S14-5
Evaluation of diagnostics and future development subjects
計測器の総合評価と今後の開発課題
Tsuyoshi Akiyama
秋山毅志
National Institute for Fusion Science322-6 Oroshi-cho, Toki, Gifu, 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
Candidates of diagnostics on a DEMO reactor such as magnetics and an interferometer/polarimeter are
listed up from requirements of plasma start-up and steady state operation scenarios. Due to the higher
radiation, the higher temperature and the limited access than present devices, there are challenges to beovercame. In this presentation, challenges of each candidate are clarified and the life time, and limitation
are estimated quantitatively and qualitatively.

1. Introduction
In S14-4, necessary plasma parameters forburning plasma control and candidates ofdiagnostics on a DEMO reactor are shown:magnetics,
interferometer/polarimeter,
microwave diagnostics, Thomson scattering
measurement,spectroscopy
andneutron
diagnostic. Due to a harsh environment, such as
high radiation level and high temperature, in theDEMO reactor, the life time of the diagnostics
and resolutions will be limited. In order to makediagnostic set which satisfies the requirements of
the reactor operation, it is important to clarify
challenges in the present diagnostic techniques
and to start the developments.
Table 1: Radiation levels in locations of diagnostic components
RegionLocation

Zone A

Behind brancket
Behindhigh-temperatureshieldBehindlow-temperatureshieldZone B

Zone C

Fast neutron
flux (cm-2s-1)
2×1013

-ray flux(MGy/h)
0.5

5×1010

0.001

3×109

0.0001

Fig. 1: Horizontal cross section of Slim-CS [2]For discussions of radiation effects on
diagnostic components, the reactor design and theradiation level of Slim-CS [1] are used as an
example of those in the DEMO reactor. The
locations where diagnostic components will be
installed are divided into three regions as shownin Table 1 and Fig. 1.
2. Magnetics
The radiation causes a temporal or permanentdegradation of the insulation performance of theceramic (RIC, RIED) and induces the electromotiveforce (RIEMF, TIEMF) in magnetics. They
determine the life time and resolutions. As for theradiation induced conductivity (RIC), the databaseof irradiation experiments [3] shows the induced
conductivity is at an acceptable level in the Zone A.
For suppression of the radiation induced electricaldegradation (RIED), magnetics in ITER aredesigned, following the guideline that the radiation
flux is smaller than 100 Gy/s [4]. The flux in Zone
A is slightly higher (140 Gy/s) than the guideline,
the RIED may occur earlier than ITER. Supposingthat the size of a flux loop in the DEMO reactor iscomparable to that in ITER, the expected
thermoelectric electromotive force (TIEMF) is
about 1 mV, is not negligible level. The neutron
irradiation test of the Hall sensor, which is analternative sensor of the magnetic field, shows a life
time of about three years behind the brancket (ZoneB). Further reduction of the radiation level toextend the life time longer than that of the brancket
replacement interval and estimation of the signal tonoise ratio at the isolated position from a plasma arenecessary. Since these radiation effects stronglydepend on the insulation and conductor materials,
the developments of materials with the small
radiation effect are also necessary. To that end, theoperation of the irradiation facility with sufficientflux and irradiation area is indispensable.
3. Laser/Microwave diagnostics
3.1. Interferometer/Polarimeter
The optimum wavelength of the probe light is at
the infrared or near infrared region in the DEMO
reactor. The candidates of the window are quartz
and sapphire for near infrared laser light. Locationsof the window will be Zone C and the reflectivity ofthe sapphire is expected to be reduced by 10% forabout one year. Since the labyrinth structure canreduce the radiation level by one order, the life timeof the window will be able to extend up to about tenyears.
The first mirrors will be installed in Zone A.
Although past irradiation tests of metallic mirrors
showed no degradation, the total fluencecorresponded to less than 100-days’ fluence in Zone
A. Hence the further irradiation test is necessary for
conformation of performance of the first mirror.
The other mirror issue is erosion, impurity
deposition and helium bubbles. Since they are also
severe problems in ITER, suppression method, suchas fin-shaped protection [5], and cleaning method
will be examined on ITER.
The density resolution required from a helical
reactor FFHR-d1 is 1017 m-3 to suppress variationsof the fusion output [6]. The present two-color
interferometer cannot realize the quite highresolution. The dispersion interferometer has higher
density resolution [7] and is one of the candidates.
Even so, the density is underestimated by about
10% due to the relativistic effect. Correction withthe temperature profile will be necessary.
3.2. Thomson scattering measurement
The spectrum of the scattered light (the incident
light: 1064 nm) broaden down to 200 nm for anelectron temperature of 40 keV. Since thetransmission degradation of the window and theoptical fiber become more significant for shorterwavelength, the upper limit of the neutron and -ray
fluences will be lower than that for theinterferometer/polarimeter. A neutron fluence of
1×1016 n/cm2 reduces the transmissivity of a quartz
window by 10% at 300 nm. The fluencecorresponds to only 0.1 year’s irradiation in Zone C.
The reduction of the radiation level about twoorders is indispensable: shielding and labyrinth duct
structure. In addition, the degradation is significantfor shorter wavelength than 500 nm, where thewavelength broaden. Hence the detected spectrumof the scattered light will change gradually and thenthe frequency of the calibration should beconsidered. The issues of the first mirror are similar
to and more significant than those for theinterferometer /polarimeter.
3.3. Reflectometer, ECE
Microwave diagnostics are relatively robust in
the DEMO reactor because a waveguide and a firstmirror/horn are not affected so much by theradiation. Hence, it can be used as reliablediagnostics of a plasma edge, alternative to themagnetics [8]. On the other hand, relativistic effectcauses underestimation of the density for thereflectometer and limitation of the measurable
region to the edge for ECE (due to downshift of theradiation frequency). As an edge measurement, they
will be reliable diagnostics in the DEMO.
4. Neutron diagnostics
A microfission chamber has a wide dynamic
range, the high resolution and the fast response. For
conversion of the local neutron yield to the fusion
output, a calibration experiment is necessary.
However, present neutron sources are too weak tocalibrate the fission chambers with a sufficientsignal to noise ratio even in ITER. Severalcalibration methods are proposed [9, 10] and will beexamined in ITER.
5. Discussion and summary
In this presentation, challenges of eachdiagnostic on ITER are shown. Irradiation tests ofcomponents are still necessary for most ofdiagnostics. However, irradiation facilities inJapan are limited now. Re-start of a fission reactor
for irradiation tests such as JMTR is desired.
The relativistic effect will be more significant in
ITER and DEMO than present devices. The
underestimation of the density and limitation of
the measurable area of the temperature cannot be
negligible. The calibration method or good
approximations should be established through
ITER experiments.
References[1] K. Tobita et. al., Nucl. Fusion 49, 075029 (2009)[2] Program Committee of Technical Study on theDiagnostics for Control of the Fusion DEMO
Reactors: NIFS-Memo-68 (2014).
[3] T. Nishitani et. al., J. Plasma Fusion Res. 84, 635(2008).
[4] G. Vayakis et. al., FST 53, 699 (2008).
[5] T. Akiyama et. al., Nucl. Fusion 52, 063014 (2012).
[6] T. Goto et. al., Fusion Eng. Des. 89, 2451 (2014).
[7] T. Akiyama et. al., Rev. Sci. Instrum 85, 11D301(2014).
[8] J. Santos, Rev. Sci. Instrum. 81, 10D926 (2010).
[9] K. Asai et. al., Rev. Sci. Instrum. 75, 3537 (2004).
[10] C.W. Bames et. al., Rev. Sci. Instrum. 68, 577(1997).