An anomalous transport in torus magnetic confinement plasma is expressed as

a product of electron density and potential fluctuations. In order to elucidate the

anomalous transport experimentally, it is necessary to measure simultaneosly lo-

cal electron density and potential fluctuation. A heavy ion beam probe (HIBP)

is only a diagnostics device to be able to measure electron density and potential

simultaneously with high temporal (～ μs) and spatial resolution (～ mm) in high

temperature plasma (1 keV ～). In the HIBP diagnostics, electron density and

potential fluctuations are measured as the fluctuations of detected beam current

and change of beam energy, respectively. However, such simultaneous measure

ments in high temperature plasma have never been performed except ISX－B [l],

TEXT(－U) [2] tokamaks.

In Compact Helical System (CHS), the HIBP has been used to measure mainly

the potential profile and its dynamics, which is much larger displacement than

fluctuation, and density fluctuations. The first purpose of this thesis is to extend

the potential ability of the HIBP and to achieve the simultaneous measurements

of electron density and potential fluctuations in all radial positions in CHS plasma

by improving its ion source of a part of the ion gun of HIBP. The second pur-

pose of this thesis is to evaluate path integral effect, which is a well-known and

long-standing problem for the HIBP diagnostics, and to reconstruct local density

fluctuation. The detected beam current fluctuation of HIBP contains information

of local electron density fluctuation at ionization point and fluctuations along pri-

mary and secondary beam trajectories. The latter effect of beam trajectory is

called a path integral effect. Several previous articles l3-6] exist on simulating

influences of its effect, assuming electron density and temperature and electron the

density fluctuation profiles. In this thesis, a method is proposed to eliminate the

path integral effect in real

tron density fluctuation profile.

2.1. Improvement oHon Source

In CHS an ion source of alkali zeolite emitter type is used for the HIBP; the

cesium ions are released from the high temperature zeolite heated up to ～1000 ℃.

Previously, in an old socket, the cesium zeolite is indirectly heated through the

ceramic case heated by a filament (Fig. 3.1(a)). The indirect heating of the

socket of this type may prevent the zeolite, from attaining the sufficiently high

temperature. Then, the structure of the ion source is newly developed to increase

the higher beam current by direct heatly heating the zeolite.

The new socket structure is shown in Fig. 3.1(b). This socket allows heating

directly the zeolite, by setting the filament inside the zeolite. Using this socket of

direct heating, ten times higher beam current compared with the old is extracted

from the present ion source.

Using the direct heating socket, simultaneous measurements of electron density

(detected beam current of HIBP) and potential fluctuations are successfully per-

formed in low density (

plasma in CHS. The major and minor radii of the CHS plasma

radius

The fluctuation distribution measurements were done with 2 mm resolution over

the whole radius form center to near the last closed field surface (LCFS) [7], as

is shown in Fig. 3.9. It is the first achievement as the simultaneous fluctuation

measurements for all radial positions in high temperature torus plasma, because

the previous works did not measure them simultaneously in center region because

of insufficient beam energy and/or detected beam current. The fluctuation spectra

measured in CHS shows broad bands characteristics to indicate turbulence nature.

It is found that fluctuation amplitudes become larger toward the plasma edge as

is similar to the observations of ISX-B and TEXT(-U).

3.1. Reconstruction local electron density fluctuation profiIe

－2Σ

+ Σ Σ

where,

rate and local electron density fluctuation rate point, ionization rate and integral

weight, respectively.Where

tron density, respectively, andδindicates fluctuation. The ionization rate can be

estimated as a function of electron density and temperature from Lotz's empirical

formula [8]. The electron density and temperature can be given with Thomson

scattering measurement. The bracket with subscript E, ＜ ＞E, represents the en-

semble average,and the terms including the bracket are the correlations between

density fluctuations at two spatial points. If the correlation terms of fluctuations

are evaluated, local electron density fluctuation is estimated by solving the above

integral equation.

