Michiaki Inomoto

Experimental investigation of three-component magnetic reconnection by use of merging spheromaks and tokamaks

A laboratory experiment of magnetic reconnection has been developed in the Tokyo University Spherical Torus (TS-3) [Y. Ono et al., Phys. Fluids B 5, 3691 (1993)] merging device, using two colliding plasma toroids with cohelicity and counterhelicity. The conventional two-component reconnection was extended experimentally to three-component reconnections by introducing a new field component BX parallel to the X-line, an external force and a reconnection-driven global equilibrium transition. Selective ion heating accompanied by a field-aligned jet was documented during the counterhelicity reconnection without BX, indicating its direct energy-conversion into the ion thermal energy. Ion heating, current-sheet resistivity and reconnection rate all increase significantly with decreasing BX and with increasing the external force, indicating three-component and driven effects of reconnection. The anomalous sheet-current dissipation and the ion heating are both found to depend on whether the current-sheet is compre...

Fast compression of a current sheet during externally driven magnetic reconnection

Magnetic reconnection of two toroidal plasmas with arbitrary q values revealed a dependence of sheet-current dissipation and ion heating on ion-gyromotion. Effective sheet-current resistivity was found to increase significantly, when an external force compressed the current sheet shorter than the ion-gyroradius, and both the reconnection speed and ion temperature increased with decreasing current sheet width and with increasing ion-gyroradius.

New relaxation of merging spheromaks to a field reversed configuration

A novel high β relaxation to a field reversed configuration (FRC) has been investigated by axially colliding two spheromaks with opposing toroidal magnetic fields. The β value of the merging toroids increases from 0.1 to 0.7-1.0 within 15 μs, indicating an equilibrium transition from the low β spheromak to the high β FRC. An important finding is that the merging spheromaks relax either to a high β FRC or to another low β spheromak, depending on whether the initial normalized magnetic helicity given to these spheromaks is smaller or larger than a threshold value. This fact suggests that the FRCs are equipped with some global stability as robust as the Taylor magnetic energy minimum state.

High-beta characteristics of first and second-stable spherical tokamaks in reconnection heating experiments of TS-3

Since 1986, the centre solenoid (CS)-less formations of ultra-high-beta CT/ST plasmas have been developed in the TS-3 merging experiment using high power heating of magnetic reconnection. In the cohelicity (Type-A) merging, two STs were merged together to build up the plasma beta to βT≈0.5. In the counterhelicity (Type-B) merging, an oblate FRC formed by two merging spheromaks with opposing toroidal field Bt, was transformed into an ultra-high-beta (βT≈0.8) ST by applying external toroidal field Bt. We made the BALLOO code stability analyses of the produced STs and concluded that formations of the first-stable/marginally second-stable STs were obtained by Type-A merging and the second-stable STs by Type-B merging and also unstable STs by both mergings. The ballooning-stable regime calculated from the experimental data was almost consistent with the measured high-n instabilities. The stable regime became larger significantly by increasing the hollowness of current profile and broadness of pressure profile. This paper also addresses normalized betas βN of thus produced STs as large as 6–17 for comparison with the Troyon scaling and a promising B2 scaling of the reconnection heating. These facts indicate that the axial merging is one of the most efficient startup method for high-beta ST without powerful CS.

Ultra-high-beta spherical tokamak formation by use of an oblate field-reversed configuration

A new slow formation of oblate field-reversed configuration (FRC) has been developed in the Tokyo University Spherical Torus No. 3 (TS-3) merging experiment using two merging spheromaks with opposing toroidal field. This unique technique was extended to a novel formation of ultra-high-beta (50%–70%) spherical tokamak (ST) by applying an external toroidal field Bt,ext to the FRC so produced. The high-beta ST was found to have a diamagnetic toroidal field in sharp contrast with low-beta STs with strong paramagnetic toroidal fields. High-beta properties of FRCs including their hollow current profile were maintained during the equilibrium transition, suggesting a close relationship between FRCs and high-beta STs in the second stability regime.

