Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks
The paper presents feasibility and design studies of the Cherenkov-type detectors designed for measurements of energetic electrons within tokamak devices. The technique in question enables the identification of electron beams, the determination of their spatial distribution, as well as the measureme...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2007
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nasplib_isofts_kiev_ua-123456789-1105802025-02-09T21:02:00Z Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks Дослідження можливості застосування та розрахунок детекторів черенкова для реєстрації швидких електронів в токамаках Исследования возможности применения и расчёт детекторов черенкова для регистрации быстрых электронов в токамаках Jakubowski, L. Rabiński, M. Stanisławski, J. Sadowski, M.J. Żebrowski, J. Plasma diagnostics The paper presents feasibility and design studies of the Cherenkov-type detectors designed for measurements of energetic electrons within tokamak devices. The technique in question enables the identification of electron beams, the determination of their spatial distribution, as well as the measurements of their temporal characteristics. On the basis of the presented analyses, i.e. heat transfer studies, a prototype of the Cherenkov measuring head has been designed, constructed and tested within CASTOR tokamak. Obtained experimental results demonstrated that relatively intense Cherenkov signals appear particularly during the final phase of the discharge, when the expanding plasma column reaches the detector. З’ясовуються можливості застосування та розрахунку детекторів Черенкова для реєстрації високоенергетичних електронів в токамаках. Метод, що розглядається, дозволяє виявляти електронні пучки, визначати їх просторовий розподіл, а також вимірювати їх часові характеристики. На основі проведеного аналізу, тобто вивчення теплопередачі, був розроблений та виготовлений прототип вимірювальної головки детектора Черенкова, який було випробувано на токамаці CASTOR. Одержані експериментальні результати показують, що досить інтенсивні сигнали з датчика Черенкова виникають, зокрема, на останній стадії розряду, коли плазмовий шнур, що розширюється, торкається детектора. Исследуются возможности применения и расчёта детекторов Черенкова для регистрации высокоэнергетических электронов в токамаках. Рассматриваемый метод позволяет идентифицировать электронные пучки, определять их распределение в пространстве, а также измерять их временные характеристики. На основе представленного анализа, то есть изучения теплопередачи, разработан и изготовлен прототип измерительной головки детектора Черенкова, который был испытан на токамаке CASTOR. Полученные экспериментальные результаты показывают, что довольно интенсивные сигналы с датчика Черенкова появляются, в частности, на конечной стадии разряда, когда расширяющийся плазменный шнур касается детектора. The reported studies were performed as the P3 task of the research programme supported by the EURATOM Community under the Contract No. FU06–CT–2004–00081 with the Association EURATOM–IPPLM, Poland. The research was also supported by the Ministry of Education and Science, Poland, under contract No.47/EURATOM/2005/7. 2007 Article Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks / L. Jakubowski, M. Rabiński, J. Stanisławski, M.J. Sadowski, J. Żebrowski // Вопросы атомной науки и техники. — 2007. — № 1. — С. 206-208. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS: 41.75.Ht, 40.60.Bq, 52.70.La https://nasplib.isofts.kiev.ua/handle/123456789/110580 en Вопросы атомной науки и техники application/pdf Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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English |
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Plasma diagnostics Plasma diagnostics |
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Plasma diagnostics Plasma diagnostics Jakubowski, L. Rabiński, M. Stanisławski, J. Sadowski, M.J. Żebrowski, J. Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks Вопросы атомной науки и техники |
| description |
The paper presents feasibility and design studies of the Cherenkov-type detectors designed for measurements of energetic electrons within tokamak devices. The technique in question enables the identification of electron beams, the determination of their spatial distribution, as well as the measurements of their temporal characteristics. On the basis of the presented analyses, i.e. heat transfer studies, a prototype of the Cherenkov measuring head has been designed, constructed and tested within CASTOR tokamak. Obtained experimental results demonstrated that relatively intense Cherenkov signals appear particularly during the final phase of the discharge, when the expanding plasma column reaches the detector. |
