Yasushi Ono

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.

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

A degradation of heating efficiency was observed during high harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) occurred. Suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high ? plasmas. In order to understand PDI, measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), arrays of RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry and fast optical diagnostics in TST-2. The frequency spectrum usually exhibits ion-cyclotron harmonic sidebands f0 ? nfci and low-frequency ion-cyclotron quasi-modes (ICQMs) nfci. PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna. The lower sideband power was found to increase quadratically with the local pump wave power. The lower sideband power relative to the local pump wave power was larger for reflectometer compared with either electrostatic or magnetic probes. The radial decay of the pump wave amplitude in the SOL was much faster for the ion saturation current than for the floating potential. These results are consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the low-frequency (ICQM). Two additional peaks were discovered between the fundamental lower sideband and the pump wave in hydrogen plasmas. The frequency differences of these peaks from the pump wave increase with the magnetic field. These decay modes may involve molecular ions or partially ionized impurity ions.

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.

Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case

The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field ( E→es) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→es around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field ...

Numerical study of energy conversion mechanism of magnetic reconnection in the presence of high guide field

The first two-dimensional particle in cell simulation of the reconnection region of two merging torus plasmas lead us to quantitative studies on the energy conversion mechanism under a high out-of-plane (guide) magnetic field condition. Even with the existence of the strong guide field, magnetic reconnection causes the efficient conversion of in-plane (poloidal) magnetic field energy. The ratio of the plasma kinetic energy flux of ions to that of electrons is roughly two to one, in agreement with the recent experimental results. Due to the suppression of nonlinear dynamics of ions motions in the vicinity of the reconnection region with guide field, the major energy flux of ions is changed to the flow energy flux. For electrons, a field-aligned acceleration caused by parallel electric field generates the non-thermal electrons through trapping (bouncing) effect, which is exhausted as the anisotropic energy flux of electrons. The inventory of the converted magnetic energy in the case with the guide field is quantitatively revealed.

Stability and confinement improvement of an oblate field-reversed configuration by using neutral beam injection

A low-energy, high-current neutral beam injection (NBI) was applied to an oblate field-reversed configuration (FRC) for the first time. The NB fast ions reduce growth rates of low-n modes dangerous for the oblate FRC, extending the FRC lifetime by a factor of 1.2. The reduced loss power of 5 MW is much higher than the NBI power of 0.5 MW, indicating that the NBI not only heats the FRC plasma but also improves its stability and transport properties. The NBI also maintains higher pressure and current density profiles of the FRC, improving its flux and energy decay times by a factor of 2.

Two-dimensional ion temperature measurement by application of tomographic reconstruction to Doppler spectroscopy

A novel 2D ion temperature measurement for toroidal plasmas has been developed by use of cost-effective discrete tomography reconstruction of 2D ion Doppler spectroscopy composed of a polychromator with an ICCD camera and optical fibres. The 2D projection of the line spectrum collected by 35 (7 × 5) optical fibres is transformed into the r–z profile of the local spectrum by means of the Abel inversion at each wavelength and finally into the 2D (r–z) profile of the ion temperature. Numerical tests of its algorithm indicate that the reconstruction error for a peaked temperature profile is smaller than 15% if the chord-integrated signals have noise smaller than 5%. This system successfully measured the peaked ion temperature profile of a torus plasma on the r–z plane under the condition of negligibly small plasma flow.

Localized electron heating by strong guide-field magnetic reconnection

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.