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Vacuum Photodiodes for Soft X

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Vacuum Photodiodes for Soft X-Ray ITER
Tomography
Yu.Gott, M.Stepanenko
10th ITPA Meeting, Moscow, 2006
The measurements of soft X-ray radiation in ITER is planned
now.
These measurements must be used, for example, for
monitoring MHD activity, for feedback control of the plasma system
and for plasma tomography.
Unfortunately semiconductor, gas and scintillation detectors
are completely inappropriate for ITER both because of sensitivity to
and damage by neutron and gamma irradiation.
So it is necessary to find an alternative.
Vacuum photodiode detector (VPD) was proposed to perform
ITER plasma tomography using the plasma own thermal x-rays.
In ITER it is planned to use
about 100-120 such detectors.
In Figure one can see
the line-of –sights of some
VPD’s.
The following design for simple VPD cell has been proposed
(Fig.1)
2
п‚·
hОЅ
3
3
1
2
п‚·
4
п‚·
Fig.1.
Anodes 2, 100 Ојm thick, are made of Be. Two Ta layers of 150 Г…
thick, are deposited on both side of Be plate 100 Ојm thick and serve
as cathodes 3. Ammetr 4 is connected between anodes and cathodes.
To eliminate the effect of charge-exchange particles flow and UV
radiation, Be filter 1 is installed at the VPD input.
The VPD detector was
design and tested and it was shown
that detector need to be optimized.
The main goal of this
optimization is to design detector
which will be as small as possible.
In the last experimental session in
the down port of T-10 was installed
the new detector. This VPD have
better design and so its dimensions
are less than previous detector has.
The photo of both detectors one can
see in Fig.2
VPD
Fig.2
VPD-2
In Fig.3 one can see the electrode system and the individual
electrode
The size of individual
electrode is
25x50 mm2,
amount of electrodes is
25,
the input surface is
25x30 mm2,
50 mm
Fig.3
the distance
electrodes is
1 mm.
between
This VPD was tested with help of X-ray tube.
The dependence of quantum effectiveness on quanta energies is
presented in Fig.4.
effectiven ess , q u an ta / electro n
2 ,5
2 ,0
1 ,5
1 ,0
0 ,5
Ta
L 1 ,L 2 ,L 3 s h e ll b in d in g e n e rg ie s
5
10
15
E hпЃ® , k e V
Fig.4
20
25
30
The angle-of-incidence dependence of quantum effectiveness is shown in
Fig.5
О±
50
e x p e rim e n t
45
I , arb. units
40
35
30
s im u la tio n
25
20
15
-2 0
-1 0
0
10
пЃЎ , d e g re e s
Fig.5
20
This detector was tested under 60Co gamma irradiation and it
was shown that on ITER the signal from gamma will be about
100 times less
than signal from the thermal X-rays.
The VPD test on T-10 tokamak
0 ,8
In Fig.6 one can
see the VPD and
the semiconductor
counter
signals
during
Ohmic
V P discharge.
D
A, rel. u n its
0 ,6
s e m ic o n d u c to r
0 ,4
c o u n te r
0 ,2
200
400
600
t , ms
Fig.6.
800
1000
2,5
VPD
c h o rd 8 .5 c m
2,0
in th e lo w fie ld d ire c tio n
A , rel.u n its
gas
2
1
1,5
1,0
c h o rd 7 .5 c m
in th e h ig h fie ld d ire c tio n
0,5
500
600
700
800
900
1000
t , ms
Fig.8. The signals from VPD (red curve) and from gas
detectors along 8.5 cm chord in the low field direction (1) and
7.5 cm in the high field direction (2) during electron cyclotron
heating and 5 pellet injection.
It is possible to see that these signals are very
similar.
The main problem which is not solved yet is
What is the minimal value of current
which can be transported
from tokamak to the control room?
We have no possibility to solve this problem
without Central Team recommendations.
If we accept the value of this current as small as
1 microampere
we can calculate, that
pencil angle in the poloidal direction must be about 30
and in the toroidal direction must be about 20 0 .
For VPD placed in the ports it is not difficult to fulfill these
conditions.
The problem will be for VPD placed on the back side of the
vacuum wall.
One of the possible solution is
to place the VPD in the crossing point
of horizontal and vertical gaps
between the blanket tiles.
The unresolved problem: The behavior of the Ta layers under
neutron irradiation.
The thermal, radiation-induced diffusion and formation of the
intermetallic compounds – beryllids (TaBe12, Ta2Be17)
can change the electrode surface composition.
To solve this problem
we intend to irradiate the electrode in nuclear power
reactor and check the electrode surface composition.
CONCLUSIONS
1. It is shown that VPD can be successfully used
thermal X-ray plasma radiation measurements.
2. The subsequent VPD design optimization is possible.
for
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