power Input HIGS Source RF Power Input Axially adjustable LaB 6 photocathode

6. Accelerating Field & Emerging Beam Cavity Oscillates in TM 010 Mode Electric field lines Magnetic field lines 2.856 GHz RF Fields Inside Cavity Backward accelerated electrons heat and can damage the cathode E max = 1 to 4 x 10 5 V/cm at the cathode surface E-Field (x10 5 V/cm) Anode t (ns) 0.5 1.0 Emerging e-beam pulses every 350 ps with LARGE energy spread Acceleration window ~ 30 ps Linac acceptance ~ 5 ps Cathode Beam micro-pulses RF/Laser pulse width time Beam macro-pulse

7. Mark III FEL Thermionic Source Mark III Linac Thermionic Source RF Accelerating Cavity α-Magnet 1 sec between RF macro-pulses Pulsed electron beam time structure 6 to 12 μs ~3000 micro-pulses per μs in each macro-pulse Beam Path Source Momentum analysis 800 keV to 1 MeV micro-pulses to linac time Δt=30 ps Δt= ~5 ps

8. HIGS Injector Electron Source The HIGS source now operates with a LaB 6 photocathode. Cathode is illuminated with N 2 infrared laser with λ=337 nm. –800 kW peak power – 1 mJoule/1ns pulse Pulsed Nitrogen Laser 250 MeV Linac

9. HIGS Beam Needs – Now and Future Present beam pulses 3 micro-pulses in each macro-pulse 1 sec between RF/Laser macro-pulses 1 ns time Need ~1 nC in each macro-pulse to obtain 0.2 nC injected into storage ring after linac Now

10. HIGS Beam Needs – Now and Future 300 micro-pulses 1 sec between RF macro-pulses 100 ns time Desired beam pulses after upgrade Present beam pulses 3 micro-pulses 1 sec between RF/Laser macro-pulses 1 ns time Need ~1 nC in each macro-pulse to obtain 0.2 nC injected into storage ring after linac Need 20 nC in each macro-pulse to obtain 4 nC injected into storage ring after linac Now Future

11. Possibilities for Upgrade Buy a longer-pulse laser … 100 ns –Flash-lamp pumped laser plus optics –4 th Harmonic of 1μm laser at ~266 nm –“Guesstimated” cost - $80K to $90K Use existing thermionic source with fast chopper

12. Possibilities for Upgrade Buy a longer-pulse laser … 100 ns Use existing thermionic source with fast chopper –Copy systems used at ANL’s Advanced Photon Source –Start with 1μs width … get 100ns after chopping –Sweep electron beam laterally at the source with fast-pulsed capacitive deflector … 8 ns and 40 ns pulses –Cheaper, but requires significant development effort

13. Possibilities for Upgrade Buy a longer-pulse laser … 100 ns Use existing thermionic source with fast chopper Develop a field-emission source using a CNT cathode

14. Applied-Nanotechnologies, Inc. 308 W. Rosemary St., Suite 209 Chapel Hill, NC 27516 Local supplier of carbon nanotube devices. –Single nanotube tips –Field-emission cathodes –Compact electron sources –X-ray tubes

15. Field Emission from Carbon Nanotubes CNTs have excellent materials properties which make them have attractive field emission characteristics. –High temperature and chemical stability –High electrical and thermal conductivity –Large aspect ratio(>1000) –Atomically sharp tips

16. What Is Field Emission? Field emission of electrons from the surface of a condensed phase into another phase, usually vacuum, occurs under the action of a high E-field (10 8 V/cm). Field emission is a quantum effect with response times to the applied field of order 10 -15 sec.

17. Fowler-Nordheim Equation* At a metal surface, electrons near the Fermi level tunnel through the energy barrier and escape to produce a field emission current density j of Here E F is the Fermi energy and Φ is the work function in eV, and E is the applied field in V/cm. * R.H. Fowler and L. Nordheim, Proc. R. Soc. London, Ser. A 119 (1928) 173.

18. From geometry, for a hemispherical tip, so for a carbon nanotube with Elocal =108 V/cm. Then for Rtip=1 nm, and α ≈10, one obtains V ≈100 volts. This leads to a relation between I and V for a single carbon nanotube tip of or This yields a linear graph vs 1/V with slope Fowler-Nordheim Equation - cont’d

19. CNT Field Emission Characteristics G.Z. Yue et al., Appl. Phys. Lett. 81 (2002) 355.

20. USAF Cathode Test Stand Cathode Insulator Anode Insulator Optical Access 4 cm Unpublished private communication of Don Schiffler

21. USAF Cathode I/V Characteristics

22. ANI CNT Cathode Performance Current (kA) 3 2 1 4 5 0 The above result implies one should achieve for HIGS a stable field emission current of 20.4 nC from a 3.2 mm diameter ANI cathode. Shot Number 0500100015002000 2500 3000

23. Current Variation Over 1 Hour at 1 Hertz 1 Trace every 10 minutes 250 kV Pulse-to-Pulse Stability

24. Long-Term Stability

25. CNT Cathode Lifetime HIGS needs –Zero maintenance in 1 year –Total ‘on-time’/year expected to be 0.32 seconds ANI Measurements –150-200 mA peak current –~2-3 A/cm 2 current density –Total "on-time" of 600 seconds. –Decay in 600 seconds is ~50% under a constant, i.e dc extraction field.

26. Outgassing Rate

27. How Will A CNT Emitter Peform?

28. CNT Emission in RF E-field When a sinusoidal voltage is applied, electrons are emitted only during the positive maxima of the RF sine wave.

29. Field Emission Time Structure Beam pulses emerge only during the accelerating half of each RF cycle Reduced back-acceleration of electrons to damage the cathode Current Pulses RF Waveform