Is a Carbon Nanotube Field- Emission Electron Source on an Upgrade Path for HIGS? Thomas B. Clegg February 2, 2004
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
HIGS Upgrade Overview Need x 20 increase in linac beam intensity to fill the booster-injector Upgrade completion in March 2006
Present Linac Electron Sources Water Cooling Lines Cathode Position Adjusting Mechanism Cathode Surface Top View Energy Electron Source RF Cavity Φ Vacuum Pumpout 500 ke. V Beam Out RF Input Waveguide EF Optimization of a thermionic microwave electron gun C. B. Mc. Kee and John M. . J. Madey, Nucl. Instr. & Meth. A 304 (1991) 386. Laser Beam Port
Electron Source and Accelerating Cavity RF Power Input Axially adjustable La. B 6 photocathode HIGS Source RF Power Input Optical Window for Laser Beam Mark III RF Cavity
Accelerating Field & Emerging Beam Emax = 1 to 4 x 105 V/cm at the cathode surface 2. 856 GHz RF Fields Inside Cavity Magnetic field lines Cavity Oscillates in TM 010 Mode Electric field lines Emerging e-beam pulses every 350 ps with LARGE energy spread Cathode Anode E-Field (x 105 V/cm) Backward accelerated electrons heat and can damage the cathode Beam macro-pulse Beam micro-pulses 0. 5 Acceleration window ~ 30 ps RF/Laser pulse width t (ns) 1. 0 Linac acceptance ~ 5 ps time
Mark III FEL Thermionic Source 800 ke. V to 1 Me. V micro-pulses to linac Δt= ~5 ps Mark III Linac Beam Path Thermionic Source α-Magnet Δt=30 ps Source RF Accelerating Cavity 6 to 12 μs Pulsed electron beam time structure Momentum analysis ~3000 micro-pulses per μs in each macro-pulse 1 sec between RF macro-pulses time
HIGS Injector Electron Source • The HIGS source now operates with a La. B 6 photocathode. • Cathode is illuminated with N 2 infrared laser with λ=337 nm. – 800 k. W peak power – 1 m. Joule/1 ns pulse 250 Me. V Linac Pulsed Nitrogen Laser
HIGS Beam Needs – Now and Future Now 1 ns Present beam pulses 3 micro-pulses in each macro-pulse 1 sec between RF/Laser macro-pulses Need ~1 n. C in each macro-pulse to obtain 0. 2 n. C injected into storage ring after linac time
HIGS Beam Needs – Now and Future Now 1 ns Present beam pulses 3 micro-pulses 1 sec between RF/Laser macro-pulses time Need ~1 n. C in each macro-pulse to obtain 0. 2 n. C injected into storage ring after linac Future 300 micro-pulses 100 ns Desired beam pulses after upgrade 1 sec between RF macro-pulses Need 20 n. C in each macro-pulse to obtain 4 n. C injected into storage ring after linac time
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
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
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
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 (108 V/cm). • Field emission is a quantum effect with response times to the applied field of order 10 -15 sec.
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 EF is the Fermi energy and Φ is the work function in e. V , and E is the applied field in V/cm. * R. H. Fowler and L. Nordheim, Proc. R. Soc. London, Ser. A 119 (1928) 173.
CNT Field Emission Characteristics G. Z. Yue et al. , Appl. Phys. Lett. 81 (2002) 355.
USAF Cathode Test Stand Unpublished private communication of Don Schiffler Optical Access Cathode 4 cm Insulator Anode Insulator
USAF Cathode I/V Characteristics
ANI CNT Cathode Performance Current (k. A) 5 4 3 2 1 0 0 500 1000 1500 2000 2500 3000 Shot Number The above result implies one should achieve for HIGS a stable field emission current of 20. 4 n. C from a 3. 2 mm diameter ANI cathode.
CNT Cathode Lifetime • HIGS needs – Zero maintenance in 1 year – Total ‘on-time’/year expected to be 0. 32 seconds • ANI Measurements – 150 -200 m. A 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.
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.
Field Emission Time Structure Current Pulses RF Waveform • Beam pulses emerge only during the accelerating half of each RF cycle • Reduced back-acceleration of electrons to damage the cathode
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