Validity Check of Thomson Scattering Data of Very High ... · 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

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.