Proceedings of ITC/ISHW2007

Measurement of radiation profile at density ramp-up phaseby using AXUV photodiode arrays in Heliotron J
S. Watanabe, K. Nagasakia, H. Matsuoka, H. Arimoto, T. Mizuuchia, H. Okadaa, S. Kobayashia,
K. Kondo, S. Yamamotoa, N. Tamurab, C. Suzukib, G. Motojima, K. Murai, F. Hamagami,
H. Yasuda, K. Mukai, A. Nakajima, D. Katayama, H. Takahashi and F. Sanoa
Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japana

Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb

National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan(Received day month year / Accepted day month year should be centered 10-point type)Measurement of radiation profiles has been made in electron cyclotron resonance heated (ECRH) plasmas on
Heliotron J by using AXUV photodiode arrays. The measured radiation in low density is attributed to the effect oflow energy photon below about 10eV at edge region, and the profile in the chord-integrated signal is found to bebroad. The radiation collapse at density ramp-up is also investigated. The radiation starts to decrease from the edge
side at the collapse, and then the reduction in the radiation gradually shifts to the core plasma during the collapse
event. The plasma starts to collapse when the edge radiation intensity reaches a certain value, independent of thedensity.
Keywords: Heliotron J, radiation profile, AXUV, soft x-ray profile, radiation collapse

1. Introduction
Radiation from plasmas plays an important role inconfinement and transport in toroidal devices. The
radiative process is related to the impurity transport,radiation collapse, and MARFE phenomenon. The energy
loss from the radiation is a key issue for high density
plasma confinement in stellarators which reach higher
densities than tokamaks. The physics of density limit havebeen investigated by several scenarios, which involve theincrease in radiated power with density [1]. The
observation in W7-AS and LHD suggests that the density
limit is determined by the power balance between the input
power and the radiative power [2, 3].
Absolute extreme ultraviolet photodiodes (AXUVD)
are widely used for plasma diagnostic to understand theradiation process. The AXUVD is capable of measuring
the radiation with high time resolution with energy
sensitivity from visible to soft-x ray region. Some of theapplications are , for example, a total radiation powermonitor from visible to soft-x region with unfiltered
AXUVD, a fast measurement of vacuum ultraviolet (VUV)
and soft-x emission with filtered AXUVD, and a fast
spectroscopic measurement of plasma impurities withAXUVD using multilayer filters [4-6]. In this paper, wepresent measurement results on radiation profiles by using
the AXUVD for ECRH plasmas in Heliotron J. The
e-mail: w.shinya@center.iae.kyoto-u.ac.jp

radiation profile in low density is shown, and the radiation
collapse event at density ramp-up phase is also reported.

2. Experimental setup
Heliotron J is a medium sized helical-axis heliotron
device with a low magnetic shear (<R0>=1.2 m, a=0.1-0.2m, B0<1.5T). The coil system consists of an l/m=1/4
continuous helical coil, two sets of toroidal coils and threepairs of vertical field coil. The combination of these coilsets produces flexible helical-axis heliotron fields. The two
sets of toroidal coils controls the bumpiness component ofthemagnetic
fieldinBoozer
coordinates,
εb=B(m/n=0/4)/B(m/n=0/0), which is an importantcomponent in the helical-axis heliotron configuration
because of its contribution to neoclassical transport andimproved particle confinement. For plasma heating,second-harmonic X-mode ECRH (70GHz 0.4 MW) isapplied. The details of Heliotron J are described in Ref. 7.
Two AXUVD arrays (AXUV 16ELO/G, IRD) havebeen installed in Heliotron J in order to measure radiation
profiles. One array views over the whole poloidal crosssection with an optical filter system, and the other views
from the plasma center to the SOL in the inboard side oftorus without optical filter. The AXUVD has a high time
resolution with the rise time of 0.5 µs. The filter system at
one photodiode array is composed of aluminum foils withProceedings of ITC/ISHW2007(a)2.0(b)

without filter(#27839)

Current [µA]

1.5
1.0

with Parylene filter(#27997)

0.5

with Al filter(#27907)

0.0

FIG. 1 Magnetic flux surface and lines of sight of AXUVD arrays(a) viewed over the whole poloidal cross section and (b) viewed
from the plasma center to the SOL in the inboard side of torus

1.01.0Parylene
0.2µm

0.20.0AXUV

10.5

Parylene
1µm

10

100

0.0

Current [µA]

AXUV

0.0
0.3

0.10.5Al 1µm15#269890.8

Al 0.2µm

0.20.1101.0Transmission

AXUV Responsivity [A/W]

0.3

1006912Channel Number

FIG. 3 Radiation profiles without the optical filter and by using
0.2 µm aluminum and 0.2 µm parylene filters

