23P124-B
Observation of Electromagnetic Fluctuation and Density Decay ofMagnetospheric Plasma in RT-1
RT-1における磁気圏型プラズマの電磁揺動と密度崩壊の観測
Haruhiko Saitoh, Zensho Yoshida, Junji Morikawa, Yoshihisa Yano and Suenori Mikami
齋藤 晴彦，吉田 善章，森川 惇二，矢野 善久，三上 季範
Graduate School of Frontier Sciences, University of Tokyo
5-1-5, Kashiwanoha,Kashiwa Chiba 277-8561, Japan
東京大学新領域創成科学研究科 〒277-8561 千葉県柏市柏の葉5-1-5
In the RT-1 device, a magnetospheric configuration generated by a levitated dipole field magnet, high beta
plasma is created by using electron cyclotron resonance heating (ECH). When the population of energetic
electrons (~30keV) is very large at low neutral gas pressure operation, a high frequency electromagnetic
fluctuation is observed. The fluctuation rotates in the electron curvature drift direction and has no clear
phase difference along magnetic field lines. The fluctuation can cause rapid loss of plasma especially in a
decay phase after the ECH power is turned off. Sufficient gas fueling suppressed the fluctuation, realizing
stable high beta confinement.

1. Introduction
The Ring Trap 1 (RT-1) device [1-2] is amagnetospheric dipole field configuration
constructed for the studies of high-beta plasma
suitable for burning advanced fuels [1-4]. The
dipole confinement concept [3] was motivated bya natural example of high-beta plasma observedin the Jovian magnetosphere. Although plasma is
trapped in bad curvature regions, it was pointed
out that dipole plasma can be stable against MHD
interchange and balooning instabilities due to theeffects of plasma compressibility [3] and magnetic
separatrix [5]. In the first series of experiments inRT-1, plasma is generated by using electron
cyclotron resonance heating (ECH). High-beta
hot-electron plasma has been successfully realized
in RT-1 [6] through the optimization of formation
conditions including the feedback-controlled
levitation of the dipole field magnet [7].
In the presence of intense energetic charged
particles in plasmas, emergence of several kinds offluctuations and instabilities are widely observed.

Fig.1. (a) Top view and (b) projection on the poloidal
cross section of the RT-1 experiment.

Existence of multiple temperature components andvelocity space anisotropy in the plasma can beenergy sources for the onset of instabilities.
Understanding of the fluctuation properties is very
important for the realization of stable confinementof high-beta plasma in the magnetosphericconfiguration. In this study, we report theobservation of high-frequency electromagnetic
fluctuation in RT-1 emerges when the population ofenergetic electrons is very large. Spatial structuresand conditions for the excitation of the fluctuation
were experimentally studied.
2. RT-1 and Experimental Setup
Figure 1 schematically shows the vacuum
chamber, magnetic surfaces, and diagnostics of
RT-1. External magnetic field is generated by a
combination of the dipole field magnet and alevitation magnet located at the top of the chamber.
Toroidal field coils are not installed at present andplasma is confined in a pure poloidal field
configuration. Hydrogen plasma was generated by
ECH with 2.45GHz microwave. Measurementswith Si(Li) and CdTe x-ray detectors, and edge
Langmuir probes show that electrons in the plasma
have multiple temperature components. Plasmapressure in the present experiments is mainly due tohot electrons of ~30keV. Due to its small crosssection of collisions with other particles, the hot
electrons have long confinement time.
For the measurements of electromagnetic
fluctuations, magnetic pickup coils (B dot probes)
are installed at an equator port (1 and 2), an upper
port (3), and a bottom port (4) of RT-1 as shown in
the figure. Pickup coils 1, 2, and 3 are located onthe same poloidal cross section. Fluctuation signals
were measured by a 500MHz broad-band
oscilloscope. The signals of magnetic probes wereconverted into frequency spectra with aconventional fast Fourier transform (FFT) method.
Diamagnetic signals and plasma pressure weremeasured by magnetic loops and Hall sensors. A
75GHz interferometer was installed for themeasurements of electron line density.
3. Observation of Electromagnetic Fluctuation
After the stop of microwave injection, theelectron density was observed to decrease withdifferent time constants. The initial rapid decreaseis due to the loss of cold electrons, and the second
slow decay is due to the effects of long-lived hotelectrons. By decreasing neutral gas pressure, very
intense hot electrons are generated by ECH. In suchcases, the plasma is sometimes unstable and theexcitation of MHz range magnetic fluctuation wasdetected in the hot electron plasma. In the slow
decay phase, burst of electromagnetic fluctuation isobserved. Temporal evolution of plasma in the
decay phase is shown in Fig.2. Just after turning off
the microwave injection at t=0s (Fig.2a), the plasma
started to decay quietly. Large magnetic fluctuation
was not observed at this point. At t=2.6ms, burst ofelectromagnetic fluctuation (Fig.2d) and rapid lossof line density (Fig.2b) were simultaneously
observed. In Fig.2c, diamagnetic signal shows thatplasma pressure decayed slowly after the onset offluctuation, suggesting that some amount of hotelectrons were trapped in this period outside thechord of the interferometer.
The fluctuation had complicated frequency
sweep behaviors, and the spectrum had several
peaks as shown in the figure. Temporal variation of
the fluctuation frequency, or chirping, with
df/dt~10MHz/ms was repeatedly observed in thefollowing phase. Emergence of the high-frequency
fluctuation was observed only when the neutral gas
pressure was low (~2mPa) and the plasma had alarge amount of hot electrons. This observationsuggests that the effects of hot electrons are
possible reasons for the onset of instability.
Electrostatic fluctuation in the similar frequency
range was reported in other experiments [8].
The spatial mode structures of the fluctuation intoroidal, vertical, and radial directions weremeasured by using two magnetic probes located atdifferent positions. The fluctuation phase had weakradial and vertical dependences when measured atedge region, suggesting that fluctuating plasma
current was located apart from the pickup coils. In
contrast, clear phase differences were observed in
the coil signals observed at different toroidal
positions. The propagation direction of thefluctuation agrees with the electron diamagnetic
motion, also suggesting that the hot electrons areenergy source for the onset of fluctuation. By using
typical parameters of field strength B=0.03T, hotelectron temperature Te=30keV, electron density
after the initial plasma decay ne~1015m-3, andassumed spatial scale length of the plasma structure
Λ=n/n'~0.1m, electron diamagnetic drift velocity is
calculated to be vD=kBTe/eBΛ~107m/s, which iscomparable to the observed toroidal propagation
velocity of the electromagnetic fluctuation.
Acknowledgments
Work supported by Grants-in-Aid for Scientific
Research (23224014) from MEXT of Japan

Fig.1. Onset of fluctuation and plasma decay. (a) Input
power of microwave power, (b) line density, (c)diamagnetic signal, (d-1) magnetic fluctuation measuredby the pickup coil, and (d-2) its power spectrum.

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