23P110-P
Measurement of Hall dynamo during CHI current driven phase on HISTHISTプラズマのCHI電流駆動過程におけるホールダイナモ計測
Kengo Ito, Takafumi Hanao, Mitsuru Ishihara, Keisuke Matsumoto, Yusuke Kikuchi
Naoyuki Fukumoto, Masayoshi Nagata
伊藤兼吾, 花尾隆史, 石原 充, 松本圭祐, 菊池祐介, 福本直之, 永田正義
Graduate School of Engineering, University of Hyogo
2167 Syosya, Himeji City, Hyogo 671-2280, Japan
兵庫県立大学 大学院工学研究科 〒671-2280 兵庫県姫路市書写2167
We have measured the dynamo electric fields necessary to balance the mean field Ohm's law in thehelicity-driven ST plasmas in double pulsing Coaxial Helicity Injection (CHI) discharges on HIST. The
dynamo measurement indicates that the Hall and MHD dynamo induced electric fields are large enough tosustain the mean toroidal current against resistive decay in the core region. Two-fluid Hall dynamo isfound to be essential to the CHI current drive mechanisms.

1. Introduction
Coaxial Helicity injection (CHI) is an efficientcurrent-drive method used in spheromak andspherical torus (ST) experiments. An anticipatedissue for CHI is achieving good energy
confinement, since it relies on the magneticrelaxation and dynamo. This is essentiallybecause CHI cannot drive a dynamo directly
inside a closed magnetic flux surface. Thus, it is
an important issue to investigate dynamo effect toexplore CHI current drive mechanisms in a new
approach such as Multi-pulsing CHI method. To
study the dynamo model with two-fluid Halleffects, we have started from the generalizedOhm law.
To study the dynamo model with two-fluideffects, we start from the generalized Ohm law,
ηj=E+v×B − j×B/en+∇pe/en, whereη is the plasma
resistivity, n the electron density and pe the electronpressure. We decompose each quantity into mean
and fluctuating part, and take the ensemble averageof the parallel component of it for turbulent
equilibrium plasmas to yield the parallel mean-field
Ohm's law ηj|| − E|| =<δv×δB>|| − <δj×δB>||/en≈<δve×δB>||, where ve is electron velocity, δ denotes afluctuating quantity, < > denotes a mean quantity [1].
The first of these terms in the right-hand side (RHS)
represents the MHD dynamo <δv×δB>||, while thesecond is the Hall-dynamo <δj×δB>||/en. In thisexperiment, we have studied each dynamo term at
the same time.
2. Diagnostics
We have measured each MHD dynamo termand Hall dynamo term separately by using Machprobe and Hall probe involving 3-axis magnetic

pick-up coils as shown by Fig.1. The Mach probe
incorporates three-axis magnetic pick-up coils tomeasure MHD dynamo induced electric field. A
pair of two tip electrodes on the Mach probe isused as a double probe to measure local electrondensity. Hall dynamo probe is covered by a glasstube with a square of 30mm × 30mm in the three
directions, and contains a Rogowski coil and 3turns loop coil in it. Thereby, we can obtain thelocal data of the current density and the magnetic
field. The Hall probe and Mach probe are inserted
at the same position in the flux conserver (FC) as
shown by Fig.2.

(a)(b)
Fig.1. Hall probe (a) and Mach probe (b)Fig.2. Mach and Hall probes inserted in the FC
3. Experimental results
Figure 3 shows the time evolution of MHD

dynamo, Hall dynamo and toroidal current density inthe central open flux column (OFC) and the coreregions. The direction and amplitude of each
dynamo are determined by the phase difference
between the fluctuating velocity or current density
and the fluctuating magnetic field. After injecting 1st
gun pulse, the toroidal current has increased and theresponse times of the MHD and Hall dynamo
induced field shows a good correlation with thepulse-like current generation. After then, the 2nd
gun-pulse seems to keep driving the current not onlyin the open flux column region but also in the coreregion. We have checked the Ohm’s law balance inthe both regions. The estimation result indicates thatthe both fluctuation-induced electromotive forces
are large enough to sustain the mean toroidal current.
The parallel mean-field Ohm's law balance isroughly satisfied both in the OFC driven-region andthe core region.
Figure 4 shows the MHD and Hall dynamo
induced electric fields measured at each radial
position. The induced field due to MHD dynamo
is found to be sufficent to sustain the mean
toroidal current against resistive decay in the coreregion. In the other hand, the anti-dynamo effectin the MHD dynamo term is observed in the OFCregion. From the viewpoint of two-fluid theory,
ion diamagnetic drift is opposite to the electron
diamagnetic drift, maybe resulting in theanti-dynamo effect. Hall dynamo may arise fromthe fluctuating electron diamagnetic current due
to high electron density gradient which is large in
the OFC region.
Note that the direction of the ion flow observed in
the OFC is opposite to that of the electron. The ion isaccelerated due to the Hall dynamo electric field andthe electron becomes slow down due to theanti-MHD dynamo. In the core region, collisionaldrag of electrons accelerates the ions and so the ion
and electron flows have the same direction. Thedynamics of the ion and electron flows may beconsistent with the electron locking model [2].
4. Summary
We have measured directly and simultaneously
the Hall and MHD dynamo spatial profile. The
relative contributions of the different dynamo
electric field on the driven current have been
examined to verify mean Ohm’s law balance. The
Hall dynamo acts to generate the mean current
density in the both regions, although anti-MHD
dynamo is effective in the OFC region. Bothdynamos play an important role in the helicity
injection current drive.

Fig.3. Time evolution of the Hall and MHD
dynamo induced electric field and the toroidal
current density in both the open flux column andthe core region.

Fig.4. Radial profiles of the MHD dynamo (red)and Hall dynamo (blue) induced electric field in thedriven phase.

This work was supported by Grant-in-Aid for
Scientific Research (B) (22360391).
References[1] H. Ji, et al., Phys. Plasmas 3, 1935 (1996).
[2] T.R. Jarboe et al., Nucl. Fusion 51, 063029(2011).