Is a Carbon Nanotube Field- Emission Electron

Source on an Upgrade Path for HIGS?

Thomas B. CleggFebruary 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

Upgrade completion in March 2006

Need x20 increase in linac beam intensity to fill the booster-injector

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 CavityElectron Source

Optimization of a thermionic microwave electron gunC.B. McKee and John M..J. Madey, Nucl. Instr. & Meth. A304 (1991) 386.

Ene

rgy Φ

EF

Electron Source and Accelerating Cavity

Mark III RF Cavity

Optical Window for Laser

Beam

RF Power Input

HIGS Source

RF Power Input

Axially adjustable

LaB6 photocathode

Accelerating Field & Emerging Beam

Cavity Oscillates in TM010 Mode

Electric field lines

Magnetic field lines

2.856 GHz RF Fields Inside Cavity

Backward accelerated electrons

heat and can damage the

cathode

Emax = 1 to 4 x 105 V/cm

at the cathode surface

E-F

ield

(x1

05

V/c

m)

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

timeBeam macro-pulse

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 μsin each macro-pulse

Beam Path

SourceMomentum

analysis

800 keV to 1 MeV micro-pulses to

linac

time

Δt=30 ps

Δt= ~5 ps

HIGS Injector Electron Source

• The HIGS source nowoperates with a LaB6

photocathode.

• Cathode is illuminated with N2 infrared laser with λ=337 nm.

– 800 kW peak power– 1 mJoule/1ns pulse

Pulsed Nitrogen Laser

250 MeV Linac

HIGS Beam Needs – Now and Future

Present beam pulses

3 micro-pulsesin 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

HIGS Beam Needs – Now and Future

300 micro-pulses

1 sec between RF macro-pulses

100 ns

timeDesired 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

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 eV , and E is the applied field in V/cm.

22

3

722

1

6 A/cmin ]108.6exp[)/(

102.6E

EE

Ej

F

F

* 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

Cathode

Insulator

Anode

Insulator

Optical Access

4 cm

Unpublished private communication of Don Schiffler

USAF Cathode I/V Characteristics

ANI CNT Cathode Performance

Cur

rent

(k

A)

3

2

1

4

5

0

The above result implies one should achievefor HIGS a stable field emission current of

20.4 nC from a 3.2 mm diameter ANI cathode.

Shot Number0 500 1000 1500 2000 2500 3000

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

• 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