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Application of Plasma Heating Technology for Frontier Science

Keishi SakamotoPlasma Heating Laboratory, Naka Fusion Research Est., Japan Atomic Energy Research Institute !"#$% !"# &311-0193 '()$%*$%+,- 801-1
Abstract
The plasma heating technology developed for fusion research, in particular power source for electron cyclotron
heating (gyrotron), and its application are discussed. Recent progress of gyrotron and related technology accelerated
the application to other field of science and technology. Some examples of application are introduced.

1. Introduction
In a fusion research, many technologies have been
newly developed. As for a plasma heating, both neutralbeam and RF technologies have contributed to theheating and current drive experiment on fusion devices.
In particular, a gyrotron, a high power mm wave sourcefor electron cyclotron heating, has been and realized astable power output at various frequency range from
28GHz to sub-millimeter region. A feature of the mm
wave is: (a) loss tangent of dielectric of most materials
increase with frequency f, and ohmic loss on the metal
surface increase with f 0.5, (b) cut-off density in the unmagnetized plasma is high, which increase as f 2, (c)power distribution of the mm wave beam can becontrolled using a combination of phase correction
mirrors, which will be a merit for plasma processing.
Previously, the gyrotron has been used only forfusion research. However, the gyrotron technology has
been progressing and the new applications of the mm
wave are proposed. In this paper, the gyrotron and its
new application to the various areas except the plasma
heating are introduced.
2. High power mm wave source (Gyrotron)
In 1990’s, major progress on the gyrotron research
was made. In JAERI, the 170GHz and 110GHz gyrotrons
with synthetic diamond window and a depressedcollector have been developed for ITER and JT-60U
application, respectively [1]. In JT-60U, the 4MW
system is under operation with four-1MW/110GHz
gyrotron, and demonstrated the electron heating(~26keV), suppression of instability (neoclassical tearingmode), current drive. Also, a 28GHz gyrotron has beendeveloped with Plasma research center, Univ. of Tsukuba.
Up to now, the power output of 0.5MW, and maximum
efficiency of 50% was obtained without depressedcollector.
In Fig.1, a picture and a configuration of the recent
gyrotron are shown. The gyrotron is installed in thesolenoid coil. A rotational electron beam is injected intoRFSolenoid
CoilWindow(Diamond)Mode
converter
CavityElectronGun

Fig.1 Picture and cross section of 170GHz gyrotron for ITER.
Hight of the gyrotron is ~3m.

a cavity, where the high power oscillation occurs with amechanism of electron cyclotron resonance maser. The
magnetic field at the cavity Bc should be setas f (GHz) ≥ 28 Bc / γ . Here, f an oscillation frequency, γ arelativistic factor of the electron beam. For example,
Bc~1.08T at 28GHz operation.
The oscillation mode is usually high order mode thathas a complex configuration of electromagnetic field,
which is converted to the parallel beam mode using aquasi-optical mode converter and outputted through thewindow. Inner surface of the converter has a small
periodic perturbation. During passing through theconverter, the many modes are excited. These modes
overlap at the end of the converter, and the single peaked
power distribution is formed at the end of the converterto suppress the diffraction loss. The RF beam can bereformed to arbitrary shape such as Gaussian, flat
distribution, multi-beams [2] with a combination ofphase correction mirrors.

3. Application of gyrotron to frontier science
1) Plasma processing
Research of plasma processing is currently carriedout at a frequency of ~2.45GHz, or MHz band, andsignificant progress has been obtained. Up to now,
example of mm wave application on plasma processing
is small. However, mm wave has a merit that a uniform
or arbitrary designed power distribution can be formed
with phase correcting mirrors. This feature has apotential to make the large sized uniform or arbitrary
shaped plasma.
2) ECR ion/plasma source
ECR (electron cyclotron resonance) ion source iscommonly used for accelerator (ECRIS) using low
frequency microwaves. Modern heavy ion accelerator
such as LHC (Large Hadron Collider) at CERN, GSI inDarmstadt, MUSES at Riken, requires an intense heavy
and multi-charged ion beam like Pb27+, at ~1mA. For thispurpose, 10kW level 28GHz gyrotron has been tested asa power source at CEA Grenoble (new generationECRIS). The RF power is injected into the mirror
configuration in parallel with the magnetic field, andpresented higher performance than lower frequency. Innext project, ion beams of U60+, Cu29+ and Kr36+(fullionized), are planned [3].
3) Gyrotron amplifier
Using a technology of gyrotron oscillator, a gyrotronamplifier (gyro-klystron) is under development. The
gyro-amplifier is useful for high gradient accelerator,
high-resolution radar, communication, etc. For next
generation linear collider (TeV collider), gyro-amplifier
is considered as a candidate of drivers. Some designs
have conducted for short pulse but high power using ahigh voltage electron beam (~0.5MeV). Design values ofpower
andfrequency
are100MW/17GHz,
55MW/34GHz, 20MW/91GHz, etc [4].
4) ESR/NMR/Plasma scattering measurement
The gyrotron frequency is extended up to 889GHz
with 3rd harmonic oscillation at Univ. of Fukui [5]. By
controlling the magnetic field, the oscillation mode shifts.
Consequently, the frequency changes in stepwise. The
high frequency (sub-mm wave) gyrotrons are applied asa power source of spectroscopy for ESR (electron spinresonance) [6], NMR (nuclear magnetic resonance)[7],and plasma scattering measurement [8].
5) Microwave beaming propulsion
By focusing the mm wave power with a parabolic
mirror, the air discharge arises. A picture of air discharge
is shown in Fig.2. Shock wave at the discharge cause adriving force on the parabolic mirror. The experiment ofmicrowave beaming propulsion was confirmed using
1MW/170GHz gyrotron under the collaboration between
Univ. of Tokyo and JAERI. Short pulse RF (~0.1msec)

Mirror(a)RF(b)Fig.2: (a) Air discharge with RF beam(170GHz/0.5MW)
focusing by parabolic mirror. (b) Experiment of
microwave beaming propulsion by injection of mm wave
(170GHz/0.5MW).

is injected to the thruster. Momentum coupling
coefficient of a conical thruster Cm defined as a ratio of
propulsive impulse to the input power was 400N/MW,
which is larger than that obtained at air discharge withlaser [9].
4. Conclusion
A major progress has been obtained in thedevelopment of ECH technologies in 1990’s with theITER and other fusion projects. The applications of mmwave to frontier science have increased with the gyrotron
progress at wide range of frequencies. Because thetechnology of the mm wave is universal, newapplications of high power mm wave will be proposed
and accelerated further.
References[1] K.Sakamoto, et al., Nucl. Fusion, 43 (2003) 729.
[2] Y.Mitunaka, et al., Proc.of 22th Int.Conf. on Infrared andmm waves, (1997) 19.
[3] A.Girard, et al., Nuclear Fusion, 43, (2003) 1520.
[4] W.Lawson, et al., Proc.of 25th Int.Conf. on Infrared and mmwaves(2000)110.
[5] T.Idehara, et al., IEEE T.Plasma Sci., 27 (1999) 340.
[6] S.Mitudo, et al., ibid., Proc.of 29th Int.Conf. on Infrared andmm waves (2004)133.
[7] M.K.Hornstein, et al., ibid., 147.
[8] I.Ogawa, et al., ibid., 263.
[9] T.Nakagawa, et al., J of Space. and Rocket, 41 (2004) 151