UCLA

LIGO-G030514-00-Z

Frequency-domain interferometry diagnostic systemfor the detection of relativistic plasma waves

Catalin V. Filip, Electrical Engineering Department, UCLA

UCLA

LIGO-G030514-00-Z

IntroductionRelativistic Plasma Waves Can Accelerate Electrons

• Relativistic Plasma Waves have phase velocities, vphase c

• Externally injected electrons can “surf” these waves and get accelerated with gradients of few GeV/m

(100 – 1000 times more than conventional accelerators)

• Plasma accelerators could be << in size than conventional accelerators (and therefore much cheaper)

• Relativistic Plasma Waves can be excited by the beat pattern of an intense, two-wavelength laser pulse

UCLA

LIGO-G030514-00-Z

IntroductionRelativistic Plasma Waves Can Accelerate Electrons

Laser

Longitudinal

Electric FieldOf the wave QuickTime™ and a

Animation decompressorare needed to see this picture.

Horizontal (mm)

Ver

tica

l (m

m)

Longit

udin

al e

lect

ric

fiel

d (

MV

/cm

)

0

-20

20

1 2 30

100µm

focus

(a)

(b)

Laser Pulse

UCLA

LIGO-G030514-00-Z

Relativistic Plasma Waves (RPWs)Detection Techniques

There are TWO major ways to detect a RPW

1 electrons accelerated electrons

PLASMAWAVE

1. by probing the RPW with electrons(the electric field of the wave is inferred)PLASMA

PLASMAWAVE

2

kprobe

kprobe

kscatter

scatter= probe epw

kscatter= kprobe kepw

2. by probing the RPW with light (photons)(the absolute density perturbation n is inferred)

UCLA

LIGO-G030514-00-Z

Detection of Relativistic Plasma Waves-Collinear Thomson Scattering

background plasma

532-nm probe beam duringand after interaction with the plasma wave

probe beam gets modulated in phase (and amplitude)

= index of refractionin a plasma:

= 1- nncrit

< 1

For a 532-nm light wave going

through a plasma a change inplasma density of

n=1014 cm-3

(1% wave at 1016 cm-3)is equivalent to a change inthe index of refraction of

10-8

UCLA

LIGO-G030514-00-Z

Detection of Relativistic Plasma Waves-Collinear Thomson Scattering

Amplitude modulation is

10-8 - 10-10

period = 1 ps

= 1 THz

~16 Å

for a plasma density of 1016 cm-3

Scattered light

P0

Ps ~ [ n (cm-3) x probe x Linter ]2

UCLA

LIGO-G030514-00-Z

Relativistic Plasma Waves (RPW) Detectionat UCLA - Goals

1. Detection of the longitudinal component of RPW2. Detection of RPW at low densities (1015 cm-3 to 1017 cm-3)3. Good signal-to-noise ratio (1000 to 1)4. Frequency and time-resolved (both red and blue shift)5. Use of independent probe to avoid stray scattering6. Easy to implement, reliable, on-line with electrons

UCLA

LIGO-G030514-00-Z

Experimental challenges

1. Scattering lasts for ~100 ps => MUST use streak camera => MUST

use visible pulses for probing => Very small scattering efficiency

Pscattered/Pprobe 10-8 - 10-10 => MUST use MW-power probe laser pulses

with duration ≥ 2ns for easy synchronization => Pulse energy is high =>

MUST use high-damage threshold optics

2. The VERY WEAK scattered light is only ~8 Å away from the

VERY INTENSE probe light

UCLA

LIGO-G030514-00-Z

Experimental considerations

frequencyor wavelength

probe lightamplitude =109

scattered lightamplitude =1

8 Å 8 Å

Etalontransmission

1. First use a “power” filter to decrease the power of the probe pulse (~1000 times) in order to use a grating afterwards

2. Use an etalon with high-damage threshold coatings as a “power” filter.

532nm = 3.51015 Hz

~1012 Hz3. Hope that the bandwidth of the probe laser pulse is narrow enough.

UCLA

LIGO-G030514-00-Z

Experimental Setup For Frequency And TimeResolved Collinear Thomson Scattering

ImagingSpectrometer

Input

Output

StreakCamera

27 reflecting surfaces,9 lenses, 5 focii

Beam Dump

Etalon (high-power filter)

100 ps CO2 pulse drivesplasma wave

F# = 2, Imax=1015 W/cm2

DCR-11Nd:YAG

Q-Switched532.1 nm

100 mJ, 5 ns

Interaction Chamber

OAP

20 MW

Beam dumpfor 532.1 nm

10 kW

< 2 mW

To positionmonitor

Grating (low-power)filter

Beam rotator

Teflondump Mirror

Mirror

Spatial filter

IP

f=1 m

UCLA

LIGO-G030514-00-Z

+ - Spectrometer

16-bit CCDWhite-light source

Spatial filterand collimator

Etalon

Spectrometerslit

a

b c

0

1000

2000

3000

4000

528 529 530 531 532 533 534 535 536

Wavelength (nm)

Inte

nsit

y (c

ount

s)

Etalon preliminary test and measurements

UCLA

LIGO-G030514-00-Z

QuickTime™ and aTIFF decompressor

are needed to see this picture.

QuickTime™ and aTIFF decompressor

are needed to see this picture.

Gree-Ne (543nm) beam

Etalon preliminary test and measurements

532nm, 100 mJ

Energy meter

Energy meter

Profile of the reflection of the543 nm Gaussian He-Ne gas laser beam

Camera

Camera

Attenuation of the probe pulse at 532.1 nm is only ~10 times!

