24C09
ICRF Heating Using Antennas with Controlled Wave-number on LHD
LHDにおける波数制御アンテナによるICRF加熱	 
Ryuhei Kumazawa, Tetso Seki, Takashi Mutoh, Kenji Saito, Hiroshi Kasahara, Goro Nomura,Fujio Shimpo, Yanping Zhao1), J.G. Kwak2) and LHD Experiment Group
熊沢隆平, 関哲夫，武藤敬，斎藤健二, 笠原寛史, 野村吾郎, 新保富士夫,
Yanping Zhao1), J.G. Kwak2), LHD実験グループ
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan1)
Institute of Plasma Physics, Academia Sinica, Hefei, Anhui 230031 China2)
Korea Atomic Energy Research Institute, Daejeon, 305-353 Korea
核融合科学研究所 〒509-5292	  岐阜県土岐市下石町322-6
The large helical device (LHD) has fifteen experiment cycles so far. The ion cyclotron range of frequencies(ICRF) heating has been carried out successfully on LHD. A long pulse plasma discharge up to about one
hour (54 minutes) has been achieved using the poloidal array (PA) antennas. In the 14th experiment cycle(2010) the toroidal array (TA) antenna was newly installed. This TA antenna consists of two antennas
arrayed in the toroidal direction and has a controllability of the wave number along the magnetic field line.
In the 15th experimental cycle (2011) the plasma with high density up to 3.6x1019m-3 was sustained with
2MW of the injected RF power using both PA and TA antennas. In this experimental cycle the optimized
plasma heating condition was precisely examined against the minority hydrogen ratio in the wide range ofthe plasma density. In addition the long pulse plasma discharge in the higher density with the higher RF
power was achieved.

1. Introduction
The large helical device (LHD) has fifteen
experiment cycles so far. Many experiment results
have been achieved, i.e., the high performance
plasmas with the high density, the high beta, andthe high ion and electron temperature. An ion
cyclotron range of frequencies (ICRF) heating has
been carried out successfully: High RF power
heating up to 1.5MW and the long pulse plasma
discharge up to about one hour (54 minutes) havebeen achieved using the poloidal array (PA)antennas [1-2]. In the 14th experiment cycle (2010)
the toroidal array (TA) antenna was installed [3].
This newly installed TA antenna consists of twoantennas arrayed in the toroidal direction. In the15th experimental cycle (2011) the poloidal array(PA) antenna was installed again.

heating experiment was carried out using fourantennas.
An RF power with frequency of 38.5MHz wassupplied to each antenna from the each RF
generator, which consists of three stage amplifiers,i.e., 5kW, 100kW and 1MW of the RF power levelin each amplifier. An impedance matching systemwith the liquid stub tuners was employed to reduce
the reflected RF power from the ICRF heatingantenna. A real-time impedance matching wascapable with changing the liquid height during theICRF heating.
A minority heating method for the ICRF heating
was employed: The minority ion was a hydrogenion with the He ion as the majority. The magnetic
field strength on the plasma axis, i.e., Rax=3.6m was
employed as B=2.75T.

2. ICRF heating antennas
The TA antenna has a controllability of the wave
number along the magnetic field line k// withchanging the phase difference between them. When
two TA antennas are seen from the plasma axis,
they look like to handshake each other and so they
are called as HAS (Handshake) antenna. In the 15thcycle another pair of the poloidal array (PA)antennas was installed again. Therefore the ICRF

3. Experimental results
3-1. Plasma discharge with higher density
The plasma was sustained with ICRF heating
power up to 0.5~2MW and sometimes with theassist of ECH power up to 0.2~0.6MW. First thetrial of achieving the higher density plasma wascarried out. In general the maximized density wasalmost proportional to the injected RF power. Theplasma collapse occurred when the radiated power
exceeds to about 40% of the absorbed RF power tothe plasma. The highest density plasma with
ne=3.6x1019m-3 was sustained for 10 seconds with
the ICRF heating power of 2.0MW and the ECH
power of 0.37MW. Then the plasma stored-energy(Wp) attained to 300kJ.
3-2. Plasma stored energy with ICRF heating
In the series of the plasma sustained only with
the ICRF heating and the ECH power, the wide
range of the plasma density, ne=0.6~3.6x1019m-3
were obtained with the injected heating RF power
PRF up to 2.35MW. The plasma stored-energy Wp
was increased with the density ne and PRF in thefollowing equation of the ISS04 scaling [4];0.39W p ∝ n e0.54 PRF

