J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)183–186
000–000

Characteristics of Electron Cyclotron Resonance Heating Plasmasand Experimental Plans for Formation of Radial Electric Field
in an Internal Coil Device Mini-RT
OHKUNI Kotaro, OGAWA Yuichi, MORIKAWA Junji, HORI Dan1, YAMAKOSHI Shigeo1,
GOTO Takuya1 and YANAGI Nagato2
1

High Temperature Plasma Center, The University of Tokyo, Tokyo 113-8656, Japan
Graduate School of Frontier Science, The University of Tokyo, Tokyo 113-0033, Japan2
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 9 December 2003 / Accepted: 27 September 2004)

Abstract
The relaxation state under the condition of a strong plasma flow is represented by the double Beltrami field. The
generalized Bernoulli condition gives a simple relation between the flow velocity and the static pressure. An internalcoil device is suitable for exploring this new relaxation state experimentally, because an appropriate radial electricfield induces a strong toroidal flow. An internal coil device, Mini-RT is constructed and the plasma production isstarted. The magnetic field strength near the floating coil is around 0.1 T, and a microwave of 2.45 GHz is applied forthe plasma production. The Mini-RT plasmas were produced with the supported coil. Several parameters have beenmeasured by Langmuir probes. We expect the longer confinement time under the low pressure or the coil levitationand plan the electrode biasing experiment in order to realize a high beta plasma.
Keywords:
high beta plasma, plasma flow, radial electric field, internal coil device, Langmuir probe measurement,biasing electrode

1. Introduction

Mini-RT first plasma is produced with the supported coil.
Several parameters have been measured by Langmuir probes.
In order to excite a strong plasma flow in a toroidal direction,
we are planning to introduce a radial electric field and to
make the E × B plasma flow. The radial electric field in theplasma would be produced by several techniques. The ECH
could produce high energy electrons more than a few tenskeV [2]. In a dipole experiment, the high-energy electrons
more than 10 keV were produced in the low backgroundpressure such as a few micro Torr [3]. Some part of theextremely high energy electrons might escape from themagnetic surface by orbit loss. This might yield the nonneutralized plasma and the build-up of the radial electric field.
Another idea is proposed for injecting electrons through theseparatrix. By utilizing the chaotic motion, some part ofelectrons can travel into the plasma [4]. A direct insertion of
the electrode inside the plasma would be a moststraightforward method to drive the radial electric field, asdemonstrated by the CCT tokamak for achieving H-mode
plasmas [5]. Figure 1 shows the schematic drawing of an idea
for confining a high beta plasma with an internal coil device.

The relaxed state developed by the new relaxation theory[1], gives the strong coupling of the static pressure over themagnetic pressure β and the plasma flow velocity v, that is, β+ (v/vA)2 = constant, where vA is Alfven velocity. To explore
this new relaxation state experimentally, a toroidal device withan internal coil is suitable. By introducing a radial electricfield with appropriate methods discussed later, we could drivea toroidal plasma flow given by vt = Er /Bp, where vt , Er and
Bp are toroidal plasma flow velocity, radial electric field andpoloidal magnetic field, respectively. If the radial electric field
is uniform, the poloidal magnetic field decreases and thetoroidal plasma flow velocity increases with the radial
direction. We expect to confine the high-beta plasma at thecore region by utilizing this toroidal flow distribution. In aninternal coil device Mini-RT, a high temperature superconductor (HTS) coil is applied. This coil, with a major radiusof 150 mm and an averaged minor radius of 40 mm, is madeof Bi-2223 tape with 428 turns and can be operated at currents
up to 50 kAturns. The magnetic field strength near the coil isaround 0.1 T, and a microwave of 2.45 GHz is applied forthe electron cyclotron heating (ECH) plasma production. The
vacuum chamber is 1 m in diameter and 1 m in height. The
Corresponding author’s e-mail: ohkuni@plasma.q.t.u-tokyo.ac.jp1183©2004 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchOhkuni K. et al., Experimental Plans for Formation of Radial Electric FieldFig. 1Fig. 2

A cross section of Mini-RT and a magnetic field
configuration with internal coil current 25 kAturns.

Fig. 3A photograph of Mini-RT plasma with the supported
internal coil.

Schematic drawing of an idea for confining a high beta
plasma with several methods to produce the radial
electric field.

2. Experimental setup
The confining magnetic field is produced by a HTS coilsupported by three poles. This experiment is carried out using
ECH plasmas produced by 2.45 GHz microwaves. The
electron density and its temperature are determined by theuse of a double probe and a triple probe. A poloidal crosssectional view of Mini-RT is shown in Fig. 2. The microwaves
were injected from upper inclined port and the probes wereinserted from horizontal ports. The triple probe and the double
probe are 135 and 90 degrees apart from the microwaves inthe toroidal direction, respectively. The density andtemperature measured by the triple probe were indicated inthis paper, however, there are the difference between thosemeasured by the double probe and those measured by thetriple probe, that is, the former are almost 3/4 and 2/3 of thelatter, respectively. The coil current and the microwave input
power are ~30 kAturns and ~2.6 kW, respectively. The
operating pressure is kept between 0.02 and 0.05 Pa, whichis measured by ionization gauge put next to the gas-supplying
port. The plasma density is about 1016 m–3 with a typicalelectron temperature range of 10 to 20 eV.

