Observation of the velocity distribution function of magnetically and electrostatically trapped ion in the tandem mirror GAMMA10

K. Hagisawa, K. Ishii, Y. Takemura, A. Kojima, Y. Miyata, T.Cho

Plasma Research Center, University of Tsukuba, Ibaraki, 305-8577, Japan

Tandem mirror device has an open end magnetic field and consists of several mirror cells having different type of rolls. The plasma is confined in the weak magnetic field region lying between the strong magnetic fields. The ratio of the strong magnetic field to the weak magnetic field decides the confinement region in the velocity space, in other words, there is a loss region. To improve the confinement, the formation of high electric potential in both of the end cells is conducted and ions are confined electrostatically. Both magnetic and electrostatic confinement is the main feature of the tandem mirror device. In the trapped region, particles are bouncing between the magnetic hills or potential hills. So that, the observation of the velocity distribution function in the trapped region gives us novel knowledge of confinement state in tandem mirror device which was ambiguous so far.
We first used the conventional gas stripping type charge exchange neutral particle analyzer and have measured the velocity distribution function of bounce ions by adjusting the mounting angle. The apparatus is installed near the strong magnetic field which decides the confinement boundary and whose measurement region is close to the loss region. The result was compared with the end loss ion energy distribution function obtained by the end loss ion component analyzer.
Next, we set focus on measuring the velocity distribution function of ions in the main confinement region. We started developing a new charge exchange neutral particle analyzer. The main criterion for the designing is to detect the energy and pitch angle distribution widely within a few plasma shots. To achieve the objective, ultra-thin stripper foil accompanied by hemispherical electrostatic energy analyzer was adopted. The properties of this combination were simulated and the probability for applying the real experiment was examined. For the estimation of angular scattering and energy straggling by the thin foil, we used the program named TRIM98 [1]. And by using the orbit calculation, the configuration of the electric analyzer was optimized. Then we found that this neutral particle analyzer can feature its compact size, yet it can cover wide range of velocity distribution function in the trapped region. The analyzer will be able to be downsized within 300mm cubic including electrostatic analyzer (main radius is 8cm), charge conversion foil, magnetic shield etc... The detectable angle width is about 35degree in one plasma shot and the analyzing energy is 1-20keV. And, furthermore, at the high energy region, more than several keV, larger detectable efficiency can be expected, compared with conventional one.

References

[1] J. F. Ziegler, J. B. Biersack, U. Littmark, The stopping and Range of Ions in Solids, vol1, (Pergamon, New York 1985)