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slides - CASA
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Progress on the Vanderbilt<br />
Table Top THz FEL<br />
Heather Andrews<br />
Department of Physics<br />
Vanderbilt University<br />
28 April 2005
• Group at Vanderbilt<br />
– Charles Brau<br />
– Chase Boulware<br />
– Jonathan Jarvis<br />
– Carlos Hernandez<br />
• Useful discussions<br />
– Hayden Brownell<br />
Acknowledgements<br />
– Jack Donohue and Jacques Gardelle
WANTED: a narrowband THz source for<br />
spectroscopy<br />
Problem Solution Requirements<br />
• Want to do frequency<br />
domain spectroscopy at<br />
THz frequencies<br />
• No existing narrowband<br />
source provides good<br />
power in THz range<br />
• Short pulse sources -<br />
good for time-domain<br />
spectroscopy<br />
* CW - good for imaging if<br />
you have enough power<br />
• Want a source which<br />
will produce<br />
– 300-1000 micron<br />
radiation (0.3-1 THz)<br />
– ~1 Watt peak power<br />
– ~5 nanosecond<br />
pulses<br />
– Narrowband
Source requires needle cathode and<br />
Smith-Purcell effect<br />
• Needle photo-cathodes<br />
provide high brightness<br />
electron beam<br />
• Smith-Purcell (SP)<br />
radiation provides a<br />
compact, tunable<br />
radiation source<br />
• Requires a high<br />
brightness beam for<br />
high power output<br />
e-beam<br />
• Voltage = 30 - 80 kV<br />
• Current = 1-10 mA<br />
THz<br />
• Brightness ~ 10 11 A/m 2 -steradian<br />
• Wavelength 300 - 1000 µm
Examine protein secondary structure<br />
• Large molecules and<br />
structures have<br />
characteristic vibrations<br />
in THz region<br />
• Reconformation could<br />
be examined using<br />
nonlinear spectroscopy<br />
• Pump-probe nonlinear<br />
spectroscopy possible<br />
using THz/THz or<br />
THz/mid-IR radiation
• Examine protein<br />
folding<br />
• High field EPR<br />
spectroscopy<br />
Additional applications
How this compares to other THz sources<br />
THz Source<br />
UCSB FEL<br />
Synchrotron<br />
sources<br />
Optically pumped<br />
FIR lasers<br />
Optical rectification<br />
techniques<br />
Backward Wave<br />
Oscillators (BWO)<br />
Comparison to SP-FEL<br />
Longer pulses (microsecond as opposed<br />
to nanosecond), higher power<br />
Lower spectral brightness, much shorter<br />
pulses, broadband<br />
Not tunable<br />
Very short pulses, low power, broadband<br />
Low power, longer wavelengths, very<br />
similar operating mechanism
TUNGSTEN<br />
NEEDLE<br />
HOLDER<br />
Experimental Set Up<br />
Nd:YAG<br />
PUMP<br />
LASER<br />
FOCUSING<br />
MAGNET<br />
R tip ~ 45 µm<br />
STEERING<br />
MAGNETS<br />
DETECTOR<br />
GRATING<br />
FARADAY<br />
CUP<br />
POWER METER
Experimental apparatus
We have achieved current densities high<br />
enough to build tabletop FELs<br />
• Photocurrent was<br />
– Up to 100 mA<br />
– Shorter pulse than<br />
laser<br />
– Limited by damage<br />
to needle<br />
• 4th harmonic Nd:YAG<br />
at 266 µm with 7 ns,<br />
200 mJ pulse<br />
• Quantum efficiency is<br />
increased by 10 2 by<br />
laser
Needle cathode e-beam has high<br />
brightness<br />
normalized brightness (A/m 2 -steradian)<br />
10 16<br />
10 14<br />
10 12<br />
10 10<br />
10 8<br />
0.0001 0.001 0.01 0.1 1 10 100 1000<br />
rf photoinjectors current (A)<br />
storage rings<br />
field emission<br />
thermionic emission<br />
photoelectric field emission
Recent needle cathode developments<br />
• Bigger needles = more current<br />
• Use 5th harmonic Nd:YAG, 5 ns pulse, maximum 50<br />
µJ/pulse
Dimensions of the aluminum grating<br />
• Gratings are fabricated out of aluminum using very<br />
thin saw blades<br />