Energy [eV]1000

30.60.4nearLCFS0.20.0246 8 10 12 14 16Channel Number

FIG. 4 Radiation profile by using AXUVD in Fig. 1(b)Wavelangth [nm]FIG. 2 AXUVD responsivity and transmission of aluminum andparylene filters

1.0 µm and 0.2 µm thicknesses, parylene foils of 1.0 µm
and 0.2 µm thicknesses, and a bandpass filter for 155 nm(FWHM 26.6 nm, FN155-N-.5D-MTD-SP, ACTON). Thetransmission characteristics in these filters and thespectral responsivity in AXUVD are shown in Fig. 2. The
aluminum foil eliminates the effect of the low energy
photon and transmits the high energy photon. On theother hand, the parylene foil eliminates the photon energy
from UV to VUV regions. The transmission curves in thefoils are also shown in Fig. 2. The radiation in soft-X andVUV region is estimated by using combination of thealuminum and parylene foils.

3. Measurement of radiation profile
3.1 Measurement with optical filter system
The radiation profiles have been measured in lowdensity ( ~0.7×1019 m-3) ECRH plasmas by using theAXUVD array to view over the whole poloidal crosssection. Shown in Fig. 3 are the radiation profiles of thechord-integrated signals without the optical filter andwith 0.2 µm aluminum and 0.2 µm parylene filters, where
8ch signal corresponds to the magnetic axis. Note that the9 and 16ch signals are not used due to a trouble in theAXUVD array system. The radiation profile in theintegrated signals without filter is rather asymmetric

which have a peak at 11ch at the inside of the torus, andresembles in shape with the parylene filter. The parylene
filter of 0.2 µm thickness eliminates the energy photon
below about 50 eV, but has the transmission
characteristics in visible light. On the other hands, theradiation profile by using the 0.2 µm aluminum filter,
which eliminates the effect of energy photon from about
80 to 200 eV and below 10 eV, is centrally peaked, andthe intensity at the edge region is low. The radiation
intensity with the aluminum filter is about 20% as low as
without the filter. The result indicates that the radiationfrom low energy photons below about 10 eV dominatesand causes the asymmetry in the measured radiation
without filter. Shown in Fig. 4 is the radiation profile in
the integrated signals without the filter by using theAXUVD illustrated in Fig. 1(b). The profile is hollow
which have a peak at 10ch near the last closed flux
surface (LCFS). The radiation intensity with thealuminum filter near LCFS (2, 14ch) in Fig. 1(a) is much
lower than without filter. The effect of low energy photon
below 10 eV should be dominant near the edge region.
3.2 Radiation profile during collapse event
In this section, the radiation profile at density ramp-up
in ECRH plasmas is studied by using two AXUVD arrays.
The measurement has been carried out in the plasma at
more than 2 × 1019 m-3 in electron density. The typical time
evolution of plasma discharge is shown in Fig. 5. The

3 #26989

100 #26545-26630, #26979-26995

2Radiation
Collapse

180

Total Curent [µA]

BOLOMETER [a.u.]Wp [kJ]

19-3

ne [ x 10 m ]Proceedings of ITC/ISHW2007

2
10.1000.050.00
160180

ECH200220
Time [msec]

20240Current [µA]141210842260

FIG. 6 Radiation profile evolution in the chord-integrated signal
with radiation collapse event. The open circles indicate themaximum intensity of the signal at each channel inside 10ch
during the collapse event.

bolometer signal is measured by a metal resistor typebolometer without metal foil filter [8]. The signal includes
the effect of the neutral particle because no filter is applied
to block off the particle. As the radiation collapse arises
after 227 ms in this discharge, the stored energy decreases
and the bolometer signal increases. Figure 6 shows theradiation profile in the chord-integrated signal measured by
the AXUVD array in Fig. 1(b). The 10ch signal viewing
near the last closed flux surface is largest and increases
with density. The signal decreases suddenly with the stored
energy at the collapse event. As the radiation collapse
proceeds, the peak of the profile moves toward the central
region. The open circles in Fig. 6 indicate the maximum
intensity of the signal at each channel inside 10ch during
the collapse event. The peak of the radiation profile shiftsgradually from edge to core plasma after near the time forthe maximum stored energy. Since no significant changes
in the signals at core region (1-5ch) are observed at t~227
ms, it can be seen that the collapse emerges from the edge
region. The collapse phenomena often occurred in wallconditioning discharges in early FY2007 campaign in

1.5

19

2.0

2.5-3

#26995
1.2 with 1.0 µm Al filter
0.8

8ch

0.4

10ch

0.0-406220
240Time [msec]

Collapse
No Collapsene [10 m ]FIG. 7 Total radiation intensity just before the collapse event(collapse case) and at time for the maximum stored energy(non-collapse case) in the early experimental campaign

16 #26989

Channel Number

40

0
1.0

FIG. 5 Typical wave forms of a radiative collapse discharge for
ECR heating plasma (shot #26989)

200

60

12ch-30-20-10∆t [msec]

01020

FIG. 8 Time series of radiation intensities at the collapse event byusing AXUVD with 1.0 µm aluminum filter. The dashed line
indicates the time for the maximum stored energy and ∆t is timeinterval from the timing.

