7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Studies of non-inductive tokamak plasma start-up with various lower hybrid
current drive scenarios
N. Tsujii1, A. Ejiri1, Y. Ko1, Y. Peng1, K. Iwasaki1, Y. Lin1, K. Shinohara1, O. Watanabe1, S. Jang1,T. Hidano1, Y. Shirasawa1, Y. Tian1, F. Adachi1 and C.P. Moeller2
1
The University of Tokyo, 2 General Atomics
e-mail (speaker): tsujii@k.u-tokyo.ac.jp

Fully non-inductive operation of a tokamak allows forremoval of the central solenoid, which may lead to acompact economical fusion reactor. Fully non-inductive
tokamak plasma start-up using lower-hybrid (LH)
current drive has been studied on the TST-2 sphericaltokamak. Start-up up to about a quarter of the Ohmicallydriven plasma current has been achieved fully
non-inductively using LH waves launched from theouter-midplane side and the top side of the plasma [1].
For the conventional outer-midplane launch scenario,
numerical simulations showed that absorption was weak.
Experimentally, uncontrolled density increase has been
observed during the rf pulse possibly due to weakabsorption and increased LH wave interactions in thescrape-off-layer (SOL). For the top launch scenario,
strong absorption was predicted due to the parallel
refractive index upshift characteristics. Density control
was also observed to be much better for the top launcher
compared to the outer-midplane launcher. However,strong thick-target x-ray radiation was observed during
the top launcher pulse. This is thought to indicate that
LH waves drive radially outward fast electron diffusionthat leads to large fast electron losses at the limiters [2].
In both these scenarios, numerical simulations showedthat LH waves were absorbed only near the edge of theplasma, limiting the current drive efficiency.
A new off-midplane LH launcher was recently
developed and installed (Fig. 1) to achieve goodabsorption nearer the magnetic axis [3]. The LH waves
launched from the outer-off-midplane side werepredicted to propagate through the magnetic axis due torefraction. The initial parallel refractive index waschosen to be relatively high (~13) so that the LH waves
could interact with the cold core electrons in the tens ofeV range.
The new LH launcher is a capacitively coupled
combline antenna with seven straps and 90 degrees
phasing between the straps [3]. Only the two elements at
the ends are connected to the transmission line. Invacuum, the power fed from the input port comes out of
the output port connected to a dummy load. In thepresence of a plasma, some of the power is radiated into
the plasma as LH waves. Since reflection is always low,the LH system is quite resilient to changes in plasma
conditions.
Using the new off-midplane LH launcher, tokamak
configuration was successfully generated. The plasma

generated by the off-midplane launcher had on-axis
electron temperature twice as high as the previous twoantennas. Density control was also better with theoff-midplane launcher. This may be due to better coreabsorption and reduced LH wave interactions with theSOL plasma. At the same time, x-ray radiation wasobserved to be much lower during the off-midplanelauncher pulse compared to the top launcher pulse. This
indicates that LH wave driven fast electron losses weresuccessfully reduced.
This work was supported by JSPS KAKENHI Grant
Numbers 22K03573 and 21226021, NIFS Collaboration
Research Programs NIFS20KUTR155 and Japan/US
Cooperation in Fusion Research and Development. Workat General Atomics was supported by US DoE contract
DE-AC03-97ER-54411.
References[1] S. Yajima et al, Plasma Fusion Res. 13, 3402114(2018).
[2] A. Ejiri et al, Plasma Fusion Res. 17, 1402037(2022).
[3] Y. Ko et al, “Development of an outer-off-midplane
lower hybrid wave launcher for improved coreabsorption in non-inductive plasma start-up onTST-2,” accepted for publication in AIP Conf. Proc.

Figure 1. The newly installed off-midplane launcher(center) with the previously installed midplane launcher(left) and top launcher (top).