Fusion Engineering

. . . . Fusion Engineering and Design 34-35 (1997)189-191 a n d Design E L S E V I E R

Two-dimensional imaging X-ray spectrometer for plasma diagnostics

T. Aota 4, N. Yamaguchi b, M. Yoshikawa 4, K. Ikeda a, T. Ishijima 4, y . Okamoto a, A. Mase 4, T. Tamano a

a Plasma Research Center, University of Tsukuba, Tsukuba 305, Japan b Toyota Technological Institute, Nagoya 468, Japan

Abstract

A two-dimensional imaging soft X-ray monochromator is now under development. The main components of this spectrometer are a channel plate collimator (CPC) and multilayer mirrors. We have applied this monochromator to obtain soft X-ray images of GAMMA10 tandem mirror plasmas. In order to obtain clear images in a low energy X-ray regime, we have utilized and tested a focusing effect of the CPC. Ray-trace simulations were carried out under the same conditions as the experiments. After comparison of these results, we have confirmed that the focusing effect of the CPC is useful in practical use. Monochromatized two-dimensional soft X-ray images have been successfully taken on the tandem mirror experiment. © 1997 Elsevier Science S.A.

1. Introduction

It has been known since 1951 that hollow capil- lary tubes can be used as waveguides for X-rays [1]. An array of glass capillaries has demonstrated its collimating [2] or converging action [3,4] in transmitted X-rays. Recently capillary-based X- ray optical systems have been investigated in di- verse fields.

A microchannel plate (MCP) consists of tiny hollow channels in lead glass. Use of MCPs as X-ray optical components originated in the work by Yamaguchi and Aoki [5,6], which demon- strated a two-dimensional X-ray imaging spec- trometer to obtain monochromat ic images of an extended X-ray source. In this type of spectrome- ter, an MCP acts as a collimator of a large effective area and an image transducer. Then an

image of the X-ray source is converted into a parallel beam by an MCP (which is called in this case a channel plate collimator or CPC) and is monochromatized by a Bragg-type reflection mir- ror.

In order to develop an imaging spectrometer as a diagnostic instrument for fusion plasmas, espe- cially for tandem mirror plasmas, there are some technical issues that should be solved. One of them is the applicability of this spectrometer for low energy X-rays. In tandem mirror plasmas an electron temperature is relatively low (about a few hundred electron volts), therefore, soft X-ray radi- ation becomes significant. For low energy X-rays, the reflection of the X-rays inside a hollow capil- lary deteriorates the collimating action. Then it leads to a degradation of its imaging property. In order to get clearer images in the soft X-ray

0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00631-X

190 T. Aota et al. /Fusion Engineering and Design 34-35 (1997) 189-191

region, we utilize a focusing effect of the CPC which is placed at the midplane between a source a n d a detector of the source-to-detector line. This focusing effect improves the deterioration of the collimating action.

Multilayer Mirror

2. Focusing effect on the CPC

The focusing action of a CPC comes from the total external reflection of the X-rays on the inner surface of a capillary wall. The angle of the nonreflecting components of the X-rays is limited by a geometrical condition. The single-reflection components are focused on t he optical axis and the depth of focus is equal to twice the length of the capillary. X-rays reflecting more than twice come out as the diffused components.

In order to investigate the precise imaging char- acteristics of a channel plate collimator a ray- trace simulation code has been developed [7], and ray-trace simulations have been carried out under the same conditions as the experiments.

3. Experiments and ray-trace simulations

We constructed a two-dimensional imaging soft X-ray monochromator specified as follows. The CPC used has an effective area of 81 x 31 mm 2 and a ratio of its channel diameter to length is 1/80. Two Mo/Si multilayer mirrors of 100 x 50 mm 2 size are mounted on a double crystal configuration which enables us to change wave- length with a fixed two-dimensional X-ray detec- tor (in our case MCPs). The Mo/Si multilayer are deposited with 120 A layer pair thickness and 20 layer pair on silicon wafers. The reflectivity of the multilayer mirror has been measured by using synchrotron radiation. The acceptance Bragg an- gle is 15-60 ° , which defines the tunable wave- length range to be from 60-220 A.

We have applied this monochromator to ob- serve soft X-ray images of GAMMA10 tandem mirror plasmas [8]. A schematic drawing of this monochromator system and the experimental configurations are shown in Fig. 1. The two-di- mensional imaging X-ray monochromator was set

Microchannel Plate ~

Fig. 1. Schematic drawing of monochromator system and the experimental configuration. The two-dimensional imaging X- ray monochrometer was set on the central cell of the GAMMA10.

on the central cell of the GAMMA10. The field of view of the imaging X-ray monochromator was screened by an r.f. antenna. The measurements were performed through a circular aperture opened in the antenna. The diameter of this aper- ture is 50 ram. The imaging X-ray monochroma- tor was expected to get a circular X-ray image through the aperture of the r.f. antenna.

