J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 45V453

Construction of a Toroidal Plasma Gonfinement Device
with a Floating Internal Coil for Studying High Beta Plasmas
OGAWA Yuichi, MORIKAWA Junji, OHKUNI Kohtaro, NIHEI Hitoshi, HORI Dan,MITO Toshiyukir, YANAGI Nagator, HISHINUMA Yoshimirsur,
IWAKUMA Masataka2, UEDE Toshio3, ITO Ikuo3, NOSE Shin-ichi3 and FUKUI Shigeoa
The University of Tolcyo, Tokyo 113-8656, JapanI National Institutefor Fusion Science, Toki 509-5292, Japan

2 Kyushu University, Fukuoka

812-8581, Japan

3 F?i Etectric210-8530, rapan!' , !:r:'o!i' Cryovac Co., Ohsaka 555-0013, Japan(Received: 11 December 2001 / Accepted: 19 June 2002)

Abstract
A new relaxation theory for high-beta plasma configuration has been proposed by Mahajan-Yoshida.
For exploring this new relaxed configuration, we are constructing an internal coil device, called Mini-RT,
with a high temperature superconductor (HTS) coil. Typical machine parameters are as follows; themajor radius of the internal coil is 0.15 m and the coil current is 50 kA.turn. The vacuum chamber is I min diameter and - 0.7 m in height. We are planning to produce a neutral plasma with 2.45 GHz Electron
Cyclotron Heating system. A strong plasma flow in toroidal direction is expected by introducing a radial
electric field d. A persistent current switch should be equipped in the floating coil, because the floating
coil current is charged with the demountable electrode. Here A Ag-sheathed Bi-2223 HTS tape with 0.3
wt%o Manganese has been successfully tested for the persistent current switch. The levitation control ofthe HTS coil is another important issue. We have fabricated a small HTS coil (0.04 m in radius, and 2.6
kA.turn), and succeeded in levitating it during a few minutes with an accuracy of 20-30 micrometers.

Keywords:
two fluid relaxation, high beta plasma, internal coil device, high temperature superconductor, levitation,
persistent current switch

1. Introduction

are expecting to induce the radial electric field by

Mahajan-Yoshida has found a new relaxation stateby taking a two-fluid effect into account, and pointed
out the possibility of a new type of configuration for ahigh beta plasma confinement []. To explore this new
relaxation theory, a toroidal device with an internal coilproducing a non-neutralized plasma.

In the past several devices with a floating coil (e.g.,Spherator, Levitron) have been constructed, where alow-temperature superconductor coil has been employed[2]. In these devices the plasma is confined at the innerside of the torus so as to study several MHD concepts
such as Min-B and so on, while in our device the plasma
is produced and confined at the outer region of the torus

is suitable, where the high beta plasma would beconfined with the dynamic pressure of the plasma flow
in toroidal direction. The toroidal plasma flow can bedriven by the radial electric field by the E x B force. WeCorre sponding author' s e-mail: o gawa@ plasma. q.t.u-tokyo.ac450jp@2002 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchOgawa Y. et al., Constrtction of a Toroidal Plasma Confinement Device with a Floating Intemal Coil for Studying High Beta Plasmas

The HTS wire is Ag-sheathed Bi-2223. Thefor exploring a new relaxed state.

specification of the floating coil is listed in Table l. Themaximum magnetic field strength appears at the surfaceof the coil and is that B,= O.75 T and B. = 0.57 T. To
evaluate the residual voltage of the HTS wire, it isassumed that the n-value defined by V = V" (Il\) is 10.

We adopt a high temperature superconductor (HTS)

wire for the floating coil, expecting easy/reliable
operation and maintenance. In addition, the HTS coil ispreferable for plasma experiments, because long pulse

and/or high power heating experiments might beOperation temperature of the HTS wire is initially 20 K,
and increases up to - 40 K. Taking the decrease of thedecay constant of the coil current at the elevatedtemperature into account, we expect to keep the floating
coil during a few hours for plasma experiments. In thisavailable due to the high thermal stability and large heat
capacity of the HTS wire.
The persistent current switch should be equipped in
the floating coil, because the floating coil current ischarged with the demountable electrode. Here the Bi2223 HTS tape with 0.3 wtTo Manganese is adopted forthe persistent current switch. The floating coil should befloating coil system we need no quench protection
system.

controlled with an accuracy of a few tens of
micrometers fbr plasma experiments. The levitation
experiments vvith a small HTS coil have been carried

3. Plasma Parameters Expected in Mini-RT
In order to produce non-neutral (or non-neutralized)
plasmas, the injection of electrons through the separatrix
has been proposed, where the inward diffusion ofout.

