HSX Thomson Scattering Experiment
K. Zhai, F.S.B. Anderson, and D.T. AndersonHSX Plasma Laboratory
University of Wisconsin-Madison,
The HSX Thomson Scattering system is based upon the design of GA divertor TS system and is constructed in collaboration with the MST group. The system is capable of 10-point profile measurement and double pulse operation that can provide measurement of rapid change of plasma parameters. Large collection optics are used in the system to get enough scattered photons from the HSX plasma with a typical density of 11012/cm3. Ten identical fiber bundles with a transmission rate of 0.6 couple the collected photons to ten polychromators that disperse the collected light. Four wavelength channels in each of the ten polychromators are optimized for temperature measurement range from 10eV-2keV. A dedicated CAMAC system is used to record the data. The initial test results of the system will be presented.
*Work supported by US DoE under grant DE-FG02-93ER54222
Incoherent Thomson Scattering
• Plasma electrons get accelerated in laser field and then emit electromagnetic radiations.
-Shape of the scattered laser light spectrum: electron temperature
-Amplitude of the scattered light: density
• Small cross section, requiring a powerful monochromatic light source.
TS spectrum for an isotropic relativistic Maxwellian electron velocity distribution(Plasma Physics, Vol. 14, pp. 783 to 791 )
700 800 900 1000 1100 1200 1300
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
am
plitu
dewavelength(nm)
Te100eV Te300eV Te500eV Te700eV Te1keV Te2keV
Theoretical relativistic scattering spectrum for a plasma diagnosed with YAG (1060nm) with a scattering angle of 90 degree
)2(sin4
)2(sin441
/2
220
2
22
2
230
2
32
01
2
a
cY
a
cY
mKTa ee
in which, A is a constant and
Theoretical TS Spectrum
)),,,(exp(),,()2/sin(
A)( 0201
aYY
aF
Schematic Diagram of the HSX Thomson Scattering System
Nd:YAG laser Beam transportation HSX vessel Beam Dump
Fiber Bundles
Collection Optics
PolychromatorsAvalanche Photodiodes
Amplifier
Data AcquisitionControl System
Basic features: 10 points (20 cm radial range), double pulse (40-100us)
Interdependent Subsystems
• Laser system• Beam transportation and stray light control• Collection optics of the scattered light• Spectrum dispersion and detection system• Signal handling and data acquisition• Control system
A commercial YAG laser is used as the scattering source.
• 10ns and 1J output pulse at the fundamental wavelength of 1.06m
• Located on optical table in clean room
• Double pulse operation
Laser System
YAG Laser
trigger
F=3.05m focus lens
CCD frame grab camera
6.2m 3.05mCeramic disc
attenuator
Laser focus spot viewed on a ceramic disc with a CCD camera and video capture card.
Laser spot size relative to the distance with the focus (cm)
Laser Focus Spot Using a 3m Focus Lens
100 200 300 400 500 600
50
100
150
200
250
300
350
400
450
-10 0 10 20 30 400.8
0.9
1.0
1.1
1.2
1.3
x-width y-width Gaussian, 2=1mm Gaussian, 2=0.95mm
Position relative to focus (cm)S
pot w
aist
(m
m)
Beam Transportation and
Stray Light Control• Beam is guided by three laser mirrors and is focused to the HSX vessel
with an f=3.05m focus lens.
• A 1/2 waveplate is used to adjust the beam polarization.
• Entrance and exit tubes are specially designed with baffles to control the stray light.
• Entrance and exit windows are Brewster angle orientated fused silica windows.
HSX TS Beam Transportation
YAG Laser
HW plate
mirror
focusing lens
Brewster window
Collection optics
Plasma region
dump
120cm
120cm
388cm 305cm
297cm
Total length from laser exit to focal point: 925cm
Lens focus length: 305cm
Entrance Tube and Exit Tube• Specially designed baffles prevent the stray light reflected from the
entrance tube wall from passing into the vessel directly. And the critical aperture will prevent the stray light originating from the entrance window from getting into the vessel.
• Fused silica windows are oriented at Brewster angle to the incident laser.
baffle
critical aperture
Layout of the collection optics with respect to plasma region
Observation vacuum window
Collection lens
Laser beam
Image planeof fiber bundlesurface
Gate valve
cmLcmn
Lnd
d
hf
EN
e
es
2,/101 312
0
10476,210
2ln2
2sin4
20
keVeVT
cm
kT
e
e
e
Collection Optics
• Collection solid angle: (2.9-3.1) 10-2
• Scattered photons:
Ns=(2.4-2.6)×104
• Spectrum width:
=17-246nm
•System Aperture: Entrance Pupil Diameter•Effective Focal Length: 16.70 cm (in image space)•Back Focal Length: 4.11 cm•Working F/#: 2.05•Image Space NA: 0.237•Object Space NA: 0.11•Paraxial Magnification: -0.459•Entrance Pupil Diameter: 10 cm•Entrance Pupil Position: 4.66 cm•Exit Pupil Diameter: 20.11cm•Exit Pupil Position: -40.03cm•Primary Wave: 1064 nm•Angular Magnification: 0.49
Optical Properties of the Collection Lens
Layout of the Collection Lens and Its Coupling to Fiber Bundles
40cm
45cm 11.56cm
19.01cm
Two-layer doublet: BK7 and SF1
Diameter:10cm
Fiber Optics
fiber length:7m
single fiber NA:0.24-0.25
Fiber Transmission Test
Expanded laser beam
Cylindrical lens
rectangularfiber end
circular fiber end
Viewing lens
detector
• Cylindrical lens linear focus the beam into fiber bundle at a given NA=0.24
• Viewing lens collect the transmitted lights at a given NA=0.25
Fiber Bundle Transmission Test Result
0 1 2 3 4 5 6 7 8 9 10 110.58
0.59
0.60
0.61
0.62
0.63
Ten fiber bundles corresponding to ten radial channels.
