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 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) hνhν 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-2 Fig.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 mm 2, amount of electrodes is 25, the input surface is 25x30 mm 2, 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. Fig.4

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

This detector was tested under 60 Co 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.

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

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 3 0 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 (TaBe 12, Ta 2 Be 17 ) 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.