Proceedings of ITC/ISHW2007

Abrupt Intense Radiation from Divertor Plates of the Large
Helical Device
K. Narihara, I. Yamada, T. Minami and H. Hayashi
National Institute for Fusion Science, Toki 509-5292, Japan
This paper examines the phenomenon of abrupt intense radiation (AIR) from a graphite divortor plate measured
by the DC-coupled Thomson scattering detectors on LHD. A time-integrated spectrum of this AIR resembles to ablack body (BB) radiation mixed with plasma light line-radiation. We estimate the BB-radiation temperature fromthe ratios among signals from a five-color-channel polychromator. The BB-radiation temperature TB, thus obtained,
abruptly rises up to ~2000K and after some duration rapidly decays usually after a small drop in NBI power. Timeevolution of BB radiation intensity calculated with this TB well reproduces the observed signals. The experimental
data are compared with 1D thermal transport simulations in graphite plate taking account of the nonlinear
diffusivity. With the conventional thermal conduction model, including non-linear thermal diffusivity, the observed
rapid TB rise cannot be reproduced. Correlations between the size of AIR and various plasma/operationalparameters are presented.

Keywords: Thomson scattering diagnostic, LHD, divertor plate

1. Introduction(A)
Te(keV), ne(arb)

Background radiation has degrading effects on
Thomson scattering (TS) diagnostics [1]: it drops S/Nratio and sometimes introduces large systematic errors
into the obtained data. In an extreme case the electron
temperature (Te) and density (ne) profiles collapse as
shown later. Even if the size of collapse is small, without
quantitative data on the radiation intensity, the confidence
on the obtained Te and ne profiles is low. In order to solve
this problem to some extent, we installed 1040-channel
scanning ADC so that the DC levels of the avalanche
photo-diodes used for detecting the scattered light can bemonitored. With this background light monitoring, wefound that an abrupt intense radiation (AIR) often appears
around divortor plates and invalidates TS data greatly.
Intending to suppress or reduce the AIR, we examine thisphenomenon.

2author’s e-mail:narihara@nifs.ac.jp

Tene0
2.50

3.25
4.00
MAJOR RADIUS (m)#073685 TIME=

4.75

1.39800 s

43(B)2102.50

2. Description of AIR
Figure 1(A) shows a Te and ne profiles with largecollapse. The collapsed region coincides roughly with thescattering region that has a divortor plate in background
scope of the light collection optics. The DC-coupled
outputs (DC_APD) from light detectors ‘seeing’ thisregion exceed greatly 0.6V, which is upper limit for
linearity for pulse response, thus invalidating the obtaineddata. (For DC response, linearity is preserved up to ~4 V.)
It should be noted that the focal points of the lightcollection optics are on the laser beam, and hence thespatial distribution of DC_APD, Fig 1. (B), is not a real
distribution but a convolution of it. Figure 2 shows#73685time=1.40048s

4

3.25
4.00
MAJOR RADIUS(m)

4.75

(C)
LASER
VIEW
SCOPE
Divertor plate
Fig. 1.

(A) Collapsed Te and ne profiles. (B) Distribution ofDC-outputs of APDs. (C) Thomson scattering
configuration [2].

Proceedings of ITC/ISHW2007
fits to the observed signals as a function of time is shownin Fig. 5(A). The upper- and –lower limits of the error
bars are given by the roots of the equation
H(TB)=2*min(H(TB)), where H is the residual square sumof the fitting. In the calculation, we set TB =0 for signal <10mV, to avoid large irregularities in TB for small signals.
Using this TB-evolution, we can calculate the time
evolution of signals as shown in Fig. 5(B), whichsurprisingly well reproduces the observed signals, Fig.
5(C) after signals reach their peaks. Here we assume thatthe BB emitting area is held constant. Although the BB
assumption seems to approximately explain theobservation, there is a difficulty in the startup phase: TheTB grows in a few ms, which is too rapidly to be realistic.

the time evolutions of the 5-color-channel DC_APD
signals from the polychromator#32 together with relevant
plasma parameters. We can see that DC_APD starts togrow after additional NBI start its operation. It is hard tofind a signal that well correlates with the startup of AIR.

