J. Plasma Fusion Res. SERIES, Vol.3 (2000) 436439

Infrared lmaging Bolometry for Long Pulse
Discharges on LHD
ASHIKAWA Naoko*, PETERSON Byron J.1), WURDEN Glen A.2),
SUDO Shigerur) and LHD Experimental groups
Graduate University for Advanced Studies, Toki-shi 509-5292 JAPANt)National InstituteFusion Science, Toki-shi 509-5292 JAPANfor2)kts Alamos National Laboratory, Ins Alamos, NM 87545 USA(Received: 18 January 2000 / Accepted: 1l April 2000)

Abstract
This diagnostic, known as the segmented mask infrared imaging bolometer (SIB), uses state-of-theart infrared (IR) imaging technology to measure the temperature rise of each pixel in a two dimensional
array mask sandwiching the metal foil. We have designed, constructed, installed and calibrated a versionof the SIB for the Large Helical Device (LHD). This diagnostic has a two dimensional view (toroidal andpoloidal) of the LHD plasma from an upper port. Through calibration, a representative decay time of r =0.13 sec., and calibration factor of K = 0.80 deg./mW were obtained. In addition, we calculate the timevarying incident radiation power at the foil using both calibration parameters. In the 3'd cycle ofexperiments on LHD, we measured the plasma radiation and show the capability of this diagnostic as asteady-state measurement system.

Keywords:
bolorneter measurement, infrared camera, plasma radiation

1. Introduction
The various widely used resistive bolometeroutside of the vessel, with no wiring across the vacuum
interface. So, this system does not have feedthrough. (2)systems are the standard radiation measurement systems

Change of the IR camera measurement mode depending

for thermal fusion plasma experiments [-3]. However,on the subject of 2-D imaging is very easy i.e. full framein the case of a future reactor many detectors will beneeded, but cables of these resistive bolometers areto the line scan mode. (3) 3-D tomography can beachieved using multiple 2-D imaging bolometers.

The first proposal to use an IR camera for a

subject to noise and vacuum leaks at the feedthrough.

The imaging bolometer is a new type of plasma
radiation measurement system which uses an infrared(IR) camera [4,5]. We refer to the segmented maskpattern type as the segmented mask infrared imaging
bolometer (SIB). In this paper we will report on the firstuse of an SIB on LHD.
The advantages of an infrared imaging bolometer(IRIB) are as follows: (1) The IR camera is installed

bolometer was by G. Apruzzese and G. Tonini [6] andthe segmented mask IR imaging bolometer wasproposed [4] and tested on Compact Helical System(CHS) t5l. In this study, about the first steady-statemeasurement, the IR imaging bolometer is designed,
installed and operated on LHD.

@2000 by The Japan Society of Plasma*C o r re sp ondin g autho r' s e - mail : a s hikaw a @ LH D. nifs. ac.j p

436

Science and Nuclear Fusion ResearchAshikawa N. et al., lnfraredlmaging Bolometry for long Pulse Discharges on LHD

2. Design
The system consists of an infrared video camera, avacuum window, and a reentrant structure that holds adetector mask and pinhole assembly. The present IR
camera is an Agema thermovision 900 LW. The spectral

response of the Mercury Cadmium Tellride (MCT)
detector is 8-12 pm. Stirling cycle cooling is used.
The camera spatial resolutionis2T2 x 136 pixels at
15 Hz sampling rate, and is reduced to 272 x 68 pixels
at 3O Hz. The nominal field of view of the IR camera isl0 x 5 degrees.
The poloidal cross-section ofthe IRIB design at theLHD 6.5-U port is shown in Fig. l. In the vacuum
vessel the light shield is installed from the inner wall
(upper end) to the LHD midplane (lower end). A mask
and a pinhole plate are fitted at the lower end. A metal
foil is sandwiched between two copper masks, whichhave a 2-dimensional hole pattern drilled i4 them. The

foil is selected according to the criterion of: (l) low
thermal conductivity for high sensitivity, (2)optimization of thickness to keep enough energetic
photons, but (3) thin as possible for high sensitivity. We

model the mask pixels as solid cylinders, using thedifferential equation shown in eq. (l).

sr lolB-xr)
T(r.t)='x
I \' 'r ./- ).." ' fb kxth fr IJB^xot
I -expl-*1B^1'"tl'--fu,--,..'Fig. 1 The lR lmaging Bolometer of field of view onpoloidal/radial d i rection.

(r)
plasma side and the mask installed inside the tube. The

where ft is the thermal conductivity, r is the thermal

IR vacuum window is installed on the vacuum vessel

diffusivity, Sy is incident power flux, p. is the m -th Je

side of the tube and we use a ZnSe window for the SIB.
The IR camera is installed outside vacuum vessel andthe view through the ZnSe window is focused on theBessel function roots normalized by the mask pixel
radius b [4]. Using this model we adjust the foil radius Dand the foil thickness th for various metals to arrive at
an optimum set of design parameters in LHD. Using thethermal parameters of different metals, rise time andtemperature for various foils are calculated.

mask. The plasma radiation is absorbed by the mask foil
and the IR camera measures the temperature rise of thisheated foil through the ZnSe window. The windowtransmission is about 95Vo in the infrared region- The
spatial resolution is different in both the poloidal andtoroidal directions. In the toroidal direction it is about 17
cm and in the poloidal direction it is about 25 cm at theplasma midplane. This gives a view which is a half field
period near the 6.5 port in LHD.

