A Photodiode for Soft X-ray Measurement

Abstract: Photodiodes with three layer zircomnium, titanium and aluminum have been used as detectors on the HBT-EP tokamak for soft X-ray radiation measurement. Signals from the photodiodes show sawteeth instabilities and MHD oscillations.

Radiation in the soft X-ray range from plasma can provide density, temperature and MHD instability information. Silicon photodiodes have been used as the soft X-ray radiation detectors. Usually, a seperately surported aluminum or berylium thin film filter is used to cut off the lower energy photons. The shortcoming of this scheme is that the thin film is very fragil. The problem is even more serious when the plasma temperature and density are not that high so that the thin film has to be very thin.

Photodiodes with Zr/Ti/C thin film filters deposited directly on the surface have been used on the HBT-EP tokamak as the detectors for the soft X-ray tomography system. This paper describes the photodiodes and presents the soft X-ray measurement results using these photodiodes.

HBT-EP tokamak was built to study MHD instabilities during high [beta] discharges.¹ It has a major radius 92 cm, and a minor radius 14 - 19.3 cm. The plasma current is usually less than 25 kA. The machine runs with a pulse length less than 10 ms. The toroidal field is about 3 kG. The line integrated plasma temperature is about 100 eV. The plasma density is 1 x 10¹³ cm^-3.

One of the unique features of HBT-EP is a segmented conducting shell which consists of 20 segments located 10 toroidal locations. Each segment is attached to a shell positioner, so the distance between the shell and the plasma can be adjusted between shots. This unique feature allows us to study the wall stabilization effect on the MHD instabilities. Another unique feature of the HBT-EP tokamak is a set of saddle coils powered by an amplifier provided by the Los Alamos National Lab. The saddle coils are located at different toroidal positions to provide a magnetic filed with m/n = 2/1 structure. They allow us to carry controlled feedback studies on the MHD modes.

A soft X-ray tomography system has been designed to study the MHD structures on HBT-EP. The system employs 32 collimated channels of silicon photodiodes manufactured by the International Radiation Detector as the soft X-ray detector. To filter out lower energy photons, we ask the photodiode manucturer to deposit thin film filters directly on the surface of the photodiodes. Berylium has been commonly used as the filter material. However, since berylium is toxic, we designed a three layer filter consisting of zircomnium, titanium and carbon.

The thicknesses of each layer were varied during the calculation to optimize the response. In the calculation, the transmission coefficients of different materials were experimental data taken by B. L. Henke, et al..² The data were downloaded from a Soft X-ray World Wide Web server, http://xray.uu.se/hypertext/henke.html. Based on the calculation, a combination of 100nm zrcomnium(Zr), 7.5 nm titanium(Ti) and 100 nm carbon(C) has been chosen. Figure 1 shows the calculated transmission of the Zr/Ti/C filter in the X-UV and soft X-ray range.

Relative response of the photodiodes to different temperature plasma has also been calculated. In the calculation, bremstraulung radiation and recombination radiation have been taken into account. A theoretical quntum efficiency of 3.63 electrons per eV photons has been assumed. Figure 2 is the calculated relative response of a photodiode with the Zr/Ti/C filter as a function of plasma temperature. In comparison, figure 2 also shows the relative responses of photodiodes with a beryllium filter and an aluminum filter as a function of plasma temperature.

Figure 3 shows the soft X-ray signals measured by the photodiodes with the Zr/Ti/C thin film filters in a HBT-EP discharge. Signals along three chords are showed here. This shot is characterized by sawteeth instabilities and MHD oscillations. The signals show the reverse sawteeth on the signal from the channel away from the center. This indicates outward movement of thermal energy during the crash phase of the sawteeth. The termination of the discharge was also accompanied by outward thermal movemen. The mode numbers of the MHD oscillations and the exact scenario of the disruption remain to be resolved by future tomography reconstructions.

The authors would like to acknowledge the help from the HBT-EP group in the Columbia University and Dr. Raj Korde from the International Radiation Detector Inc.. This work has been supported by U.S. DoE Grant DE-FGO2-86ER-53222.

Reference list:

¹T. H. Ivers, E. Eisner, A. Garofalo, R. Kombargi, M. E. Mauel, D. Maurer, D. Nadle, G. A. Navratil, M. K. V. Sankar, M. Su, E. Taylor and Q. Xiao, (to be published by Plasma Physics)

²B. L. Henke, E. M. Gullikson, and J. C. Davis, Atomic Data and Nuclear Data Tables, Vol. 54, No. 2(1993)

Figure list:

Figure 1: Transmission of the Zr/Ti/C filter in the soft X-ray range. The thickness of Zr is 100nm. The thickness of Ti is 7.5nm. The thickness of C is 100nm.

Figure 2: Relative responses of photodiodes with various filters as functions of plasma temperature.

Figure 3: Soft X-ray signals measured by photodiodes with Zr/Ti/C thin film filters. The signals exhibit sawteeth and MHD oscillations.

Fig. 1

Fig. 2

Fig. 3