Progress on the Vanderbilt Table Top THz FEL Heather Andrews Department of Physics Vanderbilt University 28 April 2005
Acknowledgements • Group at Vanderbilt – Charles Brau – Chase Boulware – Jonathan Jarvis – Carlos Hernandez • Useful discussions – Hayden Brownell – Jack Donohue and Jacques Gardelle
WANTED: a narrowband THz source for spectroscopy Problem Solution Requirements • Want to do frequency • Want a source which domain spectroscopy at will produce THz frequencies – 300-1000 micron • No existing narrowband radiation (0.3-1 THz) source provides good – ~1 Watt peak power power in THz range – ~5 nanosecond • Short pulse sources - pulses good for time-domain – Narrowband spectroscopy * CW - good for imaging if you have enough power
Source requires needle cathode and Smith-Purcell effect • Needle photo-cathodes THz THz e-beam e-beam provide high brightness electron beam • Smith-Purcell (SP) radiation provides a compact, tunable • Voltage = 30 - 80 kV radiation source • Current = 1-10 mA • Requires a high • Brightness ~ 10 11 A/m 2 -steradian brightness beam for Wavelength 300 - 1000 µ m • high power output
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
Additional applications • Examine protein folding • High field EPR spectroscopy
How this compares to other THz sources THz Source Comparison to SP-FEL UCSB FEL Longer pulses (microsecond as opposed to nanosecond), higher power Synchrotron Lower spectral brightness, much shorter sources pulses, broadband Optically pumped Not tunable FIR lasers Optical rectification Very short pulses, low power, broadband techniques Backward Wave Low power, longer wavelengths, very Oscillators (BWO) similar operating mechanism
Experimental Set Up Nd:YAG PUMP LASER DETECTOR FOCUSING TUNGSTEN MAGNET NEEDLE HOLDER GRATING STEERING FARADAY MAGNETS CUP R tip ~ 45 µ m 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 µ m with 7 ns, 200 mJ pulse • Quantum efficiency is increased by 10 2 by laser
Needle cathode e-beam has high brightness 10 16 2 -steradian) 10 14 normalized brightness (A/m 10 12 10 10 10 8 0.0001 0.001 0.01 0.1 1 10 100 1000 current (A) rf photoinjectors 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 µ J/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 µ m period, 150 µ m grove width and 200 µ m depth • 173 µ m period, 62 µ m width and 100 µ m depth used for calculations* 250 µ m 150 µ m 200 µ m 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 1 β − cos θ n Radiating wave Evanescent θ wave Electron beam
Details of laser radiation • Evanescent wave: – Phase velocity parallel to electon Radiating harmonic waves beam, group velocity opposite – Bunches electrons Bound wave – Produces harmonics within the SP spectrum which radiate Electrons bunching
Expected wavelengths 1 10 3 Laser fundamental wavelength 800 Wavelength (microns) 1st Smith-Purcell range 600 Laser 2nd harmonic 400 Second SP range 200 Laser 3rd harmonic Laser 4th harmonic 0 20 25 30 35 40 Beam Voltage (kV)
Comparison of gain theories 600 -1 ) Amplitude growth rate, µ (m 500 Schaechter and Ron 400 300 Present theory 200 100 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 and attenuation peak at v g =0 ( ) 1 3 Attenuation ∝ 1 v g Gain ∝ 1 v g 10 5 Gain/attenuation (m -1 ) 10 4 10 3 Gain 100 Attenuation 10 20 40 60 80 100 120 140 160 Electron energy (keV)
Net gain peaks before v g =0 200 150 Net gain (m -1 ) 100 50 0 20 40 60 80 100 120 140 160 Electron energy (keV) Net gain = gain - attenuation
Refresher on Smith-Purcell parameters φ λ = l 1 β − cos θ θ n • n = order number, θ = angle from electron beam, φ = azimuthal angle
Spontaneous azimuthal power 160 θ = 90 ° 140 Angular power (nW/steradian) -1 order 120 100 80 60 40 -3 order -2 order 20 0 0 20 40 60 80 100 Azimuthal angle (degrees)
Spontaneous power peaks near 90 degrees φ = 0 ° 200 Angular power (nW/steradian) -1 order 150 100 50 -3 order -2 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
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