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Edge electron temperature fluctuation is evaluated by the use of fast voltage sweeping technique on TST-2. The validity of obtained current-voltage characteristic curve was checked by comparing the time evolutions of floating potential between that obtained from the fast voltage sweeping technique and that measured with floating probe method. Good agreement between them was confirmed. We also found that fitting errors in the evaluation of the electron temperature itself are less than 10% of fluctuation levels of the electron temperature. Therefore the accuracy of the technique is applicable to study of plasma fluctuations.

A double-pass Thomson scattering system, in which a laser pulse makes a round trip through the plasma, was constructed. Using the same optics and a fast detection unit, we can resolve backward and forward scattering pulses in the signal. Because these scatterings reflect velocity distribution along different directions, electron temperature anisotropy can be estimated from the double-pass Thomson scattering system.

The amplitudes of electron temperature fluctuations are evaluated in the edge plasma of TST-2 using a new technique. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the magnitude of the floating potential. High/low electron temperatures are obtained in low/high floating-potential phases. The relationship between the electron temperature fluctuations and the time-averaged electron temperature gradient is discussed.

The spatial distribution of the radiofrequency (RF) magnetic field associated with a high harmonic fast wave (HHFW) was measured using an array of magnetic probes in the plasma inside the University of Tokyo Spherical Tokamak (UTST). Data obtained from 25 probes (19 locations for toroidal polarization and 6 locations for vertical polarization) distributed along the poloidal cross section were analyzed. The RF magnetic field is polarized in the toroidal direction, indicating that the HHFW is excited in the plasma. The RF field is weak on the inboard side. Analysis of the group delay suggests that the waves travel long distances in the plasma, probably because of poor absorption. No indication of parametric decay was observed up to 80 kW of injected RF power.

A new 50GHz microwave interferometer was installed, and non-inductively sustained spherical tokamak plasmas in the TST-2 device were measured. The line integrated electron densities (nels) on five chords were compared with visible CCD camera image and equilibrium analysis. It is concluded that the high-density region has a C-like shape along the outboard boundary. This shape suggests that the electron density is not constant on the magnetic flux surface. c

Electron temperature is one of most important parameters in all fusion devices, including spherical tokamaks (STs). STs can generate high pressure with a smaller magnetic field compared with conventional tokamaks. However, STs are characterized by MHD events which cause significant distortion in the temperature profile[1]. Thomson scattering measurement is a highly reliable measurement of electron temperature, and many devices are equipped with a system. Recently the system in TST-2 was upgraded and more accurate and new results are reported in this paper. This system consists of the following three components, incident optics, light collection optics, spectroscopic system. We use a compact Newtonian mirror system consists of a primary spherical mirror of diameter 254 mm and a small secondary flat mirror for the collection optics[2]. The system has large solid angle (88 msrad). For incoherent Thomson scattering, we use Nd:YAG laser with a wavelength of 1064 nm, with a pulse width of 10 ns, with a repetition rate of 10 Hz. The former low energy (0.45 J) laser has been replaced by a new high energy (1.6 J) laser, and the signal to noise ratio was improved from 5 to 10. The Calibration method of the polychrometar was modified, and the wavelength resolution was improved from 2.5 nm to 0.24 nm as a result. Recently, we introduce an 8-channel high speed oscilloscope, and thus all 5 wavelength channels can be recorded simultaneously, and fitting becomes more accurate. Rayleigh scattering is used for the calibration of density, and multi spatial point density (and temperature) measurement is possible. We measured intensity of scattering light and stray light in the calibration. Typical results are show in the following. The typical central electron temperature and the electron density are 150 eV and 1.5 × 10 19 m −3 , respectively. The temperature profile has a maximum and flat near the center of the plasma, the density profile also has an maximum near the center. When instability is observed in the radiation signal, the temperature becomes low. Dependence of the temperature on other parameter will be reported.

A new technique is used to estimate the amplitude of temperature fluctuation in the edge plasma on TST-2. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the intensity of the floating potential. High/low electron temperatures are obtained in low/high floating potential phases. As a result, normalized temperature fluctuation levels are found to be about 19 % or larger.

In the TST-2 spherical tokamak, non-inductive start-up experiment using electron cyclotron heating (ECH) at 2.45 GHz are performed. Hydrogen and deuterium discharges were compared after sufficient wall cleaning by Ohmic discharges to ensure discharge reproducibility. After the wall cleaning, wall recycling seemed to be reduced, because additional gas puffing was necessary to sustain a discharge. Even in that case, hydrogen emission measurement revealed the existence of finite wall recycling source. A current jump occurred earlier in the deuterium plasma than in the hydrogen plasma. Except for the timing of the jump, the discharge time evolution and various values such as the plasma current were almost the same for the deuterium and hydrogen plasmas.