Progress on the Vanderbilt Table Top THz FEL Heather Andrews - - PowerPoint PPT Presentation

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

• 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 Problem Solution Requirements

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

• Voltage = 30 - 80 kV
• Current = 1-10 mA
• Brightness ~ 1011 A/m2-steradian
• Wavelength 300 - 1000 µm

e-beam e-beam THz THz

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

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

Experimental Set Up

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

FARADAY CUP

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 102 by laser

Needle cathode e-beam has high brightness

108 1010 1012 1014 1016 0.0001 0.001 0.01 0.1 1 10 100 1000 rf photoinjectors storage rings field emission thermionic emission photoelectric field emission normalized brightness (A/m

2-steradian)

current (A)

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*

12.5 mm 200 µm 250 µm 150 µm

*Parameters used at Dartmouth, Urata et. al., PRL, 80, 516, (1998)

Evanescent wave Electron beam Radiating wave

Smith-Purcell laser produces two types of radiation

λ = l n 1 β − cosθ      

Below threshold current: Above threshold current:

θ

Details of laser radiation

• Evanescent wave:

– Phase velocity parallel to electon beam, group velocity

• pposite

– Bunches electrons – Produces harmonics within the SP spectrum which radiate

Electrons bunching Bound wave Radiating harmonic waves

Expected wavelengths

200 400 600 800 1 103 20 25 30 35 40 Wavelength (microns) Beam Voltage (kV) Laser fundamental wavelength 1st Smith-Purcell range Second SP range Laser 2nd harmonic Laser 3rd harmonic Laser 4th harmonic

Comparison of gain theories

100 200 300 400 500 600 28 30 32 34 36 38 40 42

Amplitude growth rate, µ (m

• 1)

Electron energy, V (keV)

Schaechter and Ron Present theory Kim and Song

Schaechter and Ron, Phys. Rev., A40, 876 (1989) Kim and Song, Nucl. Inst. Meth., A475, 159 (2001)

Gain and attenuation peak at vg=0

10 100 103 104 105 20 40 60 80 100 120 140 160 Gain/attenuation (m-1) Electron energy (keV)

Gain

Attenuation

Gain ∝ 1 vg

( )

1 3 Attenuation ∝1 vg

Net gain peaks before vg=0

Net gain = gain - attenuation

50 100 150 200 20 40 60 80 100 120 140 160 Net gain (m-1) Electron energy (keV)

Refresher on Smith-Purcell parameters

• n = order number, θ = angle from electron beam,

φ = azimuthal angle

λ = l n 1 β − cosθ      

θ φ

Spontaneous azimuthal power

20 40 60 80 100 120 140 160 20 40 60 80 100 Angular power (nW/steradian) Azimuthal angle (degrees)

• 1 order
• 2 order
• 3 order

θ = 90°

Spontaneous power peaks near 90 degrees

50 100 150 200 50 100 150 200 Angular power (nW/steradian) Angle from beam (degrees)

• 1 order
• 2 order
• 3 order

φ = 0°

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 vg=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