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Improving the Thomson scattering Diagnostic Improving the Thomson scattering Diagnostic installed on the Large Helical Deviceinstalled on the Large Helical Device
(changed)(changed)
Validity Check of Thomson Scattering Validity Check of Thomson Scattering Data of Very High Density Plasmas in Data of Very High Density Plasmas in
LHDLHD
K.K. NariharaNarihara, I. Yamada, H. Hayashi and H. , I. Yamada, H. Hayashi and H. FunabaFunaba
National Institute for Fusion ScienceNational Institute for Fusion Science509509--5292 Toki, Japan5292 Toki, Japan
P8-24
MotivationsMotivationsHow confident is the Thomson scattering data for a How confident is the Thomson scattering data for a very high density plasma, which accompanies very very high density plasma, which accompanies very intense back/foreintense back/fore--ground plasma light?ground plasma light?We examined the effect of back/foreWe examined the effect of back/fore--ground plasma ground plasma light on the performance of an avalanche photodiode light on the performance of an avalanche photodiode (APD) used for the detecting the Thomson(APD) used for the detecting the Thomson--scattered scattered light, and established a criterion for the validity.light, and established a criterion for the validity.We newly installed thirteen 80We newly installed thirteen 80--channel scanning ADCs channel scanning ADCs to monitor the DC level of to monitor the DC level of APDsAPDs, which is a measure , which is a measure of the intensity of the plasma light.of the intensity of the plasma light.
Scattering Configuration of LHD TSScattering Configuration of LHD TS
Highly oblique back scattering
Whole plasma region along a major radius on the mid plane
200-spatial channels (the plasma is covered by 130 channels)
Segmented mirror
Tips of fiber array
Scattering Parameters as a Function of the Scattering Position
PolychromatorPolychromator
LHD_polychromator#36
400 600 800 1000 1200WAVELENGTH(nm)
0.0
0.2
0.4
0.6
0.8
1.0 YAGH_aH_b HeI
1keV
2keV
4keV
8keV
16keV
Expected Plasma light
Thomson scattered light
ch5
ch4ch3
ch2ch1
_+
R2
R1100kΩ
1nF
APD
Vb
47Ω
10kΩ
1881M FASTBUS ADC(1024ch)
NI6225 scanning ADC(1040ch)
-6C1=10nF
Circuit Diagram of APD and Data Acquisition System
1 of 1000
EG& C30950 CD1161
Newly installed
Setup for Test MeasurementSetup for Test Measurement
Pulse generator
DC-powerYHP-LED5082-4850 APD
HighVoltage Vb
50ns pulse light
APD: EG&G C30950-CD1161 S/NC2087DOperating Voltage Vr=307V @ R=675kV/WDark current Id= 140 nABand Width BW=25 MHz
Output Voltage as a Function of Bias VoltageOutput Voltage as a Function of Bias Voltage
APD S/N=C2087-D
100 150 200 250 300 350Bias Voltage (V)
0
50
100
150
200
250
Out
put(m
V)
Vr
Vb=0.9Vr
0 20 40 60LED current (mA)
024
DC
Out
(V)
Vb=0.5Vr
0 20 40 60LED current (mA)
024
DC
Out
(V)
LED-CURRENT vs DC-OUTPUT for two different high voltage
Pulse Output as a Function of DCPulse Output as a Function of DC--level for a Fixed level for a Fixed Pulse Light Intensity:Pulse Light Intensity:
10%
Guaranteed range
APD S/N=C2087-D
0 1 2 3DC Voltage (V)
0
150
300
Out
put(m
V)
Vb=0.9Vr
Vb=0.5Vr
10%
Lower Vb is favorable for measuring denser plasma:
But too low Vb (eg. 0.4 Vr) causes the frequency response of the APD slow (the depression layer width becomes narrower and the APD has a higher capacity.)
FASTBUS1881M
pulseAmp
NIM modules
pulseAmp
pulseAmp
Pxi-6225
pulseAmp pulseAmp
Example #69360:Example #69360:
#69360time=1.03355s
3.20 3.38 3.56 3.74 3.92 4.10 4.28 4.46 4.64 4.82 5.00 MAJOR RADIUS (m)
0.0
0.2
0.4
0.6
0.8
1.0
Te(
keV
), ne
(10^
21 m
^-3)
Te
ne
0.0 0.5 1.0 1.5 2.0 2.5TIME(s)
-0.05
0.00
0.05
0.100.15
volt
0 200 400 600 800channel#
-0.10-0.050.000.050.100.150.20
volt
DC
OU
T(V
)D
C O
UT
(V)
ch404
Time evolution of DC-level
1.03s
We assumed that all APD’s gain depend on Vb/Vr similarly, which is not yet checked.Density calibration is not yet completed.
DC-level of all APDs are low enough toguarantee the linearity.
VbVb=0.5 =0.5 VrVr; Window fully opened.; Window fully opened.
Comment on the 9Comment on the 9thth Campaign DataCampaign Data
APD S/N=C2087-D
0 1 2 3DC Voltage (V)
0
150
300
Out
put(m
V)
Vb=0.9Vr
Vb=0.5Vr
Vb=0.9Vr; window 10% open
We have no information on interferometer and VDC
Suppositions:
1: if VDC<0.8V for all APD: error on ne & Te would be less than 30% ::OK
2. If VDC > 2.5V for most APD: Te profile would be irregular and unnaturalWe would disregard it:: OK (We would not be deceived).
3. If VDC < 2 V for all APDs: signal error <30%; ne & Te profile may be smooth in shape but have error 60~90%; highly deformed profile. We should be much careful not to be deceived.
ConclusionsConclusions1. 1. We evaluated the effect of the plasma light on the We evaluated the effect of the plasma light on the pulsepulse--response of response of APDs APDs used for detecting the used for detecting the Thomson scattered light.Thomson scattered light.2. For accuracy better than 10%, the DC2. For accuracy better than 10%, the DC--outputs outputs induced by plasma light should be less than ~0.8 V, induced by plasma light should be less than ~0.8 V, for for VbVb=0.9Vr; and ~1.6V for =0.9Vr; and ~1.6V for VbVb=0.5 =0.5 VrVr..3. The newly installed 133. The newly installed 13--80ch scanning ADCs 80ch scanning ADCs routinely monitor the DCroutinely monitor the DC--levels of all APD outputs, levels of all APD outputs, thus enabling the linearitythus enabling the linearity--check on the acquired data.check on the acquired data.4. For measurement under super bright radiation, 4. For measurement under super bright radiation, combined optimization ofcombined optimization of VbVb and the windowsand the windows’’aperture will be needed. aperture will be needed.