These data are plotted in the plain of Wp/PRF0.39 vs.
ne. The envelope of data points has a scaling ofne0.54, but many data were found below the envelopein the€ higher (> 2.5x1019m-3) and the lower (<1x1019m-3) density. It was thought the smaller Wp
was caused with the non-optimized ratio of theminority hydrogen ion as described later.
3-3. ICRF heating efficiency and coefficient ofISS04
It was thought that the ICRF heating efficiency
should have been a key parameter to determine theattained Wp. It mainly depends on the minorityhydrogen ion ratio to the bulk helium ion.
Employing ISS04 the ICRF heating efficiency ηwas derived as follows;−1η ∝ W p2.56 n e−1.38 PRF

In the wide range of ne (0.6~3.8x1019m-3), Wp(70kJ~300kJ) and PRF(0.5MW~2.35MW), η wereplotted against H/(H+He). Here the intensity ratio€of the visible Hα to that of HeI at the periphery ofthe plasma [5] was employed to evaluate the η inplace of minority hydrogen ion ratio. The calculatedη was an arbitral unit, but it increased from 1.0~2.5
with H/(H+He), i.e., 10~30%. On the other hand theηRF was deduced with the break-in slope of Wp andthe RF power modulation method. However thedependence of ηRF on H/(H+He) was opposite tothat of η.
Then it was proposed that the ISS04 scaling
might have another coefficient, referred to as Ceff,
which depended on H/(H+He) in the ICRF heating.
These data were plotted again against H/(H+He) in
accordance with
0.39
−0.39
Ceff ηRF∝ W p n e−0.54 PRF
The Ceff indicates how the plasma heating method
depends on the energy confinement time. The
minority ion heating was employed in this ICRF
€heating scenario. First the high-energy minorityhydrogen ion is produced with the RF electric field.

This energy is transferred to the bulk electron andbulk helium ion. This power fraction depends on
the relation between the energy distribution of theminority hydrogen ion and the electron temperature.
When the density of the minority hydrogen ion nH isincreased, i.e., the larger H/(H+He), the tailhydrogen ion temperature decreases with nH, andthe power from the hydrogen ion with thehigh-energy flows mainly to the bulk helium ion. It
was thought that the above Ceff was improved with
the increase in the heating power flow to the bulkions.
3-4. Fueling hydrogen to ICRF heated plasma
Three methods were succeeded in fuelinghydrogen to the RF plasma; one was a conventional
puffing of the mixed helium gas with hydrogen gas,
and the second and the third were the hydrogenpellet injection and the super sonic hydrogen
injection with the repetition rate of a few Hz.
3-5. Long pulse plasma discharge
A long pulse plasma discharge was carried out
using only RF power, e.i., ICRF heating power andECH power. The plasma of ne=2x1019m-3 andWp=170kJ was sustained for 93 seconds with
PICH=1.6MW and PECH=0.6MW. The plasma of
ne=1.4x1019m-3 and Wp=130kJ was sustained for
320 seconds with PICH=0.8MW and PECH=0.3MW.
Summary

The ICRF heating was carried out using the newTA antenna and the PA antenna. The higher densityof 3.6x1019m-3 was sustained for 10 seconds with
PICH=2MW and PECH=0.37MW. The higher plasma
stored-energy was found at ~20% of H/(H+He). In
this range of H/(H+He) the ICRF heating efficiency
was degraded, but the coefficient of the energy
confinement was much improved in ~20% ofH/(H+He). The duration time of the long pulse
discharge was extended in the higher density withthe higher RF heating power.
Acknowledgments
This work was supported by NIFS grants,
10ULRR003, 11ULRR013 and 11ULRR019.
References[1] T.Mutoh R.Kumazawa et al., Nuclear Fusion,47(2007), 1250.
[2] R.Kumazawa et al., Proceeding of 22th IAEA Fusion
Energy Conference (2008), IAEA-EX/P6-29.
[3] H.Kasahara, et al., J. Plasma Fusion Res., Vol. 5,S2090 (2010).
[4] H.Yamada et al., Nuclear Fusion 45(2005) 1684.
[5] M.Goto et al., Physics of Plasmas 10(2003) 1402.