In this manuscript, the construction of the internalcoil device called Mini-RT is described, and expected
plasma parameters and experimental plan are discussed.

The development of the persistent current switch andlevitation experiments of the HTS coil are described, aswell. This is a first challenge to apply the HTS coil for aplasma confinement device, as well as the levitation
experiment of the HTS coil.

2. Design of a Floating Coil Ddevice(Mini-RTl
Schematic drawing of the internal floating coildevice Mini-RT is shown in Fig. 1, where a vacuum
chamber is 0.5 m in radius and - 0.7 m in height. A
levitation coil is put at the top of the vacuum vessel.
Since the floating coil is unstable to the vertical motion,
the levitation coil current is feedback-controlled to keep
the floating coil position. The floating coil is re-cooledat the bottom of the vacuum vessel through thedemountable pipe, and the coil current is recharged

Fig. 1 Schematic Drawing of the internal coil device
Mini-RT, where a floating coil are re-cooled andcharged at the bottom of the torus.

directly by the power supply with the demountableelectrode, as u'ell.

Table 1 The Specification of the Floating Coil Made of HTS Wire.
ValuesParameters

R=0.15 manda =0.028m
I"P = ll7 AMajor and minor radii
Operation current of the coil

Critical cunent of the HTS wire1., = 50,000 A (428 tums)/. = 108 A at77 K, s.f., I trrVlcm

Residual voltage and inductance ofthe coil

Ro = 1.66 x 10-5 V, L = 0.091 H

Magneto-motive force (total coil current)

W-20kgWeieht of the coil451

Ogawa Y. et al., Constntction of a Toroidal Plasma Confinement Device with a Floating Internal Coil for Studying High Beta Plasmas

electrons is expected at the separatrix, by taking thechaotic motion of electrons at the magnetic null point
into account [3].
We are planning to produce a neutral plasma in therange of n - l}r6-ri m-3 with 2.45 GHz Electron
Cyclotron Heating system. As shown in Fig. 2, thecontour of the magnetic field strength of 0.1 T is locatedaround the outer side of the floating coil. We expect theneutral plasma will be produced in the neighborhood of
the contour ofB = 0.1 T. Since the ECH could produce
high energy electrons more than a few tens keV [4], thismight be helpful to produce a high beta plasma. Inaddition, some part of the extremely high energy
electrons might escape from the magnetic surface
through the separatrix region. This might yield a bulk
plasma to non-neutralize. To examine the compatibility
between high energy electron production and nonneutralization by the orbit loss, an orbit of a high energy
electron has been calculated for the magnetics.ts-t}.1-6. !

00.1

Q-8.

S.3

C,4

r. I

Fig. 2 Magnetic field configuration only with the levita-tion coil and the contour of the magnetic fieldstrength. At the hatched region a high energyconfiguration given in Fig. 2. An electron with aperpendicular energy of a few tens keV is initiallyelectron with a perpendicular energy more than
80 keV is lost.

launched at the resonance position (i.e., at the locationof B, - 0.0875 T). Only at the magnetic surface far awayfrom the internal coil, a direct orbit loss of a high energy
electron takes place; for example, an electron with anenergy more than 80 keV is not confined at a hatchedregion shown in Fig. 2.