We picked the foil material in this case to bealuminum based on its rise time and thermal sensitivity.
Additionally, aluminum is available commercially asvery thin and large size foils. The resulting foil
segments have b = 5.25 mm and th = 0.8 ptm, and thetotal number of pixels on the mask is 118.
The field of view in the poloidal direction of theIRIB as installed in LHD is shown in Fig. l The light3. Calibration
Calibration is necessary because the response of themetal foil surface using the pixel mask is not uniform.

shielding tube (which prevents stray IR light from otherparts of the vessel from coming into the camera field ofview) is installed inside the vacuum vessel and this tube
has a slit plate installed on the end of the tube on theThis is due to (l) variation in thickness due tomanufacturing, (2) the effect of uneven deposition ofcarbon on the foil and (3) uneven clamping of the foil437

Ashikawa N. et al., Infrared Imaging Bolometry for Long Pulse Discharges on LHD

between the masks at the pixel edge resulting in an
effective radius of the pixel which is larger than themask pixel size. Each pixel needs to be checked bydetermining the decay time, ?, and the temperature rise
relative to the mask temperature, AT due to a unit heatsource. The plasma radiation, P,aa, calr then becalculated as shown in eq. (2)p^o=I(^'. " #)Using this calibration data, we can then estimate
the incident plasma radiation power at a foil pixel. Theradiation power is calculated by eq.(2). dTldt isestimated using the three point Lagrangian method. The

radiation powers on three pixels are shown in Fig. 3(a).
When the plasma discharge is started, the pixels
near the mask edge have large signals. Pixel Rl0Pl
decays long after the discharge because clamping of themask and foil is not good, so the actual pixel area islarger than the mask pixel area and copper masks do nota)

Where K is the calibration coefficient. A He-Ne
laser is used for this calibration whose power is about10 mW. z is decided by the exponential rise time of thehave a strong cooling effect. But later in time, pixel
signals fit well compared to the resistive bolometer data.

foil temperature when a chopper cuts the laser light. K isdetermined from the temperature rise divided by thethan the IRIB, fast transient pulse signals can not be

Because the resistive bolometer time resolution is better

detected by the IRIB, but for the total radiation
measurement still has sufficient response.

incident laser power. To steer the laser light, we used
two mirrors and the laser power arriving at the mask isabout 807o. The decay time varies from a pixel to apixel, depending on clamping of the mask and foil, andtypically is about 0.13 seconds. The fitting analysis isshown in Fig. 2. In additional, the averaged K is 0.80

f8864
1.5

deg./mW.

tI

Some of the pixels of the mask have crosstalk witheneighboring pixels, in spite of the thick copper mask.
Evidently the clamping is imperfect in some spots on
the mask (mostly near the edge), causing both the pixel
to be effectively larger than the design value, and alsoi,IIaiLl,"* too-,nf:i!" Il)t i:
plIIt,l lall,ir,iH} xiII.Eo.sItIIi\r

I b'E r:it....i.1 .-r' j.....
It, ,t\IOEJ.

iI:

fiIt,hEi'!,t)iI'causing some thermal leakage. Consequently, we havemade a calibration table for each of the I 18 pixels.

(a)

I!

f \ l"..

ai0 0.5 r 1.5 2 2.5 3 3.5Time (sec. )R2P93.5,^3tl' e. sE;2:1.5(!
blo.

3 ,.oFG 1.05.0

Pellets injection
(b)

2.5

Y r.s

6 0.50-0.5

il I

00.5

0.50.or

1.5

Time (sec. )or23time (s)Fig. 3 (a) The incident radiation powers at three different
Fig. 2 The fitting analysis for a cooling decay time at afoil pixels as a function of time. (b) The plasma

pixel.

radiation power by the resistive bolometer.
438

Ashikawa N. et al., lnfraredlmaging Bolometry for l,ong pulse Discharges on LHD

measurement and have demonstrated the capability

4. The magnetic Field Test of the lRof this diagnostic as a steady-state measurement

Camerasystem.

The present IR camera has several components
which might be susceptible to magnetic fields. At aminimum, it has four electric motors. That is a coolingmotor, a lens focus motor, and two motors for the x andThe acquisition of data during long plasma shotswith the IRIB system is very easy because the storable

perturbation on electric motors by the change in LHD

time duration is dependent only on the hard disk size.
We have measured long discharge plasmas up to 70 sec
at a 15 Hz sampling rate.
It is has been shown that an IR camera system ismagnetic fields is very weak because LHD has avery useful, but installation near a fusion reactor

superconducting coil and therefore essentially steadymagnetic fields.
We tested the IR camera in a magnetic field below
600 G and have operational conditions of the IR camera

operating in steady-state has restrictions caused both byy scanning mirrors. From our experience, themagnetic interference and future nuclear radiationdamage issues.

Future work on this technique will automate theapplication of calibration factors to 2-dimensional arrayas follows:(1) IR camera was shielded by an iron box with adata, and therefore a better understanding of the plasma
radiation profile will result when used in conjunction
with classical bolometry arrays.

thickness of th6o, = 6 mm,(2) The highest magnetic field outside the iron box isunder 400G, as determined bv calculations andfield measurements.
(3) The optical fiber relay unit (component of thecamera) was also installed at a low magnetic field

Acknowledgment
This work has been under development through acollaboration between the National Institute for Fusion
Science (NIFS)(Japan) and Los Alamos Nationalarea.

Laboratory QSA).

5. Summary
In the LHD 3'd cycle, we have design a segmented
mask infrared imaging bolometer system and installed it

References

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