On the right-hand-side of Eq. (1), the second and third terms are effects around

ionization point and along the beam trajectories, respectively. They are under-

stood if assuming a limiting case that the fluctuation should have the infinitesimal

short correlation length and the ensemble averaged terms should be expressed as

theδ-function. In addition to this, several simplification makes the Eq. (1) take

the followlng form, as

where the first , second and third terms represent the local density fluctuation, the

screening effect and the accumulating effect. From this simplification, the path

integral effect is found to be composed of the screening and accumulating effects.

The degree of the path integral effect, whose coefficient ζ, is estimated from the

sum of the second and third terms in Eq. (2).

Fig. 4.2(a) shows the dependence of the coefficient on electron density and

temperature, using cesium ion beam. The coefficient is calculated for our HIBP

geometry and CHS plasma assuming that the correlation length is 1 cm and the

used beam is cesium ion. Here, the primary and secondary beam trajectory lengths

are 0.2 m, corresponding to the CHS plasma radius. In this calculation, the

coefficient is 0.1 when

path integral effect can be almost negligible. On the other hand, the coefficient

is 1 when Te ～ 1 keV and

fluctuation amplitude is twice as local density fluctuation, and the path integral

effect is dominant.

The ensemble averaged terms can be evaluated if the fluctuations correlation be-

tween two spatial points is assumed as the Gauss function, ƒ( Δ

where

nels, so that the correlation length

of the channels, Δ

tion function into Eq. (2), the local electron density fluctuation amplitude profile

can be reconstructed. Fig. 5.6(b) shows an example of the reconstruction for the

case that the electron density and temperature are ～ 10

spectively. Here, the integral equation is solved after several times iteration. The

detected beam current and the local electron density fluctuation amplitudes are

within the range of error in the outer region (

region (

the local fluctuation amplitude.

The reconstruction method of local density fluctuation is extended to the esti-

mation of density fluctuation spectra. It is apparent that the power spectra can be

evaluated simply by appling the above method to the fluctuation power at each

frequency. However, the integral equation for low frequency (< 50 kHz) cannot be

often solved with iteration method. This is caused by the fact that the correlation

length appears to become longer than the actual value.

This is considered to result from the path integral effect on the correlation length.

The fluctuations of local three channels are strongly affected by the fluctuation

in the outer plasma regions, and the fluctuations at local three channels become

to show quite similar behavior. A technique is derived to correct this longer

correlation length. Thanks to the technique, the corrected correlation length is

evaluated and the spectra of local density fluctuation are successfully obtained.

Fig. 6.3(b) shows one example of the reconstructed spectrum of local electron

density fluctuation, whose position is

The result shows that the path integral effect is larger in low frequency than that

in high frequency.

As a result of the estimation of local density, the detected beam fluctuation

can be roughly regarded as the local density fluctuation in low density regimes of

ne ～ 5 × 10

3.9 reflects local density fluctuation. A rough comparison between the density

fluctuation and potential fluctuation (normalized by electron temperature) allows

examining the validity of the Boltzmann relationship. Fig. 7.1(a) shows the nor-

malized density fluctuation as a function of normalized potential fluctuations. In

Fig. 7.1(a), the different marks correspond to the different ranges of radial posi-

tion of the plasma; red and green and blue plots are data point in 0 <

0.3 <

squared method. The results show that the Boltzma- relationship should be

valid, although tendency is found that the normalized density fluctuation should

be larger than the normalized potential fluctuation.

The simultaneous measurements of electron density and potential fluctuations

are successfully made with heavy ion beam probe after the development of a new

ion source. The measurement is achieved in a wide range of radial region with

spatial resolution of 2 mm. A method is proposed to evaluate the path integral

effect that is a long-standing problem for density fluctuation measurement with

the HIBPs. Applying this method on the CHS fluctuation measurements, the local

density fluctuation and its power spectrum are successfully evaluated., 総研大甲第927号}, title = {重イオンビームプローブを用いた揺動分布計測と経路積分効果の評価法の確立}, year = {} }