An example of the monochromatic X-ray image is shown in Fig. 2a as a contour plot. The wave- length of this X-ray image is tuned to about 20 nm. The photographic film density is transformed

- . . . . . . . . . . . . . . . . . . i

. . . . . . + . . . . . . . . . . . . . 4 ................... i . . . . . . . . . . .

9~ ............ i E ........ i .............

O I , r . . i . . . . I . . . . i . . . . i . . . . i . . . . , . . . . ~ . . . . ~

-40 -20 0 20 40 Position (ram)

Fig. 2. Experimental data of the monochromatic X-ray image taken under de-focused condition. (a) Contoured monochro- matic X-ray image. (b) Relative X-ray intensity profile plotted along the line denoted by an arrow. (c) Simulated X-ray intensity profile from ray-tracing calculation.

T. Aota et a l . / Fusion Engineering and Design 34-35 (1997) 189-191 191

~ , (a)

~ i .... I .... I .... I .... i .... i b "

1 i 1 i i i

o [ _ _ i ........... i ............... ........... i ............

~ I ! i i i " 1

I o i 1 7 1 i . . . . J . . . . . . . . t . . . . i . . . . . . . .

-40 -20 0 20 40 Position (mm)

Fig. 3. Experimental data of the monochromatic X-ray image taken under near-focused condition. (a) Contoured monochro- matic X-ray image. (b) Relative X-ray intensity profile plotted along the line denoted by an arrow. (c) Simulated X-ray intensity profile from ray-tracing calcualtion.

into the relative X-ray intensity profile plot (Fig. 2b). The separation of the circular aperture and the CPC was 1280 mm. The separation of the CPC and the MCP was 433 ram. This X-ray image seems faint. This degradation of the X-ray image is caused by the reflection of the X-rays inside the hollow capillary of the CPC.

The focusing action of the CPC can improve the X-ray image quality. An experiment under a neat-focusing condition was performed. The sepa- ration of the circular aperture and the CPC is 1190 ram, and that of the CPC and the MCP is 1043 mm. Experimental results under the near-fo- cusing condition are shown in Fig. 3a and b. The results indicate that the X-ray image under the near-focusing condition has a sharper cut around the edge of the aperture than that of the unfocus- ing one, and even the distance from the X-ray source to MCP under the focusing condition is longer than that under the unfocusing condition by about 60%. In order to investigate the perfor- mance of the focusing action, the ray-trace simu- lations have been carried out under the same conditions as the experiments. In this simulation code, X-rays from a point source are traced through a CPC to a detector plane. The calcu- lated point source image was convolved over the circular aperture of 50 mm diameter. The calcu- lated intensity profiles under both experimental

conditions are shown in Fig. 2c and Fig. 3c. Comparing these simulation results, one can find that the image at the near-focusing condition has a column-like intensity profile and on the other hand the image at the unfocusing condition has a conical intensity profile. Then the near-focusing configuration would provide sharper image char- acteristics.

4. C o n c l u s i o n

We have constructed a two-dimensional imag- ing soft X-ray monochromator and applied it to obtain soft X-ray images of GAMMA10 tandem mirror plasmas. Monochromatized two-dimen- sional soft X-ray images have been taken success- fully on the tandem mirror. We have confirmed that the focusing effect of the CPC is useful for practical use. We could get sharper X-ray images by setting the focusing condition more precisely.

A c k n o w l e d g e m e n t s

The authors acknowledge the GAMMA10 group members of the University of Tsukuba for their collaborations. This work has been sup- ported in part by a Grant-in-Aid for Scientific Research from the Ministry of Eduction, Science and Culture, Japan. A part of the work has been performed under permission of the Photon Fac- tory Advisory Committee (Proposal No. 93G323).

R e f e r e n c e s

[1] P.B. Hirsh and J.N. Keller, Phys. Soc. (London), (1951) B64. [2] D. Mosher and S.J. Stephanakis, Appl. Phys. Lett., 29 (1976)

105. [3] S. W. Wilkins, A.W. Stevenson et al., Rev. Sci. Instrum., 60

(1989) 1026. [4] M. Kumakhov and F.F. Kumanov, Phys. Rep., 191 (1990)

290. [5] N. Yamaguchi and S. Aoki, Jpn. J. Appl. Phys., 26 (1987)

2111. [6] N. Yamaguchi, S. Aoki and S. Miyoshi, Rev. Sci. Instrum.,

58 (1987) 43. [7] T. Aota, N. Yamaguchi et al., in preparation. [8] M. Inutake et al., Proc. 14th Int. Conf. Plasma Phys. Contr.

Nucl. Fusion Res., Wfizburg. 1992 (IAEA, 1993), Nucl. Fusion Suppl. Vol. 2, p. 651.