The University of Tokyo Spherical Tokamak (UTST) was constructed to explore the formation of ultrahighbeta spherical tokamak (ST) plasmas using double null plasma merging. The main feature of the UTST is that the poloidal field coils are located outside the vacuum vessel to demonstrate startup in a reactor-relevant situation. Initial operations used partially completed power supplies to investigate the appropriate conditions for plasma merging. The plasma current of the merged ST reached 100 kA when the central solenoid coil was used to assist plasma formation. Merging of two ST plasmas through magnetic reconnection was successfully observed using two-dimensional pickup coil arrays, which directly measure the toroidal and axial magnetic fields inside the UTST vacuum vessel. The resistivity of the current sheet was found to be anomalously high during merging.

We successfully generated two spherical tokamaks (ST) at two null points by using ex-vessel poloidal field (PF) coils only and succeeded in merging them. This scheme is called a double null merging (DNM) scheme. First, two pairs of ex-vessel PF coils generate two null points where the poloidal field is zero at the upper and lower regions inside the vacuum vessel. Then a poloidal flux swing generates two STs at two null points, because the distance to the wall along the magnetic field is long at the null points. Finally, the coil currents push two STs toward the mid-plane and merge them into a single ST. Since a magnetic reconnection during merging transforms magnetic energy into thermal energy, this merged ST plasma is expected to have a high beta. It must be noted that the DNM scheme generates an ST without a center solenoid coil. The DNM scheme was demonstrated on the TS-3/4 (Japan) and MAST (UKAEA). However, these devices have all PF coils inside the vacuum vessels, and the initial plasmas were generated around the PF coils, not the null points. Since internal coils are not feasible in a fusion reactor due to high neutron flux, it is important to demonstrate the DNM scheme by using ex-vessel PF coils.

SUMMARYThe UTST (University of Tokyo Spherical Tokamak) device was constructed for the purpose of exploring the formation of ultrahigh-beta ST (spherical tokamak) plasma using the double null plasma merging method. When two plasmas merge together to form a single plasma, magnetic field lines reconnect, and magnetic field energy is converted to plasma kinetic energy, increasing the plasma beta. Merging start-up has been demonstrated in the TS-3/4, START, and MAST devices using coils inside the vacuum vessel, and the TS-3 plasma obtained a 50% beta. In order to demonstrate start-up in a more reactor-relevant situation, UTST has all poloidal field (PF) coils outside the vacuum vessel. The first plasma experiment on the UTST was begun in December 2007. In the results, the plasma obtained 10 kA by using only the outer PF coils and a single ST was generated in the lower area (z = -0.3 to -1.0 m) close to a washer gun. This result suggests that another washer gun on the top of the UTST can allow the generation of ST in the upper area and merging start-up by using outer PF coils.

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 high harmonic fast wave (HHFW) is used to heat electrons and sustain the plasma after spherical tokamak (ST) formation by the double null merging technique in the University of Tokyo Spherical Tokamak (UTST) [1]. HHFW is suitable for electron heating of high-β ST plasmas [2]. Efficient electron heating was confirmed experimentally with the National Spherical Torus Experiment (NSTX) [3]. However, parametric decay instability (PDI) in the edge plasma region and degradation of electron heating were observed with NSTX and the Tokyo Spherical Tokamak-2 (TST-2) [4,5]. It is important to confirm the excitation and propagation of HHFW in the plasma core region. In this paper, results of direct measurements of the radiofrequency (RF) magnetic field associated with a HHFW are presented.UTST is an ST device with the following typical parameters: R 0 (major radius) = 0.43 m, a (minor radius) = 0.17 m, B t (toroidal magnetic field) = 0.15 T, and I p (plasma current) = 100 kA. The frequency of the externally applied RF power (21 MHz) is more than ten times the hydrogen ion cyclotron frequency. The HHFW antenna consists of two poloidal current straps separated in the toroidal direction by 22.5• . When they are excited out of phase, the toroidal mode number of the excited HHFW is ±8 (which corresponds to the toroidal wavenumber of k φ = ±13 m −1 ). This is referred to as double-strap excitation. When only one strap is excited (single-strap exci- tation), a broader toroidal wavenumber spectrum centered around k φ = 0 is excited. A 2-dimensional (2-D) 9×9 magnetic probe array is inserted into the plasma at the poloidal section that is 45• away from the antenna toroidally. Magnetic field components in the toroidal direction (B φ ) and vertical direction (B z ) can be measured at each probe location. Data from only 25 probes (8 toroidal fields and 6 vertical fields on the midplane, and 11 toroidal fields off the midplane) were taken simultaneously in this experiment due to limitation of number of digitizers. Probes are separated from each other by 70 mm in the radial direction and 60 mm in the vertical direction. Figure 1 depicts the 2-D (z-φ) wave polarization angle (defined as tan −1 [B z /B φ ]) profile on the midplane. In these

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