| format |
Article |
| author |
Jakubowski, L. Rabiński, M. Stanisławski, J. Sadowski, M.J. Żebrowski, J. |
| author_facet |
Jakubowski, L. Rabiński, M. Stanisławski, J. Sadowski, M.J. Żebrowski, J. |
| author_sort |
Jakubowski, L. |
| title |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| title_short |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| title_full |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| title_fullStr |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| title_full_unstemmed |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| title_sort |
feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2007 |
| topic_facet |
Plasma diagnostics |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/110580 |
| citation_txt |
Feasibility study and design of cherenkov-type detectors for measurements of fast electrons within tokamaks / L. Jakubowski, M. Rabiński, J. Stanisławski, M.J. Sadowski, J. Żebrowski // Вопросы атомной науки и техники. — 2007. — № 1. — С. 206-208. — Бібліогр.: 6 назв. — англ. |
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Вопросы атомной науки и техники |
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2025-11-30T17:34:36Z |
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2025-11-30T17:34:36Z |
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PLASMA DIAGNOSTICS
206 Problems of Atomic Science and Technology. 2007, 1. Series: Plasma Physics (13), p. 206-208
FEASIBILITY STUDY AND DESIGN OF CHERENKOV-TYPE
DETECTORS FOR MEASUREMENTS OF FAST ELECTRONS
WITHIN TOKAMAKS
L. Jakubowski, M. Rabi ski, J. Stanis awski, M.J. Sadowski, J. ebrowski
The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Poland
The paper presents feasibility and design studies of the Cherenkov-type detectors designed for measurements of
energetic electrons within tokamak devices. The technique in question enables the identification of electron beams, the
determination of their spatial distribution, as well as the measurements of their temporal characteristics. On the basis of
the presented analyses, i.e. heat transfer studies, a prototype of the Cherenkov measuring head has been designed,
constructed and tested within CASTOR tokamak. Obtained experimental results demonstrated that relatively intense
Cherenkov signals appear particularly during the final phase of the discharge, when the expanding plasma column
reaches the detector.
PACS: 41.75.Ht, 40.60.Bq, 52.70.La
1. INTRODUCTION
The Cherenkov radiation is emitted by a charged
particle moving through a transparent medium with a
velocity higher than the phase velocity of light in this
medium. Emitted energy increases with an increase in a
particle velocity and it is larger for a medium with a
larger refraction coefficient. From comparison of
refraction index values and corresponding minimal energy
values for different materials one can conclude that to
record electron beams of lower energy it is necessary to
use radiators made of diamond or rutil.
Studies of pulsed streams of fast electrons in various
experimental facilities, operated at the IPJ in Swierk,
Poland, have been carried out by means of Cherenkov-
type detectors for many years [1]. Thus, it was decided to
use the accumulated experience for measurements of the
ripple-born electrons inside tokamaks, e.g. CASTOR and
TORE SUPRA facilities.
Measurements of fast electrons produced and escaping
from tokamak-type facilities appeared to be of particular
interest due to the fact that such electrons inform about
non-linear processes occurring inside plasma. The IPJ
team, operating in a frame of the Association
EURATOM/IPPLM, proposed Cherenkov-type probes for
measurements of fast electrons within tokamaks because
of their high spatial- and temporal-resolutions. To lower
the energy threshold of the electron detection the authors
proposed to use radiators with the highest values of the
refractive index. The practical application requires also
the selection of appropriate radiators and consideration of
geometrical as well as thermal limitations.
2. DESIGN ASSUMPTIONS
On the basis of measurements and theoretical analyses
performed so far one can conclude that electron streams
in question can be produced during the additional RF
heating of plasma. The fast electrons and ions can drift
along the izo-B surface deeply inside the volumes of the
vertical diagnostic ports.
Energy spectrum of such electrons ranges from about
50 keV to above 300 keV, and the electron current density
amounts to about 2 mA/cm2. A pulsed stream of such
electrons can bombard the internal surface of the tokamak
vacuum chamber and deposit about 500 W/cm2, what in
turn can cause local destructions of the chamber walls and
induce serious exploitation problems, e.g. vacuum or
cooling-medium leaks.
