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icc2011_proceeding_inomoto.pdf2011-08-19 12:03:12Michiaki Inomoto
icc2011_inomoto.pdf2011-08-19 10:36:45Michiaki Inomoto

Neutral Beam Injection to Oblate FRC Formed by Plasma Merging Method

Author: Michiaki Inomoto
Requested Type: Consider for Invited
Submitted: 2011-06-10 02:56:12

Co-authors: T. Ii, K. Gi, T. Umezawa, Y. Ono, T. Asai, S. Okada

Contact Info:
The University of Tokyo
Kibantou 415, GSFS, Univ. Toky
Kashiwa, none   277-856

Abstract Text:
Counter-helicity merging of spheromaks is an alternative method to form a high-beta field-reversed configuration (FRC) plasma [1]. Pronounced features of the merging method are occurrence of significant ion heating due to magnetic reconnection, applicability of a center solenoid current drive, and oblate plasma shape with large major radius. These features enhance the potential for neutral beam injection (NBI) heating in high-beta FRC plasma. Large plasma radius and large magnetic flux are essential for efficient ionization of beam neutrals and confinement of produced fast ions. Effective tangential injection of 15 keV hydrogen beam requires target FRC plasma with magnetic flux more than 20 mWb. The NBI is also expected to improve the global stability of FRC plasma by providing azimuthally rotating fast ions near the magnetic null [2].
In the TS-4 device, large FRC plasma with major radius of 45 cm is produced by the counter-helicity plasma merging method. Because of the large S* number (ratio of minor radius to ion skin depth), the hydrogen FRC in the TS-4 device usually shows relaxation to spheromak equilibrium driven by low-n instabilities [3]. A washer gun type neutral beam source [4] was developed and installed on the TS-4 device. Primary results from NBI experiments show that the stability of hydrogen FRC was drastically improved by the tangential injection of 15 keV hydrogen beam with peak input power of 500 kW, resulting in a formation of hydrogen FRC with trapped flux of 15 mWb. NBI also reduces the flux decay rates of FRC plasmas with various gases such as hydrogen, helium, and argon. Since the decrease of energy loss rate is much larger than the input NBI power, the stabilization of the low-n modes is possibly responsible for the longer decay time observed in the NB-injected FRC plasmas.
Recently, two other ion sources with acceleration voltage and current of 15 keV and 20 A, respectively, were moved from Osaka University (FIX device) and equipped on the TS-4 device for tangential injection on the midplane. Full-scale NBI experiments with injection power up to 1 MW are in preparation.

[1] Y. Ono, et al., Nucl. Fusion 39, 2001 (1999).
[2] E. Belova, et al., Phys. Plasmas 13, 056115 (2006).
[3] E. Kawamori and Y. Ono, Phys. Rev. Lett. 95, 085003 (2005).
[4] T. Asai, et al., Rev. Sci. Instrum. 79, 063502 (2008).

Characterization: A2,D2

I appreciate if you arrange my presentation on the beginning three days of the workshop, since I will leave on Friday. Thank you for your help.

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011