Heliotron J. Figure 7 shows the density dependence of thetotal radiation intensity. Here the total radiation isestimated by summing the intensities in all the channels at
the time for the maximum stored energy. The radiation
collapse occurs when the total signal current reaches
between about 65 and 90 µA, independent of the electron
density. Since the density is lower than the ECRH cut-off
density, the collapse should be triggered by radiation loss
above certain radiation intensity.
The change in radiation during the collapse event
was also measured by the AXUVD array with the opticalfilter system shown in the Fig. 1(a). Similarly as Fig. 6,
the radiation intensity without filter firstly decreases fromthe edge side and then the reduction region shifts to thecore region at same time scale as Fig. 6. On the otherhands, the measurement by the array with 1µm aluminum
filter was carry out in order to eliminate the energy
photon below about 400 eV in the discharge with thecollapse as shown in Fig. 8, where ∆t is time intervalfrom timing in the maximum stored energy. No clear shiftof reduction region of radiation intensity by using thealuminum filter is observed during the collapse event.
Figure 9 shows the time in peak (over 95% of maximum)of radiation intensity at each channel. The time in shift ofProceedings of ITC/ISHW2007

15 #26989

4. Summary#26989without filter

BOLOMETER
Maximum time∆t [msec]

105#26995
with Al filter

0-5

36

912

15

Channel Number
FIG. 9 Time region in the peak (over 95% of maximum) of theradiation intensity at each channel in AXUVD array
190.4

1.4

core

10.2

ECEOV0.70.0-302edge

VISIBLE [a.u.](CIII, OV)

ECE [a.u.]-3#26602 (ne~1.6x10 m at ∆t=0)0.1CIII-20

Hα (edge) [a.u.]Hα (core) [a.u.]

0.8-10010200.030∆t [msec]
FIG. 10 Time series of the signals from Hα, OV, CIII and ECEat the collapse eventthe reduction region exceeds 10 ms in non-filter case, but
a few ms in the aluminum filter case. The timing at
reduction of the central channel (8ch) signal in theradiation profile without filter is later than with the filter.
The change in radiation profile during the collapse event
should be the effect of shift of emission region fromenergy photon below about 400 eV. The signals of Hα atedge and core region, OV and CIII viewed whole
poloidal cross section are shown in Fig. 10. Similarly inthe signals of AXUVD, the timing at reduction in Hαsignal at the core is later than at the edge, and the signal
at the core is kept increasing to near ∆t=15 ms. On theother hands, the OV and CIII signals increase at thecollapse event, and the OV signal drops at ∆t~15 ms. The
electron cyclotron emission (ECE) signal, shown in Fig.
10, decreases after ∆t=0 ms. Since the density is lower
than the ECRH cut-off density, it suggests that theelectron temperature decrease during the collapse event.
As the ionization potential of CIII and OV are
respectively 47.9 and 114 eV, the signal drop in OV isdue to cooling of the plasma. As a result of energy loss in
the radiation collapse, the radiation from the energy
photon below 400 eV should be dominant, and increases
by reduction in the electron temperature.

Measurement of radiation profiles was carried out for
ECRH plasmas in Heliotron J by using two AXUVD
arrays. The radiation profile in the line-integrated signals
is rather asymmetric, which has a peak in inside of thetorus in low density plasma of~0.7×1019 m-3.
Comparison between the radiation profiles with andwithout the filter showed that the radiation from low
energy photons below about 10eV dominated and caused
the asymmetry. The radiation intensity near LCFS is high,and the radiation from the low energy photon was foundstrong at the edge region.
The radiation collapse at density ramp-up( >2×1019 m-3) was also investigated. The peak ofradiation moves from the edge side during the collapse
event. No significant changes in the radiation at the coreregion are observed just after the collapse event, and thecollapse emerges from the edge region. The radiation
collapse occurs when the emission intensity reaches athreshold, independent of the electron density, suggestingthat the collapse should be triggered by radiation-loss
above certain radiation intensity. The radiation intensitywith 1 µm aluminum filter decreases at each channel neartiming of maximum in the stored energy. The reduction
timing for radiation without filter is later than that withthe filter. The ECE signal decreases with increasing thedensity, and the signals from Hα at the core, OV and CIII
signals increase during the collapse. These measurement
results suggest that the electron temperature decreases
from the edge region, and the low temperature region
spreads into the core region.

Acknowledgements
The authors are grateful to the Heliotron J team fortheir excellent arrangement of the experiments.

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