The chosen Cherenkov-type radiator, which must be
shielded by an appropriate absorption filter protecting the
radiator against the plasma interaction, should be placed
inside the tokamak vacuum chamber. Fast electron
streams, which might penetrate through the filter and the
radiator, induce the Cherenkov radiation if they have
energy above the threshold value (determined by
characteristics of the applied radiator and filters). The
emitted radiation can be collected and delivered through
optical cables to a control room, where it can be detected
by means of fast photomultipliers and recorded within the
electronic equipment.
The realization of the measuring scheme might
induce, however, some serious problems. The main
problem is an effective transfer of heat through the
radiator and a shielding body, i.e. the formation of an
appropriate heat sink as well as keeping temperatures of
the radiator and its shield below the maximum admissible
values. One should note that the power flux of about
500 W/cm2, which might be brought by the investigated
ripple-born electrons, is deposited mainly in a radiator
surface layer of a few hundreds micrometers in the depth.
The deposited heat must be dissipated quickly in order to
eliminate local destructions of the radiator surface. The
electron-induced intense heat stream requires the use of
materials resistant to high temperature, the application of
materials with high thermal conductivity, and the
performance of all measurements during a relatively short
time.
On the basis of a comparative analysis it was decided
to apply the radiator made of a diamond crystal, which
has an excellent thermal conductivity (four times higher
than that of copper) enabling to dissipate heat deposited
upon the radiator surface. Its refractive index is high
enough to observe the Cherenkov radiation from electrons
of energy above 50 keV. It was also decided to split the
whole electron energy spectrum into four energy
channels. Each channel should have the lower energy
threshold determined by a thickness of the applied
absorption foil filter, which must be placed in front of the
207
radiator. As the absorption filter material was chosen
molybdenum (Mo), due to its good thermal diffusivity
and resistance to the sputtering.
Another problem for measurements of ripple-born
electrons in TORE SUPRA facility constitutes trajectories
of the investigated electrons, which are almost tangential
to the front detector surface. It complicates the detector
construction, since in typical constructions the radiator is
usually placed behind a window in the shielding body,
what limits the bombardment of the radiator by electrons
and facilitates the dispersion of heat. In the considered
case the radiator must be shifted behind the shielding. For
such a configuration the heat transfer in the radiator itself
must be very effective to limit a temperature rise.
Fig. 1 presents the radiators made of pure diamond
plates, polished upon whole surfaces and coated by
several metal layers, except for one corner which will be
used to lead out the generated radiation. The shape of the
radiator plates, as well as the polishing and multi-layer
coatings assure effective internal reflections of the
Cherenkov radiation and its collection at the uncoated
corners. According to the proposed design four diamond
plates should be assembled together with some small
(~5o) angular shift which enables four optical cables to be
connected vertically, as shown in Fig. 2.
Fig.1. A single diamond radiator designed for TORE
SUPRA experiments. The metal coatings (grey) should be
deposited upon all the radiator surfaces, except for the
part marked in white. The front (rounded) corner of the
diamond radiator should be coated by a Mo-filter
Fig.2. Spatial arrangement of four diamond radiators
and separate optical fibres of a 4-channel Cherenkov-type
detector proposed for TORE SUPRA experiments
3. THERMAL ANALYSIS
The design of Cherenkov-type detectors, which are to
be exposed to intense electron streams within tokamaks,
requires an analysis of thermal effects connected with the
deposition and dispersion of relatively high power fluxes.
Modelling of the transient temperature distributions in
structural components of measurement head requires the
use a computational programme [3] solving the above
problem numerically. Detailed computations of a heat
transport inside the Mo filters, diamond radiators and
shielding body were performed for the assumed power
flux equal to 500 W/cm2. Different constructions of the
detector head were considered.
With neglecting the anisotropy of the considered body
in the direction perpendicular to main heat-conduction
gradient, the thermal problem can be reduced to the one-
dimensional. Nevertheless thermal properties of the
modelled structure are depth dependent. Since the
majority of the heat dissipated by the structure will flow
through the substrate into the heat sink located at the
distance peripheries, the heat loss through the thermally
isolated side surfaces may be assumed as negligible. As a
result of that assumption the thermal system can be
approximated by a one-dimensional slab sandwich
composed of outer filter, diamond and bulk metal plate.