UCLA

LIGO-G030514-00-Z

Multi-passing of the Etalon

50 µm

1

2 BeamDump

3

10%

1%0.1%

To grating

Multi-passing of the etalon is achieved by sending theprobe beam on consequent transmission maxima.

Top view

UCLA

LIGO-G030514-00-Z

Grating preliminary test and measurements

0th order

Ist orderMonitor

Signal

Razor blade on atranslation stage

The grating can reduce theprobe light ~107 times withoutattenuating the scattered light

532nm~5 mJ

Beam splitter

UCLA

LIGO-G030514-00-Z

0

10

20

30

40

531 532 533 534

Ne lines

Wavelength (nm)

Rel

ativ

e T

rans

mis

sion

531 533 535

0.0

2.5

5.0

7.5

50 150 250

Dis

tanc

e (m

m)

Ne lines

Wavelength (nm)

Intensity (counts)

(a)(b)

White light measurements

Etalonattenuation

at probe

Signal at probe+8Å

Signal at probe– 8Å

UCLA

LIGO-G030514-00-Z

1. Plasma is produced by tunneling ionization of backfill gas- H2, He, Ar from 1015 cm-3 to 1017 cm-3

2. CO2 laser parameters:- ≤100 ps FWHM, 2-, 10.3µm and 10.6 µm- energy, 10.6/10.360J/25J, maximum 1 TW- beam size (dia.) from 1.5” to 3.4”,- OAP focussing down to 2 w0 120 µm- max. 6 1015 W/cm2, linearly polarized

3. Probe pulse parameters:- visible, Q-switched, Nd:YAG, 532.1 nm- ~100 mJ in 5 ns, 1011 W/cm2 in plasma

4. Thomson scattering parameters- collinear (0 degree) geometry, and k-matched for relativistic plasma waves- interaction length ~2 zR 2 mm

- frequency resolution 0.4 Å, time resolution ~100 ps (grating limited)

Experimental Parameters

CO2

probe

Plasma

Probebeam

CO2 laserbeam

Plasma

3mm

35mm

UCLA

LIGO-G030514-00-Z

250 750 1250

531.0

531.5

532.0

532.5

533.0

125 250 375 500 625 750

200 900 1600 2300

530.00

531.25

532.50

533.75

535.00

20 90 160 230 300

11341,322 mTorr He 11358,1.73 Torr He

Blue shifted sidebands (in He)

Red shifted sidebands

10 x attn.

Time-Resolved Spectra FromCollinear Thomson Scattering

n = 1.07 nres n = 5.76 nres

Wav

elen

gth

Wav

elen

gth

Time (ps)

Line of Simultaneous

events

C. V. Filip et al., Rev. Sci. Instr. 74, 3576 (2003)also in July 2003 Virtual Journal of Ultrafast Science, http://www.vjultrafast.org.

UCLA

LIGO-G030514-00-Z

160 ps

1500 2000 2500

532.0

532.5

533.0

533.5

534.0

534.5

535.0

40 50 60 70 80 900

0

100

200

300

400

500

400 600 800 1000 1200 1400

Time (ps)

Inte

nsi

ty (

cou

nts

)

Lineout of the red shifted2nd harmonic beatwave component

10x attn.

Beatwave

Time (ps)

Intensity (arb. units)

Wav

elen

gth

(n

m)

2nd harmonic

3rd harmonic

Probe pulseat 532.1 nm

2Torr H2; n=13.3 nres

Ratio of intensityof 1st/2nd harmonic

R 20

Relativistic Plasma Wave Detection in H2

Off Resonance

CO2 pulse ~ 100 ps

UCLA

LIGO-G030514-00-Z

c Æ

d

~ 3 cm

~ 3

cm

Incoming pulse

Outgoing pulse in I-st order of grating

Temporal resolution of thediagnostic system 100 ps

∆d=

i

DNprobe

cCos(i)

D

100ps

UCLA

LIGO-G030514-00-Z

Time Resolved Spectra FromF/18 Collinear Thomson Scattering, n=nres

0.0 3.0 6.0 9.0 12.0

532.0

533.0

534.0

Time (ns)

Wav

elen

gth

(nm

)

133 227 320 413 507 600Intensity (counts)

0.00 3.00 6.00 9.00 12.000

175

350

525

700

0

175

350

525

700

Time (ns)

Int.

(cou

nts)

PROBE

-

100x

UCLA

LIGO-G030514-00-Z

Acceleration of injected electrons in H2

at n=3.3nres

10

100

1000

104

105

106

107

108

109

15 20 25

Ele

ctro

ns/M

eV

Energy (MeV)

Noise level

12 13 14 15Energy (MeV)

12 MeV,injectionenergy

3

Single frequency

Shot #11999

UCLA

LIGO-G030514-00-Z

Conclusions

A Thomson scattering diagnostic system has been used to detectrelativistic plasma waves excited at low plasma densities (0.1% critical).

The probing was done collinearly with an independent probe laser pulse.

A novel spatial-spectral notch filter based on a triple-pass Fabry-Perot etalonwas utilized to simultaneously attenuate the probe light ~1012 times

and collect the weak, 100-ps scattered light which is shifted by only ~8 Å.

Both the red and blue-shifted scattered light up to the third harmonicwas recorded in time and frequency. Using this system, non-resonantly

excited plasma waves were characterized.