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)

hν

1

2

2

3

3 4

Anodes 2, 100 μm thick, are made of Be. Two Ta layers of 150 Å thick, are deposited on both side of Be plate 100 μm 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.

Fig.1.

VPD VPD-2Fig.2

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

In Fig.3 one can see the electrode system and the individual electrode

Fig.3

The size of individual electrode is

25x50 mm2,

amount of electrodes is 25,

the input surface is 25x30 mm2,

the distance between electrodes is

1 mm.

50 mm

This VPD was tested with help of X-ray tube.

The dependence of quantum effectiveness on quanta energies is presented in Fig.4.

5 10 15 20 25 30

0,5

1,0

1,5

2,0

2,5

Ta

L1,L

2,L

3 shell binding energies

eff

ecti

ven

ess

, qu

anta

/ el

ectr

on

Eh

, keV

Fig.4

-20 -10 0 10 20

15

20

25

30

35

40

45

50

simulation

experiment

degrees

I , arb

. u

nit

s

,

The angle-of-incidence dependence of quantum effectiveness is shown in Fig.5

Fig.5

α

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.

200 400 600 800 1000

0,2

0,4

0,6

0,8

VPD

semiconductor counter

A, r

el. u

nit

s

t , ms

Fig.6.

In Fig.6 one can see the VPD and the semiconductor counter signals during Ohmic discharge.

The VPD test on T-10 tokamak

500 600 700 800 900 1000

0,5

1,0

1,5

2,0

2,5

21

gas

chord 7.5 cm in the high field direction

chord 8.5 cm in the low field direction

VPD

A ,

rel.u

nit

s

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 for thermal X-ray plasma radiation measurements.

2. The subsequent VPD design optimization is possible.