shot=073685 poly#=000032

NBIHaPradWpne_bar

10

APD DC-OUTS3000K

6There seems to be a tendency, however, that an AIR
sometimes appears at the time when the diamagnetic
signal Wp approaches its peak. From the energy-flow
equation: Pheat= dW/dt + Pout + Prad, where Pheat is heating
power, Pout is the wall loading power carried by particles
and Prad is radiation loss from plasma, the above
statement leads us to an assumption that the AIR appears
when Pout becomes large. This assumption is further
supported by facts that there is a lower threshold in NBI
power for AIR to occur and that the higher Prad often
reduces the size of AIR. Figure 3 (A) shows atime-integrated spectrum of the AIR appeared on thesame shot as in Fig. 2 measured by a fiber-multi-channelspectrometer. The 3073K black body radiation spectrum
from a standard lamp measured by the same spectrometeris shown in Fig. 3(B) for a reference. The spectrum of theAIR resembles well black body radiation.
poly32

2000

(A)

F5

20600

700

F4F3

F2 F1

800
900
1000
WAVELENGTH(nm)1100

POLY#34
F3/F1

F4/F1

F5/F1

2000

00246RATIO

810Fig. 4. (A) Responsivities of 5-color- polychromator #34 andBB radiation spectra of various temperatures. (B) Signal ratios
among the five outputs vs. temperature.
3000(A)

Poly348(B) signals expected from BB radiation

642

2000

0
1.0

1.5(C)

2.0

2.5

3.0

2.5

3.0

Observed signal

41000
2000
1.0

2TIME(s)1.5

2.0
TIME(s)4Fig. 5. (A) Deduced temperature evolution. (B). Calculated
signal sizes from BB radiation of temperature shown left. (C):The observed signal size. Both waveforms are similar in shape.

3170K Black body radiation
4000

Ha2900K

4Time evolutions of DC_APD together with thecord-averaged plasma density ne_bar, the diamagnetic
energy Wp, the bolometer signal Prad, H-alpha, andtotal NBI power.

2500

F2/F1

8

5.0

Effective Temp.(K)Fig. 2.

2.5TIME(s)(B)

POLY#34
3100K

Temperature(1000K)0.0(A)

(B)

1500
2000

3. 1D simulation

1000#73685500
0500

Fig. 3.

#73688700
900
wavelength(nm)

1100

0500

700900wavelenght(nm)1100(A) Time integrated multi-channel-spectrum of thelight from the fiber connected to poly32 with/without
AIR. (B) A 3170K BB radiation spectrum measured
by the same spectrometer.

Assuming the black body radiation, we can estimate
effective temperature of the AIR using the currently used
polychromators, the spectrum of which is shown in Fig.4(A) together with the BB radiation of varioustemperatures. The ratios F5/F1, F4/F1, F3/F1, F2/F1 as afunction of TB are shown in Fig. 4(B). The TB that best

In order to examine the above fast rise of TB, weconsider 1D heat conduction model described below. One
side of a graphite (x=0) is exposed to a heat flux q andemits BB-radiation σTs4, where Ts is the surface
temperature, and other end surface (x=15mm) is kept at aconstant temperature T(15)=300K. We assume the heat isdeposited in the surface layer of thickness τ. The heatdiffusivity decreases rapidly as temperature rises asshown in Fig.6(B) [3], the non-linearity of which is
expected to accelerate the temperature rise. An exampleof the numerical solution of the heat conduction equation

Proceedings of ITC/ISHW2007∂T∂ 2T= α (T ) 2∂t∂x ,APD_DC(V)with boundary conditions∂Ts∂T
4= q − σTs + k s∂t∂xcρτandRaxis vs APD_DC

4203.50 3.60 3.70 3.80 3.90 4.00
MAJOR RADIUS(m)T (15, t ) = 300 ,Fig. 7. Major radius vs. AIR size.

where c is heat capacity, ρ density, τ the thickness of heat
deposition, σ Stefan-Boltzmann coefficient, k the heatconductivity, is shown in Fig. 6(A). We can see that as qincreases the rise time becomes faster and the attainabletemperature is higher. Within the above conventionalmodel of heat conduction, we cannot reproduce theobserved behavior of Ts. An exotic model such as bubbles
or a crack formation beneath the heated surface, whichswitches off the heat conduction, may reproduce theobservation. With this fast rise problem, we now hesitate
to conclude the AIR is BB radiation. Further diagnostics
such as IR camera will help us draw a conclusion.

