slides - CASA

Progress on the Vanderbilt
Table Top THz FEL
Heather Andrews
Department of Physics
Vanderbilt University
28 April 2005

• Group at Vanderbilt
– Charles Brau
– Chase Boulware
– Jonathan Jarvis
– Carlos Hernandez
• Useful discussions
– Hayden Brownell
Acknowledgements
– Jack Donohue and Jacques Gardelle

WANTED: a narrowband THz source for
spectroscopy
Problem Solution Requirements
• Want to do frequency
domain spectroscopy at
THz frequencies
• No existing narrowband
source provides good
power in THz range
• Short pulse sources -
good for time-domain
spectroscopy
* CW - good for imaging if
you have enough power
• Want a source which
will produce
– 300-1000 micron
radiation (0.3-1 THz)
– ~1 Watt peak power
– ~5 nanosecond
pulses
– Narrowband

Source requires needle cathode and
Smith-Purcell effect
• Needle photo-cathodes
provide high brightness
electron beam
• Smith-Purcell (SP)
radiation provides a
compact, tunable
radiation source
• Requires a high
brightness beam for
high power output
e-beam
• Voltage = 30 - 80 kV
• Current = 1-10 mA
THz
• Brightness ~ 10 11 A/m 2 -steradian
• Wavelength 300 - 1000 µm

Examine protein secondary structure
• Large molecules and
structures have
characteristic vibrations
in THz region
• Reconformation could
be examined using
nonlinear spectroscopy
• Pump-probe nonlinear
spectroscopy possible
using THz/THz or
THz/mid-IR radiation

• Examine protein
folding
• High field EPR
spectroscopy
Additional applications

How this compares to other THz sources
THz Source
UCSB FEL
Synchrotron
sources
Optically pumped
FIR lasers
Optical rectification
techniques
Backward Wave
Oscillators (BWO)
Comparison to SP-FEL
Longer pulses (microsecond as opposed
to nanosecond), higher power
Lower spectral brightness, much shorter
pulses, broadband
Not tunable
Very short pulses, low power, broadband
Low power, longer wavelengths, very
similar operating mechanism

TUNGSTEN
NEEDLE
HOLDER
Experimental Set Up
Nd:YAG
PUMP
LASER
FOCUSING
MAGNET
R tip ~ 45 µm
STEERING
MAGNETS
DETECTOR
GRATING
FARADAY
CUP
POWER METER

Experimental apparatus

We have achieved current densities high
enough to build tabletop FELs
• Photocurrent was
– Up to 100 mA
– Shorter pulse than
laser
– Limited by damage
to needle
• 4th harmonic Nd:YAG
at 266 µm with 7 ns,
200 mJ pulse
• Quantum efficiency is
increased by 10 2 by
laser

Needle cathode e-beam has high
brightness
normalized brightness (A/m 2 -steradian)
10 16
10 14
10 12
10 10
10 8
0.0001 0.001 0.01 0.1 1 10 100 1000
rf photoinjectors current (A)
storage rings
field emission
thermionic emission
photoelectric field emission

Recent needle cathode developments
• Bigger needles = more current
• Use 5th harmonic Nd:YAG, 5 ns pulse, maximum 50
µJ/pulse

Dimensions of the aluminum grating
• Gratings are fabricated out of aluminum using very
thin saw blades
• Dimensions will be 12.5 mm long (1/2 inch), with 250
µm period, 150 µm grove width and 200 µm depth
• 173 µm period, 62 µm width and 100 µm depth used
for calculations*
200 µm
250 µm 150 µm
12.5 mm
*Parameters used at Dartmouth, Urata et. al., PRL, 80, 516, (1998)

Smith-Purcell laser produces two types of
radiation
Below threshold current: Above threshold current:
λ = l
n
⎛ 1 ⎞
− cosθ
⎝
⎜
β ⎠
⎟
θ
Electron
beam
Radiating
wave
Evanescent
wave

• Evanescent wave:
Details of laser radiation
– Phase velocity
parallel to electon
beam, group velocity
opposite
– Bunches electrons
– Produces harmonics
within the SP
spectrum which
radiate
Bound wave
Radiating harmonic waves
Electrons bunching

Wavelength (microns)
1 10 3
800
600
400
200
Expected wavelengths
1st Smith-Purcell range
Second SP range
Laser fundamental wavelength
Laser 2nd harmonic
Laser 4th harmonic
Laser 3rd harmonic
0
20 25 30
Beam Voltage (kV)
35 40

Amplitude growth rate, µ (m -1 )
600
500
400
300
200
100
Comparison of gain theories
Schaechter and Ron
Present theory
Kim and Song
0
28 30 32 34 36 38 40 42
Electron energy, V (keV)
Schaechter and Ron, Phys. Rev., A40, 876 (1989)
Kim and Song, Nucl. Inst. Meth., A475, 159 (2001)

Gain/attenuation (m -1 )
10 5
10 4
10 3
100
Gain and attenuation peak at v g=0
Gain
Gain ∝ ( 1 v ) g
1
3 Attenuation ∝1 vg Attenuation
10
20 40 60 80 100 120 140 160
Electron energy (keV)

Net gain (m -1 )
200
150
100
50
Net gain peaks before v g=0
0
20 40 60 80 100 120 140 160
Electron energy (keV)
Net gain = gain - attenuation

Refresher on Smith-Purcell parameters
λ = l
n
⎛ 1 ⎞
− cosθ
⎝
⎜
β ⎠
⎟ θ
• n = order number, θ = angle from electron beam,
φ = azimuthal angle
φ

Angular power (nW/steradian)
160
140
120
100
80
60
40
20
Spontaneous azimuthal power
-1 order
-3 order
-2 order
0
0 20 40 60 80 100
Azimuthal angle (degrees)
θ = 90°

Spontaneous power peaks near 90 degrees
Angular power (nW/steradian)
200
150
100
50
φ = 0°
-2 order
-1 order
-3 order
0 50 100 150 200
Angle from beam (degrees)

We will conduct the following experiments
• Verify that we observe the laser emission
– Hard to see because it will scatter
– Should be strongest radiation in the chamber
• Observe harmonics of laser radiation
– Do angular intensity scan - verify increase in
intensity at predicted harmonic angles
– Verify wavelengths of emission at different angles
• Observe intensity as a function of beam voltage
– Look for peak intensity at voltage for peak gain
– Look for intensity drop near voltage for v g=0

Summary
• Vanderbilt table top THz source will produce:
– wavelengths of 300-1000 microns (0.3-1 THz)
– peak power ~1 W
– pulse length ~5 ns
• Will use the Smith-Purcell effect in conjunction with a
high brightness tungsten needle cathode
• Expect to be able to confirm recent theoretical
developments experimentally
• Eventually will use device in conjunction with other
sources available at Vanderbilt FEL Center