Tot
al tr
ansm
issi
on
• The transmission of ten fiber bundles is within the range from 0.625-0.585, comparing the ideal transmission of 0.63.
Coupling to Fiber Bundles
0 2 4 6 8 10-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Length unit: cm
Each square corresponds to an individual fiber bundle’s rectangular surface of 0.8mm*7mm
Las
er b
eam
imag
e on
the
fibe
r su
rfac
e
leng
th u
nit:m
m
• Ten identical polychromators designed and manufactured by GA.
• Four wavelength channels in each polychromator optimized for the measurement of the electron temperature range from 10eV to 2keV.
• Silicon avalanche photodiode detector ( EG&G C30956E ) and amplifier provided by GA are attached to the polychromators.
• Output from the amplifier range from 0.0 to –1.0 volt.
Spectrum Dispersion and Detection System
Polychromator Calibration
• The spectral calibration determines the response of the detection system to a radiation source of constant spectral emissivity.The result of spectral calibration will be used to build a look-up-table for electron temperature measurement.
Spectral calibration of each channels in a polychromator can be measured separately in the lab.
• Absolute calibration of the absolute sensitivity of the complete detection system can only be performed in site on HSX machine to get electron density measurement.
Experimental Setup for Spectral Calibration
Tungsten lamp
monochromator
Reference detector
Fiber opticspolychromator
Absolutely calibrated detector
DATA acquisitionComputer
control
850 900 950 1000 1050 1100-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
ch1 ch2 ch3 ch4 Te=50eV Te=100eV Te=200eV Te=500eV Te=1000eV te=1500eV te=2000eV
Wavelength (nm)
Spe
ctra
l res
pons
e fu
ncti
on a
ndT
S s
catt
erin
g sp
ectr
um f
or d
iffe
rent
ele
ctro
n te
mpe
ratu
re
Measured Polychromator (SN:39024-124) Spectral Response Function Together with the Scattering Form Factor S (Te, =90°)
0 500 1000 1500 20000
5
10
15
20
ratio ch2/ch1 ratio ch3/ch1 ratio ch4/ch1 ratio ch3/ch2 ratio ch4/ch2 ratio ch4/ch3
0 500 1000 1500 20000
20
40
60
80
100
ratio ch2/ch1 ratio ch3/ch1 ratio ch4/ch1 ratio ch3/ch2 ratio ch4/ch2 ratio ch4/ch3
Electron Temperature (eV)Rat
io o
f si
gnal
s in
dif
fere
nt s
pect
ral c
hann
els
Conversion Function Based on the Ratios of Signals in Different Spectral Channels
• A computer controlled CAMAC system dedicated for HSX Thomson scattering experiment.
– A GPIB crate controller from KINETICS SYSTEM is used to communicate between the CAMAC crate and the computer.
– The signal is recorded by gating Leroy Model 2250 charge integrating digitizer. These digitizers have a sensitivity of 0.5pC/count, with a range of 512 counts.
– LabView program ready.
• System synchronized with HSX timing with a NI6602 timing card and a DG535 digital pulse generator from Stanford Research Systems.
– LabView program ready.
Signal Handling DATA System and Control system
Overview of the Thomson Scattering Timing Sequence During HSX Discharge
Plasma time
Laser emission
Scattering light
Digitizer gating
0 500 1000~800-850 Time:s
Stray light background
HSX master trigger TS timing Laser trigger
Digital delay
Gating digitizerPersonal computer LabVIEW control
Acquired Signal During a Thomson Scattering Experiment
Type of signalSample
realizationSampling time
Stray light 1~300-400ms, before plasma breakdown
Scattering light 1~800-850ms, during
plasma discharge
Pedestal subtraction
Fluctuation(noise)5 After plasma
• Weighted average of electron Temperature and its error:
Computation of the Weighted Average Electron Temperature
2
1
2,,,
2
12
,,
1
1
qfinaleqesyste
q qestate
TTN
T
TT
,where
• Error analysis
212
,2
,2
, )( jscjstrayjbgjC
i i
j jq
C C
C CR
min max,
)()(5.0 minmax, qeqeqe RTRTT
1. Uncertainty of the scattered signal in spectral channel j,
2. Upper and lower limits of spectral channel signal ratio,
3. Electron temperature range associated with the ratio limits
systestatefinale
q qe
qe
qqe
finale
TTT
T
T
TT
,,,
2,
,
2,
,
5.05.0
)()/1(
1
Flow Chart of Thomson Scattering Data Processing Program
Get raw data
Separate raw data:•Get stray light•Get scattering light•Get pedestal subtraction
• Subtract pedestal• Subtract stray light• Background pedestal analysis
compute standard error of pedestal, stray light, and scattering
• Build ratios• Compute error in channel ratio
based on deviation of stray light, scattering light, and background noise
• Get Te for different channel ratios from look-up table
• Compute final Te from weighted average
• Computer error from weighted average
END
• 10-point, double-pulse Thomson scattering system optimized for the measurement of HSX plasma parameters of electron temperature of 10eV-2keV and electron density of 1012/cm3 or higher.
• While all the subsystem function properly, successful operation also require precise alignment of the whole system, the input optics and collection optics. One point measurement of system is expected to operate in this year.
Summary of the System and its Operation Schedule
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