Table 2 Typical Plasma Parameters to study High Beta
plasma Experiments.
Beta value

Temperature Radial electric field

0.ll
T = l24keY
0.1T) T = 12.4 keV
lffiVo k:tB = 0.01T) T = l.24keY
lovo 1atB = 0.01D T = 0.12 kev

Mahajan-Yoshida has found a MHD relaxation
state in two-fluid plasmas, and derived the following

Beltrami/Bernoulli condition []; i.e., B + (VplV^)2 =constant, where B is the ratio of the plasma pressure to
the magnetic pressure, and the velocities Vp and Va areplasma flow velocity and Alfven velocity, respectively.
When the plasma velocity is increasing as the plasma
radius, high beta plasmas can be confined at the coreregion of the plasma column. If the radial electric field

1007o (at B =690 kV/ml07o (at B =22okYlm
6.9 kV/m

2.2kYlmz.

The low density plasma needs a high plasma
temperature and a large radial electric field. We might
expect a high beta plasma at the outer region of theplasma column, because the magnetic field strength

is introduced in the torus plasma with the intemal coil,the E x B drift velocity is an increasing function of thedrastically decreases, as shown inFig.2.
It is not so easy to predict the plasma confinement

plasma minor radius. This is the reason why the intemal

time in this internal coil device. Here we roughly
estimate the energy confinement time necessary tocoil device is suitable for studying high beta plasmas
based on two-fluid MHD relaxation theory.

By assuming that p = 0 at the plasma surface, therelation between the plasma beta value and the radial
electric field can be derived as follows;achieve these plasma parameters. When the ECH power

is assumed to be 10 kW, the energy confinement time toachieve the plasma with p = l00%o is estimated to be 4.1
msec.

E_^[_{B - Y ponrm, {F '4. Engineering Progress of the HTS CoilWe employ the method to drive the HTS coilwhere n; and mi are ion density and mass, respectively.
This gives plasma parameters necessary for studying
high beta plasmas, and typical values are listed in Table

current through the demountable electrode. This has
some advantages in comparison with the induction452

Ogawa Y. et al., Constuction of a Toroidal Plasma Confinement Device with a Floating Internal Coil for Studying High Beta Plasmas

drive; e.g., the induction coil is not necessary, and thepower supply system becomes quite simple. However, itis necessary to introduce the persistent current switch(PCS) made ol the HTS wire in the floating coil. Thereradius and 2.6 kA.turn) and carried out the levitation
experiment [6]. The coil is covered with the casing
made of polycarbonate and immersed in liquid nitrogen.
The total weight is approximately 321 g with liquid
nitrogen. The HTS coil current is induced by the fieldcooling method. The detected signals of the floating coil

are few experiences of the PCS with the HTS wire inthe past. We use the Ag-sheathedBi-2223 tape with 0.3
wtvo Manganese, which gives high electric resistance atthe elevated temperature (the turn-off resistance of 0.21

position are transferred to the feedback system, which isQ at the critical temperature). We have measured thecharacteristics of the HTS-PCS tape for various

control (the so-called PD feedback control). The HTS
coil position can successfully be controlled within anaccuracy of 25-30 pm, and levitated for 4 min or more.
This work was supported by a Grant-in-Aid for
Scientific Research (No. 12308019) from the Ministryof Education, Culture, Sports, Science and Technologyof Japan.

composed of a proportional and differential feedback

temperatures, and confirmed the performance as thePCS for the floating coil [5].

The levitation of the HTS coil is another importantissue. We cannot expect the pinning effect for the HTS

coil, because the HTS wire is composed of multifilaments, in comparison with the bulk HTS material.
The feedback control is, therefore, indispensable. InReferences[1] S.M. Mahajan and Z. Yoshida, Phys. Rev. Lett. El,

addition, we particularly pay attention to the position
control of the floating coil, because the fluctuation ofthe coil position induces an error field, resulting in the
disturbance of the magnetic surface for the plasma
confinement. In plasma experiments, the position of thefloating coil should be controlled within a sufficiently

4863 (1998).

[2] S. Yoshikawa, Nuclear Fusion 13,433 (1973).
[3] Z. Yoshidaet al., Phys. Rev. Lett.81, 2458 (1998).
t4l H. Ikegami et al.,Phys. Rev. Lett. 19,778 (1967).
t5l N. Yanagi et al.,MT-ll, Geneva,2001.
[6] J. Morikawa, Jpn. J. Appl. Phys.40, Lt029 (2001).

high accuracy (e.g., less than 100 micrometers).

We have fabricated a small HTS coil (4 cm in453