• Dimensions will be 12.5 mm long (1/2 inch), with 250<br />
µm period, 150 µm grove width and 200 µm depth<br />
• 173 µm period, 62 µm width and 100 µm depth used<br />
for calculations*<br />
200 µm<br />
250 µm 150 µm<br />
12.5 mm<br />
*Parameters used at Dartmouth, Urata et. al., PRL, 80, 516, (1998)
Smith-Purcell laser produces two types of<br />
radiation<br />
Below threshold current: Above threshold current:<br />
λ = l<br />
n<br />
⎛ 1 ⎞<br />
− cosθ<br />
⎝<br />
⎜<br />
β ⎠<br />
⎟<br />
θ<br />
Electron<br />
beam<br />
Radiating<br />
wave<br />
Evanescent<br />
wave
• Evanescent wave:<br />
Details of laser radiation<br />
– Phase velocity<br />
parallel to electon<br />
beam, group velocity<br />
opposite<br />
– Bunches electrons<br />
– Produces harmonics<br />
within the SP<br />
spectrum which<br />
radiate<br />
Bound wave<br />
Radiating harmonic waves<br />
Electrons bunching
Wavelength (microns)<br />
1 10 3<br />
800<br />
600<br />
400<br />
200<br />
Expected wavelengths<br />
1st Smith-Purcell range<br />
Second SP range<br />
Laser fundamental wavelength<br />
Laser 2nd harmonic<br />
Laser 4th harmonic<br />
Laser 3rd harmonic<br />
0<br />
20 25 30<br />
Beam Voltage (kV)<br />
35 40
Amplitude growth rate, µ (m -1 )<br />
600<br />
500<br />
400<br />
300<br />
200<br />
100<br />
Comparison of gain theories<br />
Schaechter and Ron<br />
Present theory<br />
Kim and Song<br />
0<br />
28 30 32 34 36 38 40 42<br />
Electron energy, V (keV)<br />
Schaechter and Ron, Phys. Rev., A40, 876 (1989)<br />
Kim and Song, Nucl. Inst. Meth., A475, 159 (2001)
Gain/attenuation (m -1 )<br />
10 5<br />
10 4<br />
10 3<br />
100<br />
Gain and attenuation peak at v g=0<br />
Gain<br />
Gain ∝ ( 1 v ) g<br />
1<br />
3 Attenuation ∝1 vg Attenuation<br />
10<br />
20 40 60 80 100 120 140 160<br />
Electron energy (keV)
Net gain (m -1 )<br />
200<br />
150<br />
100<br />
50<br />
Net gain peaks before v g=0<br />
0<br />
20 40 60 80 100 120 140 160<br />
Electron energy (keV)<br />
Net gain = gain - attenuation
Refresher on Smith-Purcell parameters<br />
λ = l<br />
n<br />
⎛ 1 ⎞<br />
− cosθ<br />
⎝<br />
⎜<br />
β ⎠<br />
⎟ θ<br />
• n = order number, θ = angle from electron beam,<br />
φ = azimuthal angle<br />
φ
Angular power (nW/steradian)<br />
160<br />
140<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
Spontaneous azimuthal power<br />
-1 order<br />
-3 order<br />
-2 order<br />
0<br />
0 20 40 60 80 100<br />
Azimuthal angle (degrees)<br />
θ = 90°
Spontaneous power peaks near 90 degrees<br />
Angular power (nW/steradian)<br />
200<br />
150<br />
100<br />
50<br />
φ = 0°<br />
-2 order<br />
-1 order<br />
-3 order<br />
0 50 100 150 200<br />
Angle from beam (degrees)
We will conduct the following experiments<br />
• Verify that we observe the laser emission<br />
– Hard to see because it will scatter<br />
– Should be strongest radiation in the chamber<br />
• Observe harmonics of laser radiation<br />
– Do angular intensity scan - verify increase in<br />
intensity at predicted harmonic angles<br />
– Verify wavelengths of emission at different angles<br />
• Observe intensity as a function of beam voltage<br />
– Look for peak intensity at voltage for peak gain<br />
– Look for intensity drop near voltage for v g=0
Summary<br />
• Vanderbilt table top THz source will produce:<br />
– wavelengths of 300-1000 microns (0.3-1 THz)<br />
– peak power ~1 W<br />
– pulse length ~5 ns<br />
• Will use the Smith-Purcell effect in conjunction with a<br />
high brightness tungsten needle cathode<br />
• Expect to be able to confirm recent theoretical<br />
developments experimentally<br />
• Eventually will use device in conjunction with other<br />
sources available at Vanderbilt FEL Center