The analysed sandwich plate is assumed to be insulated
along the side surface except the heat source area at the
front end and the bottom, where the radiative heat transfer
boundary condition of Stefan-Boltzman type has been
applied. The value of the panchromatic emissivity of the
surface has been selected of the order typical for oxidized
metals (i.e. 0.6).
The simulation for a sequence of six 0.5-second-long
heat loads is presented in Fig. 3. The presented analysis
considers the disk-shaped sandwich composed of a thin
metallic filter of 100 µm in thickness, 1-cm-thick
diamond plate and bulk metal (Mo), with the heat flux
acting on the top surface. Nonlinearity of the heat
conduction problem causes that with the shortest heat
pulses one can observe the slightly lower temperatures
achieved at the front end surface in comparison with one
heat load lasting for 3 seconds, although energy passed to
sandwich plate is the same. In general the simulations
confirm the positive effect of the proposed solution. From
a point of view of heat engineering it seems to be
promising to replace the continuous Cherenkov
measurement with the sequence of short pulses, i.e. to
introduce and withdraw the detector from plasma.
Fig.3. Transient temperature distributions in the analysed
structure
208
4. EXPERIMENTAL RESULTS
Experimental studies of generated electron beams
have been performed within CASTOR facility (major
radius = 40 cm, limiter radius = 8.5 cm, minor chamber
radius = 10 cm) operating at a magnetic field B ranging
from 0.8 T to 1.6 T, plasma current Ip = 10 kA, the line-
averaged electron density ne ~ 1019 m-3 and electron
temperature equal to about 200 eV. The electron and ion
temperatures at the plasma edge were of the order of
10 eV, and the edge plasma density was below 1018 m-3.
To carry out the fast-electron measurements, a
movable support with the Cherenkov radiation detection-
system was installed upon an upper diagnostic port
situated 135° toroidally from the limiter. It enabled the
measurements in different positions along the minor
radius to be performed. In the most outer position, the
detector was well hidden inside the diagnostic port, then it
could be moved to the shadow of the limiter, and finally
after a deeper insertion, it could reach a plasma region.
Fig.4. Typical traces recorded during tests of the
Cherenkov detector in the CASTOR facility
The detection head contained a Cherenkov-radiator
made of a small aluminium-nitride (AlN) crystal
protected from the visible light by a Ti-layer of about
10 µm in thickness. The AlN crystal radiator was chosen
due to its relatively low energy threshold, good thermal
conductivity and a relatively low price. The traces
presented in Fig. 4 show typical results. The difference
between the Cherenkov- and Hα-signals confirms that the
Cherenkov detection head was well protected against the
visible radiation.
The obtained results show that the Cherenkov
measuring circuit might be influenced by intense hard
X-rays. To eliminate the X-ray influence, some changes
of the detector head might be required, e.g. in the
construction of the light-pipe and shielding of a photo-
multiplier.
CONCLUSIONS
An analysis of thermal loads, which are produced by
pulsed electron streams within tokamak facilities, has
been performed. In particular the thermal loads expected
within the CASTOR and TORE SUPRA facilities have
been analysed numerically.
The design and preliminary tests of the Cherenkov
detector, used for measurements of fast electrons within
the CASTOR tokamak have been reported and some
results of such measurements have been presented.
A technical design of a prototype Cherenkov detector
head, which was designed especially for TORE SUPRA
experiments, has been described.
ACKNOWLEDGEMENTS
The reported studies were performed as the P3 task of
the research programme supported by the EURATOM
Community under the Contract No. FU06–CT–2004–
00081 with the Association EURATOM–IPPLM, Poland.
The research was also supported by the Ministry of
Education and Science, Poland, under contract No.
47/EURATOM/2005/7.
REFERENCES
1. L. Jakubowski, M. Sadowski, J. Zebrowski // J. Tech.
Phys. 1997, v. 38, N1, p. 141-150.
2. M. Rabi ski; INR 1917/XXIV/PP/A, Warsaw 1981.
3. L. Jakubowski et. al.// Czech. J. Phys. 2006, v. 56B,
p. B98-B103.
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