APD_DC(V)4

B vs APD_DC

20-4-202
Magnetic Field (T)4Fig. 8. Magnetic field intensity on the axis vs. AIR size.
thickness=50um, Q=100+400*J

5000

4APD_DC(V)

1.0

4000

0.8

3000
0.6

2000
0.4

1000

20.20200

400

600
TIME(ms)

Fig. 6.

800

1000

0.002000

4000
6000
TEMPERATURE(K)8000

Right: thermal heat diffusivity of graphite, which ishighly non-linear function of temperature. Left:
Examples of surface temperature evolutions forvarious heat flux. Heat deposition layer thickness is
50 μm. The top curve is Ts for q=3.7 kW/cm2: From
the top, q decreases by 400W/cm2 from curve tocurve.

4. Statistics
In order to get clues to the question what the AIR reallyis, we took statistics of the size of the AIR as a functionof various operation/plasma parameters. Figure 7 shows
the DC_APD distribution as a function of the majorradius. The larger DC_APD appear in the range 3.6m≦
Rax ≦3.75m. Although the shot numbers on bins arelargely different, they are large enough, except extremelyinwards shifted cases, to show the general tendency. Thistendency may be explained by fact that the particle/heatflux to inner-divertors is smaller for the outwards shifted
configurations. Figure 8 shows DC_APD as a function ofmagnetic field intensity on the axis. There is a clear
tendency that at higher magnetic fields, AIR is larger.
This suggests that a stronger magnetic field may focus
plasma flow more effectively to the divertor plate. Figure
9, bolometer signal vs. the size of AIR, indicates theyanti-correlate. This seems to support our assumption that
AIR is caused by heat flux to the divertor plate:

0

10000

020
BOLO(kW)

Fig. 9. Bolometer signal vs. AIR size.

4APD_DC(V)0W p v s A P D _D C200 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4
W p (M J )Fig. 10. Wp vs. AIR size.

4APD_DC(V)surface temp.(K)Bolo vs APD_DC

thermal diffusivity of graphite(cm^2/s
1.2

Heat flux on

NB4 vs APD_DC

200.0 0.1 0.2 0.3 0.4 0.5
NBI4(Arb)

Fig. 11. NB4 injection power vs. AIR size.

Proceedings of ITC/ISHW2007
higher radiation power lessens the wall-loading power,
provided that the total loss power is held constant. Figure
10, Wp vs. AIR size relation, shows no correlation
between them.
NB4 is injected in the radial direction, whereas theother NB, NB1, NB2 and NB3, are injected in tangential
directions. It is expected that appreciably amount of theradially injected NB is promptly lost and hit divertor plate.
In this respect, it is curious to investigate the correlation
between NB4 power and AIR size. Figure 11 gives this.
No correlation can be seen.
If the AIR is plasma light generated around thegraphite divertor plate, it will accompany appreciable
signal on the CIII-line-monitor installed on 1-O-port.
Figure12, CIII vs. AIR size relation, however, reveals nopositive correlation.

APD_DC(V)4

CIII vs APD_DC

200.0

0.2

0.4
0.6
CIII(Arb)

0.8

Fig. 12. CIII radiation vs. AIR size.

Figure 13 shows the distribution of DC_APD as afunction of line averaged plasma density. There seems ahigh occurrence region in the lower density, but its
significance is not yet clear.

APD_DC(V)4

ne_bar vs APD_DC

20020
ne_bar(Arb)

Fig. 13. ne_bar vs. AIR size.

5. Discussions and summary
The AIR poses a serious problem to the Thomson
scattering diagnostic, but it seems that it has noappreciable adverse effect on the core plasma
confinement. If the AIR is only black body radiation fromthe heated graphite plate, it is quite natural. On the otherhand, if the AIR is radiation from plasma created on thesurface of the divortor plate, there may be carbon
impurity influx to the core plasma. But, up to now, nosuch indication has been observed. Even if the AIR isplasma radiation, the existence of the so-called stochastic
region will shield the impurity influx efficiently.
We have not yet obtained a definite conclusion what
the AIR really is. Probably, it is black body radiationfrom the heated divertor plate. But its very rapid rise
makes us to hesitate to take this simple view. We areplanning to replace the divertor plate with a new one withmuch higher heat conductance.
References[1] K.Narihara, et al., in ITC-16 proceedings, (2006).
[2] K.Narihara, et al., Rev. Sci. Instrum. 72, 1122(2001).
[3] Thermo-physical properties of matter, Volume 10. Thermal
Diffusivity, IFI/Plenum, New York-Washington 1970.