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Here we report our latest investigation of global ion heating/transport process during magnetic reconnection in the central solenoid (CS)-free merging spherical tokamak (ST) formation experiment on TS-6. Using a new 96CH/320CH ion Doppler tomography diagnostics, our full-2D imaging measurement clearly revealed that (i) magnetic reconnection initially forms localized hot spots in the downstream region of outflow jet with inboard/outboard asymmetry, but (ii) the continuous accumulation of the heating coupled with transport process expands the high temperature region globally on the field line direction. (iii) The dynamic heat transport process is also affected by the polarity of toroidal magnetic field and poloidally rotating global structure formation has experimentally found for the first time.
Magnetic reconnection is a fundamental process which converts magnetic energy of reconnecting field $B_{rec}$ to kinetic and thermal energy of plasma. Application of the high power heating for CS-free startup was pioneered on START and TS-3 in 1990's, and high field machines such as MAST and ST40 demonstrated high ion temperature ($T_i$) plasma formation in keV regime in the last decade$^{[1][2]}$. Fundamental energy conversion mechanism has long been investigated in U-Tokyo laboratory experiments using 2D internal magnetic probe array and ion Doppler tomography diagnostics and it is known that the order of reconnection heating could be scaled by magnetic field $B_p$ or plasma current $I_p: \Delta T_i \propto B_{rec}^2 \sim B_p^2 \propto I_p^2$$^{[1][2][3]}$. Merging startup plasma typically has radially two hot spots both inside and outside magnetic axis after merging (doubly-peaked/hollow $T_i$ profile) based on outflow heating mechanism: energy conversion from kinetic energy of outflow jet into ion thermal energy downstream$^{[3]}$.
The doubly-peaked/hollow $T_i$ profile was typically interpreted as a flux function in a longer time scale but here we present more dynamic heating/transport process as shown in Fig.1 (full-2D profile measurement of $T_i$ ($r-z$) during merging in TS-6). As illustrated in the schematic model in Fig.1 (left), two upstream plasma rings at the top and bottom vertically approach toward midplane ($z \sim 0$m), and then drive reconnection heating during merging. At the beginning of merging ($t=465{\rm \mu}$s), initial upstream plasmas simply have peaked $T_i$ profile around their magnetic axes; however, magnetic reconnection immediately changes the profile to have hot spots in the outflow region: radially inside ($r \sim 0.12$m) and outside ($r \sim 0.26$m) of $X$-point ($R_{X-point}\sim0.2$m). Accumulation of outflow heating continue to increase $T_i$ and the hot spot at $r \sim 0.12$m forms $T_i \sim 60$eV at $t = 480{\rm \mu}$s as shown in the 1D vertical profile in Fig.1 (right): average $T_i$ in 0.10m$ < r < $ 0.14m at each $z$ position. After merging at $t = 485{\rm \mu}$s, the peak $T_i$ starts to be decreased and ion heat flux $q = - \kappa^i_{\parallel}\nabla_{\parallel}T_i - \kappa^i_\perp\nabla_\perp T_i + \kappa^i_\wedge(B/|B|)\times\nabla T_i$ mostly propagates to field line direction because the ratio of ion heat conductivity satisfies $\kappa^i_\parallel/\kappa^i_\perp \sim 2(\omega_{ci}\tau_{ii})^2 > 3000$ and $\kappa^i_\parallel/\kappa^i_\wedge \sim 0.8\omega_{ci}\tau_{ii} > 30$ in the heating region. Ion thermal velocity at the hot spot is roughly $v^i_{th}\sim 100$km/s = 0.1m/$\rm \mu$s and its propagation in 5$\rm \mu$s is $\sim 0.5$m but the ratio of toroidal/poloidal magnetic field $4 < B_t/B_p < 5$ leads to longer orbit including $2\pi r_{r\sim{\rm 0.12m}}$ and finite delay time of parallel heat transport in poloidal direction is clearly visualized in the full-2D measurement.
In the low aspect ratio configuration with aspect ratio $A\sim 1.5$, the difference of $B_t$ in high field side and low field side is not necessarily negligible. For example, $B_t > 0.2$T in high field side but it is $\sim 0.1$T in low field side in the experiment on Fig.2. In high field side, both $\kappa^i_\parallel/\kappa^i_\perp >> 1$ and $\kappa^i_\parallel/\kappa^i_\wedge >> 1$ satisfied but the latter condition is not in low field side: $1 < \kappa^i_\parallel/\kappa^i_\wedge < 10$. Figure 2 shows the comparison of its contribution to global heat transport process by changing negative/ positive polarity of $B_t$. In $\kappa^i_\wedge(B/|B|)\times\nabla T_i$ term, $B_p\times\nabla T_i$ is in toroidal direction and only $B_t\times\nabla T_i$ contributes the heat transport process, leading to the polarity reversal of $T_i$ profile in clockwise/anti-clockwise direction.
Figure 3 shows comparison of reference polarity by Hall-effect$^{[3]}$ and $(B/|B|)\times\nabla T_i$, $T_i$ profile in negative/positive polarity of $B_t$, floating potential $\phi_{floating}$ with electric field vector $E$ and $E\times B$ drift velocity profile at $t = 475{\rm \mu}$s. Ion-electron decoupling around $X$-point forms quadrupole potential profile and there is Hall electric field toward lower (negative) potential region but its contribution is mainly on inboard-outboard asymmetry of $T_i$ (higher $E$ and $V_{E \times B}$ in high field side) and the globally rotating $T_i$ profile is rather in the opposite polarity to Hall electric field which is in poloidally clockwise direction. The global $T_i$ profile shows that the polarity is poloidally anti-clockwise direction with the polarity in $(B/|B|)\times \nabla T_i$ and the structure reversed when the polarity of toroidal field $B_t$ is reversed: a new feature of structure formation process has been experimentally found for the first time.
$[1]$ M. Gryaznevich et al., Nucl. Fusion 57 (2017) 072003
$[2]$ H. Tanabe et al., Nucl. Fusion 57 (2017) 056037
$[3]$ H. Tanabe et al., Nucl. Fusion 59 (2019) 086041
| Affiliation | Graduate school of frontier sciences, university of Tokyo |
|---|---|
| Country or International Organization | Japan |
