A centre-solenoid-free merging start-up scheme for spherical tokamak plasmas was developed in a University of Tokyo spherical tokamak (UTST) experiment by using outer poloidal field coils. Torus breakdown was initiated at null points and two spherical tokamak plasmas with a total current up to 80 kA were generated inductively. Their merging process provided substantial ion and electron heating by magnetic reconnection. The obtained dependence of heating on plasma current suggests that high-temperature and high-current plasma suitable for neutral beam injection is attainable under the realistic conditions in the merging start-up method.
Floating potential profile was measured around the X-point during high guide field reconnection in UTST merging experiment where the ratio of guide field (Bg) to reconnecting magnetic field (Brec) is Bg/Brec>10. Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the X-point during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the X-point. While field-aligned components of inductive electric field (E∥ind) and electrostatic electric field (E∥es) cancel out with each other away from the X-point, E∥ind exceeds E∥es around the X-point, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field and electric field profile, our particle trajectory analysis indicates that fast electrons with energies over 300 eV are produced within 1 μs around the X-point in the non-ideal MHD region. These results indicate that production of fast electrons or electron heating are expected to be observed in the vicinity of the X-point.
The axial merging method is one of the candidates to provide a center-solenoid-free start-up of high-beta spherical tokamak (ST) plasma. Two initially formed STs merge through magnetic reconnection in the presence of the guide (toroidal) magnetic field, which is perpendicular to the reconnection (poloidal) magnetic field. During ST merging start-up, electrons are effectively accelerated near the reconnection point where the reconnection electric field is almost parallel to the magnetic field. In order to evaluate the effectivity of this acceleration process on electron heating, the temporal and spatial distributions of generated energetic electrons are observed by a soft x-ray fast imaging system equipped on the UTST device. The energetic electrons were generated not only in the vicinity of the reconnection point but transiently in the inboard-side downstream region until the static electric field by charge separation grew to cancel the reconnection electric field component parallel to the magnetic field line. Adequate control of the downstream condition could enhance the generation of energetic electrons and provide a more effective conversion from the released magnetic energy to electron energy.
Bursts of bremsstrahlung soft X-ray (SXR) emission (> 200 eV) were observed during spherical tokamak merging in the University of Tokyo Spherical Tokamak (UTST) experiment. SXR signal waveforms coincided well with the time evolution of a reconnection electric field, and their intensity shows a clear dependence on the strength of a toroidal magnetic field. This result suggests that electrons near the X-point are effectively accelerated in the toroidal direction by a parallel electric field during plasma merging in the presence of a strong toroidal magnetic field.
Localized electron heating of magnetic reconnection was studied under strong guide-field using two merging spherical tokamak plasmas in the University of Tokyo Spherical Tokamak experiment. Our new slide-type two-dimensional Thomson scattering system is documented for the first time the electron heating localized around the X-point. Shape of the high electron temperature area does not agree with that of energy dissipation term Et·jt. If we include a guide-field effect term Bt/(Bp+αBt) for Et·jt, the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point.
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