Merging Startup Experiments on the UTST Spherical Tokamak

Abstract

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.

Electron and Ion Heating Characteristics during Magnetic Reconnection in the MAST Spherical Tokamak

“…However, those heating characteristics of reconnection are still under serious discussion, indicating that direct evidence for the reconnection heating mechanisms should be provided by a proper laboratory experiment. Since 1986 the merging of two toroidal plasmas (flux tubes) has been studied in a number of experiments: TS-3 [6] [7], START [8], MRX [9], SSX [10], VTF [11], TS-4 [12], UTST [13] [14], and MAST [15]. For those laboratory experiments, evidence of plasma acceleration toward outflow direction were observed as split line-integrated distribution function in 0D [16], 1D and 2D bidirectional toroidal acceleration during counter helicity spheromak merging [17] [18], and in-plane Mach probe measurement around X point with and without guide field [19][20] [21].…”

Development of a Quasi-Steady Equilibrium Field System for Plasma Merging ST Startup Experiments on the UTST Device

“…The UTST device is currently in the preliminary experimental stage and operated with a very short pulse length (< 1 ms) with peak plasma current < 150 kA [5]. The next upgrade in the power supply system is scheduled to achieve ST discharge with plasma currents > 200 kA and flat-top times > 10 ms for neutral beam injection heating with 2 MW injection power.…”

“…PF coils #1 and #4 were employed to form two initial STs by the double null merging method [5]. After the formation of two STs, PF coil #2 was energized to accelerate them for the mid-plane of the device to merge.…”

“…Various methods of non-inductive start-ups of ST plasma, such as RF current drive [2], coaxial helicity injection [3], and plasma merging [4], are intensively investigated worldwide in ST devices. The University of Tokyo Spherical Tokamak (UTST) [5] device has been designed for high-beta ST start-up using the plasma merging method in which magnetic reconnection [6] is utilized as an effective high-power heating source to establish high-beta equilibrium in a short period.…”

Magnetic Helicity Injection Mechanism for Double-Null Startup of the UTST Spherical Tokamak Plasmas

“…In the late phase of author's e-mail: watanabe@ts.t.u-tokyo.ac.jp DNM formation, a single ST is connected to the external poloidal coil flux. Merging of two ST plasmas through magnetic reconnection was successfully observed by twodimensional pickup coil arrays, which are located in the r-z plane of the vacuum vessel [14]. On the basis of the measured 2-D magnetic field profile, poloidal magnetic flux Ψ , toroidal current density j t and toroidal and poloidal eigen values λ t and λ p respectively are calculated as follows: [15], and,…”