microwave input power were 28 kAturns and 550 W,
respectively. The triple probe was fixed at 240 mm, that is,
30 mm apart from the internal coil. In this condition, theelectron density peaking around 0.03 Pa is about 2 × 1016
m–3 and its temperature is almost constant at 18 eV. The
density decreases toward the lower pressure region under 0.01
Pa, where we challenged the plasma production with apreliminary ionization. The ionization ratio at the maximum
density is 0.1% in this condition. Figure 5 shows themicrowave input power dependence of electron temperatureand its density. The coil current, the operating pressure andthe probe position were 25 kAturns, 0.02 Pa and 240 mm.
The density increases with the input power and about 5 ×
1016 m–3 at 2.6 kW, which is the highest density in this present
experimental series. The temperature is mostly constant or
gradually decreases form 18 to 12 eV. Figure 6 shows theradial profiles of electron temperature and its density. The coil

3. Experimental results
A photograph of Mini-RT plasma with the supported coilis shown in Fig. 3. We examined the characteristics of theplasmas about the pressure dependence, the power
dependence and the radial profiles of electron temperature anddensity. Figure 4 shows the pressure dependence of electron
temperature and its density. The coil current and the2184

Ohkuni K. et al., Experimental Plans for Formation of Radial Electric Field

current, the microwave input power and the operating pressure
were 22 kAturns, 1.3 kW and 0.02 Pa, respectively. The triple
probe moved from 230 mm to 400 mm. The density decreases
with the radial direction, and the temperature is almost
constant at 18 eV.

4. Discussions
4.1 Confinement time

Fig. 4

Energy confinement time τE ~ neTV/P derived from themicrowave input power P ~ 103 W, the density n ~ 1016 m–3,
the temperature T ~ 10 eV and the plasma volume V ~ 10–1
m–3 is about 10–6 s. We would like to compare it with otherspecific time. Electron-neutral collision time is 10–6 s, because
the ionization ratio is 0.1% in this experiment. The grad-B
and curvature drift speed is about 102 – 103 m/s when thetemperature T and the magnetic field B are about 10 eV and
10–3 – 10–2 T, respectively. The averaged time of toroidal drift
electrons to conflict with the coil-supporting poles is about
10–4 s. At present, the energy confinement time would begoverned by the electron-neutral collision time. If the plasma
could be produced at the lower pressure, the increase of theenergy confinement time would be expected because of thereduction of the electron-neutral collision, and would befollowed by the limitation due to the drift conflict time to thecoil-supporting poles. We expect the longer confinement timeat the lower pressure and the coil levitation. In the past
spherator experiment, the confinement time at low pressure
is ten times as long as that at high pressure, and more, theconfinement time with a levitated condition is two times aslong as that at low pressure [6,7].

Pressure dependence of electron density and electrontemperature.

4.2 Electrode biasing for radial electric field

Fig. 5

We plan to have a toroidal flow produced by the radialelectric field. A plasma potential (Vs) is estimated by anelectrode floating potential (Vf ) and an electron temperature(Te), such as Vs = Vf + 3Te. In this experiment, the plasma
potential has 30 to 40 V and almost same shape with theelectron temperature profile shown in Fig. 6. As we need toenhance the radial electric field, the electrode biasing method
is prepared. The toroidal velocity is approximated by Er/Bpand the damping is balanced by the toroidal component of
the J × B force [8]. The biasing electrode has a cylinder form
with 60 mm diameter and 60 mm height.
The electrode is considered a single probe, and then theelectron and the ion saturation current are 85 mA/cm2 and3 mA/cm2 at the electron temperature 10 eV and the electron
density 1016 m–3, respectively. The electron saturation current
is drawn by the 100 V bias voltage. We intend to prepare anelectrode with the surface area 100 cm2 and a power supply
with more than 100 V – 10 A specifications.

Input power dependence of electron density andelectron temperature.

5. SummaryFig. 6The internal coil device Mini-RT is constructed toexplore the new relaxation state experimentally, and toproduce the high-beta plasma. The Mini-RT plasmas havebeen produced with the supported coil by using 2.45 GHz

Radial profiles of electron density and electrontemperature.

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Ohkuni K. et al., Experimental Plans for Formation of Radial Electric Field

radial electric field by the electrode biasing or other methods.

microwave and several parameters have been measured byLangmuir probes. The electron density is peaking around 0.03
Pa and its temperature is almost constant in the pressure
alteration experiment. The ionization ratio is 0.1% in thiscondition. The density increases with the input power andabout 5 × 1016 m–3 at 2.6 kW, which is the highest density inMini-RT at the present. The density decreases and thetemperature is almost constant with the radial direction.
Energy confinement time derived from the microwave input
power is about 10–6 s, and we expect the longer confinement
time under the low pressure or the coil levitation in the next
experimental programs. As the plasma potential is almost
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