Transcript Applied-Nanotechnologies, Inc.

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 x20 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 keV
Beam
Out
RF Input
Waveguide
EF
Optimization of a thermionic microwave electron gun
C.B. McKee and John M..J. Madey,
Nucl. Instr. & Meth. A304 (1991) 386.
Laser
Beam Port
Electron Source
and Accelerating Cavity
RF Power
Input
Axially
adjustable
LaB6
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
TM010 Mode
Electric field lines
Emerging e-beam pulses
every 350 ps with
LARGE energy spread
Cathode
Anode
1.5
E-Field (x105 V/cm)
Backward
accelerated
electrons
heat and can
damage the
cathode
1
0.5
0
0.5
t (ns)
1.0
-0.5
-1
-1.5
Beam macro-pulse
Beam micro-pulses
Acceleration
window ~ 30 ps
RF/Laser pulse width
Linac acceptance
~ 5 ps
time
Mark III FEL Thermionic Source
800 keV to 1 MeV
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 LaB6
photocathode.
• Cathode is illuminated
with N2 infrared laser
with λ=337 nm.
– 800 kW peak power
– 1 mJoule/1ns pulse
250 MeV
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 nC in each macro-pulse to obtain 0.2 nC
injected into storage ring after linac
time
HIGS Beam Needs – Now and Future
3 micro-pulses
Now
1 ns
Present beam pulses
1 sec between
RF/Laser macro-pulses
time
Need ~1 nC in each macro-pulse to obtain 0.2 nC
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 nC in each macro-pulse to obtain 4 nC
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
1
3
2
( EF / )
2
7 
j  6.2 10
E exp[6.8 10
] in A/cm2
EF  
E
6
2
Here EF 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.
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 (kA)
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 nC 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 mA peak current
~2-3 A/cm2 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
16000
14000
12000
Current Pulses
10000
8000
i
v
6000
4000
RF Waveform
2000
0
0
2E-10
4E-10
6E-10
8E-10
1E-09
1.2E-09
-2000
• Beam pulses emerge only during the accelerating half of each RF cycle
• Reduced back-acceleration of electrons to damage the cathode