Ion and electron heating characteristics of magnetic reconnection in tokamak plasma merging experiments

Recently, the TS-3 and TS-4 tokamak merging experiments revealed significant plasma heating during magnetic reconnection. A key question is how and where ions and electrons are heated during magnetic reconnection. Two-dimensional measurements of ion and electron temperatures and plasma flow made clear that electrons are heated inside the current sheet mainly by the Ohmic heating and ions are heated in the downstream areas mainly by the reconnection outflows. The outflow kinetic energy is thermalized by the fast shock formation and viscous damping. The magnetic reconnection converts the reconnecting magnetic field energy mostly to the ion thermal energy in the outflow region whose size is much larger than the current sheet size for electron heating. The ion heating energy is proportional to the square of the reconnection magnetic field component . This scaling of reconnection heating indicates the significant ion heating effect of magnetic reconnection, which leads to a new high-field reconnection heating experiment for fusion plasmas.

Intermittent magnetic reconnection in TS-3 merging experiment

Ejection of current sheet with plasma mass causes impulsive and intermittent magnetic reconnection in the TS-3 spherical tokamak (ST) merging experiment. Under high guide toroidal field, the sheet resistivity is almost classical due to the sheet thickness much longer than the ion gyroradius. Large inflow flux and low current-sheet resistivity result in flux and plasma pileup followed by rapid growth of the current sheet. When the pileup exceeds a critical limit, the sheet is ejected mechanically from the squeezed X-point area. The reconnection (outflow) speed is slow during the flux/plasma pileup and is fast during the ejection, suggesting that intermittent reconnection similar to the solar flare increases the averaged reconnection speed. These transient effects enable the merging tokamaks to have the fast reconnection as well as the high-power reconnection heating, even when their current-sheet resistivity is low under high guide field.

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

Electron and ion heating characteristics during merging reconnection start-up on the MAST spherical tokamak have been revealed in detail using a 130 channel yttrium aluminum garnet (YAG) and a 300 channel Ruby-Thomson scattering system and a new 32 chord ion Doppler tomography diagnostic. Detailed 2D profile measurements of electron and ion temperature together with electron density have been achieved for the first time and it is found that electron temperature forms a highly localized hot spot at the X point and ion temperature globally increases downstream. For the push merging experiment when the guide field is more than 3 times the reconnecting field, a thick layer of a closed flux surface form by the reconnected field sustains the temperature profile for longer than the electron and ion energy relaxation time ~4-10 ms, both characteristic profiles finally forming a triple peak structure at the X point and downstream. An increase in the toroidal guide field results in a more peaked electron temperature profile at the X point, and also produces higher ion temperatures at this point, but the ion temperature profile in the downstream region is unaffected.

A LABORATORY EXPERIMENT OF MAGNETIC RECONNECTION: OUTFLOWS, HEATING, AND WAVES IN CHROMOSPHERIC JETS

Hinode observations have revealed intermittent recurrent plasma ejections/jets in the chromosphere. These are interpreted as a result of non-perfectly anti-parallel magnetic reconnection, i.e. component reconnection, between a twisted magnetic flux tube and the pre-existing coronal/chromospheric magnetic field, though the fundamental physics of component reconnection is unrevealed. In this paper, we experimentally reproduced the magnetic configuration and investigated the dynamics of plasma ejections, heating and wave generation triggered by component reconnection in the chromosphere. We set plasma parameters as in the chromosphere (density 10^14 cm^-3, temperature 5-10 eV, i.e. (5-10)x10^4 K, and reconnection magnetic field 200 G) using argon plasma. Our experiment shows bi-directional outflows with the speed of 5 km/s at maximum, ion heating in the downstream area over 30 eV and magnetic fluctuations mainly at 5-10 us period. We succeeded in qualitatively reproducing chromospheric jets, but quantitatively we still have some differences between observations and experiments such as jet velocity, total energy and wave frequency. Some of them can be explained by the scale gap between solar and laboratory plasma, while the others probably by the difference of microscopy and macroscopy, collisionality and the degree of ionization, which have not been achieved in our experiment.

Development of multi-channel Doppler spectroscopic measurement system using 8 × 8 multianode photomultiplier tube assembly.

Using an 8 × 8 channel photomultiplier tube assembly and a single Czerny-Turner monochromator, we have developed a novel Doppler spectroscopic system which can measure the time evolutions of spectral distribution of plasma emission from eight different lines of sight simultaneously. An optical lens system is employed to couple the output of the monochromator with the detector assembly, resulting in small cross-talks less than 5% in spatial distribution together with large magnification of up to 50 in wavelength direction. The suggested system yields cost-effective polychromatic measurements of eight spatial channels with uniform optical and electrical characteristics.