23P115-P
Neutral Beam Injection to a Field-Reversed Configuration
磁場反転配位への中性粒子ビーム入射	 Toru Ii1, Toshiyuki Umezawa2, Keii Gi2, Michiaki Inomoto2, and Yasushi Ono2
伊井	  亨1, 梅澤俊之2，魏	  啓為2，井	  通暁2，小野	  靖2Graduate School of Engineering, The University of Tokyo1
Graduate School of Frontier Sciences, The University of Tokyo22-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan東京大学 〒113-0032	  東京都文京区弥生2-11-16
The effect of energetic beam ions on oblate field-reversed configurations (FRCs) has been studied experimentally in the TS-4 plasma merging device. In order to examine its kinetic effects, we developed a novel
economical high-power pulsed neutral beam injection (NBI) system by use of a washer gun as a plasma
source. We found for the first time that two merging high-s hydrogen spheromaks with opposite helicities
relaxed into the large scale FRC with poloidal flux as high as 15 mWb under the assistance of the 0.6 MW
NBI. This fact indicates importance of ion kinetic effects for the FRC stability.

1. Introduction
A Field-reversed configuration (FRC) is acompact toroid with small or zero toroidal magnetic field and dominant poloidal field. The FRC
has some potential for future fusion reactors due
to its high beta and simple geometry. Plasma
heating and current drive by neutral beam injection (NBI) are important research subjects for
FRC. Although the NBI for prolate FRCs wasintroduced sorely to the FIX (FRC Injection Experiment) device at Osaka University [1], its
heating efficiency was low since the theta pinch
formation method with poor formation efficiency
could not produce the poloidal flux large enoughto trap fast beam ions inside the FRC. On theother hand, oblate FRCs formed by merging
spheromaks have an advantage in the large
amount of poloidal flux which can be amplified
easily by a center solenoid coil.
It is important to experimentally elucidate thephysics of kinetic effects of energetic beam ionsin oblate FRCs. A numerical study shows that
NBI is useful not only for heating but also for an
improvement of stability by the energetic ions [2].
The stabilization effects of kinetic ions have been
investigated in our TS-4 plasma merging experiments. We found for the first time that twomerging high-s (average number of ion gyroradiiin the separatrix) hydrogen spheromaks with opposite helicities relaxed into the large scale FRC
with poloidal flux as high as 15 mWb under theassistance of the 0.6 MW NBI. This fact suggeststhat some ion kinetic effects are essential to theFRC stability.
2. Plasma Merging Device TS-4

The plasma merging device, TS-4, was utilized toproduce two spheromaks with opposing helicities,
to merge them axially by means of magnetic reconnection, and to relax to an oblate FRC [3]. Itscylindrical vessel with a diameter of 1.6 m and alength of 2.2 m has two flux cores whose internal
poloidal and toroidal field coils produce poloidal
and toroidal magnetic fields of the two initial spheromaks, respectively. The center conductor,
equipped with center solenoid coils, is locatedalong the center axis to reduce toroidal mode n = 1(tilt, shift) activities. Its typical FRC plasma parameters are as follows: Ψ ~ 15 mWb, Ip ~ 30 kA, ne ~1020 m-3, Te ~ 10 eV, Ti ~ 40 eV, R ~ 0.5 m, and A ~1.5.
3. NBI System with Washer Gun Plasma Source
In order to investigate the kinetic effects, we developed a novel high-power pulsed NBI system. In
the conventional plasma sources, the neutral gas is
pre-ionized by hot electron emission from filaments.
Their arc discharge requires high-capacity water-cooling system to remove heavy heat loads tothe permanent magnets on the exterior wall of thevessel. In the new system, we replaced the filaments by a gas-injected stainless-steel washer gun
placed on the bottom of the vessel as a plasma
source. It enables us to remove the conventional
filaments and water cooler from the system and toadopt neodymium magnets for the cusp field.
We already completed the development of theNBI system. It is observed that the beam current
exceeds 20 A over approximately 0.5 ms, which issuitable for our target plasma in TS-4. Our experimental results reveal the successful beam extraction
up to 15 kV, 40 A and 0.6 MW. Recently, two new

NB sources with acceleration voltage and current of
15 kV and 20 A are also under development.
4. Numerical Simulation of the NB in an Oblate
FRCs
Numerical orbit calculation has been performed
for the purpose of checking whether the newly developed NBI system can be applicable to the oblate
FRC in TS-4 and whether fast neutral beam particles can yield the kinetic energy to the plasma. Theionization positions of tangentially injected 15 keV
neutral beam particles are calculated on the midplane of TS-4 using the Monte Carlo code. Figure 1
shows the particle trap rate in the FRC, indicatingthat most of fast beam ions are trapped between theaxis and the separatrix due to the gyroradius of thebeam ion as long as the separatrix radius and theradially inward Lorenz force v × B.

Fig.1. Particle trap rate calculated by the orbit code.

5. Initial Results of Merging Formation ofLarge-size FRCs with the Assistance of NBI
The neutral beam injection was performed during
the merging formation of an FRC in TS-4. Poloidal
flux and plasma current were increased as much as
possible by a solenoid coil for the purpose of trapping the beam ions efficiently. Figure 2 shows poloidal flux contours and toroidal field of merging
spheromaks with and without NBI. Figure 3 alsoshows their radial profiles of electron densities as afunction of time. A new finding is that two merging
high-s hydrogen spheromaks with opposing helicities relax into the large scale FRC with poloidal
flux as high as 15 mWb under the assistance of the0.6 MW NBI. However, they did not relax to an
FRC without the assistance of the NBI. Although
the beam ion density was almost a few percent of
the plasma density, ne around the magnetic axis of
the FRC with NBI was almost twice as much as thatof the FRC without NBI. These facts suggest thatsome ion kinetics effects such as toroidal ion flow
are essential to FRC stability.

Fig.2. R-Z contours of poloidal flux and toroidal field(in color) of counter-helicity merging spheromaks (a)without and (b) with NBI. In the case (b), the merging
spheromaks relax into a single FRC.

(a) FRC without NBI(b) FRC with NBI

Fig.3. Radial profiles of electron density ne of hydrogen FRCs without NB and with NB as a function oftime.

6. Summary
The effect of energetic beam ions on oblate
FRCs was studied experimentally for the first timein TS-4. We found that two merging high-s hydrogen spheromaks with opposite helicities relaxed into the large scale FRC with poloidal flux
as high as 15 mWb under the assistance of thenewly developed NBI with the beam power of 0.6
MW. This fact suggests that kinetic effects are essential to FRC stability. We will perform the upgraded FRC experiments using three NB sources
under development whose total output power exceeds 1 MW.
Acknowledgments
This work was partially supported byGrant-in-Aid for JSPS Fellows 23-2462 and JSPS
Core-to-Core Program 22001.
References[1] T. Asai et al.: Phys. Plasmas 7 (2000) 2294.
[2] E.V. Belova et al.: Phys. Plasmas 13 (2006) 056115.
[3] E. Kawamori et al.: Nucl. Fusion 47 (2007) 1232.