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

Diagnosing fast electrons interacted with kinetic waves on spherical tokamak

R. Ikezoe1, H. Idei1, T. Onchi1, M. Hasegawa1, Y. Nagashima1, K. Hanada1, T. Ido1, F. Zennifa1,T. Yamaguchi2, R. Miyata2, S. Tai2, Y. Shirai2, Y. Wang2
1
Research Institute for Applied Mechanics, Kyushu University2Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
e-mail (speaker): ikezoe@triam.kyushu-u.ac.jp
Recently, ubiquitous feature of the interactions among
high-frequency waves and fast electrons is getting
spotlight both for the geomagnetic space and magnetic
bottles for fusion energy development. Both cases sharethe same physics basis but have largely different
characteristic scales of the systems. Resultantly, available
diagnostics and relevant challenges are different. In thispresentation, several problems of interest for laboratory
magnetic confinement of fusion plasmas that is related to
fast electrons is introduced. Then, recent works performedon the QUEST spherical tokamak (Kyushu Univ., Japan)
for diagnosing fast electron behavior interacted withkinetic waves will be presented.
It is recognized that energetic/fast electrons
confined in tokamaks and spherical tokamaks (STs) arehighly anisotropic due to those low collisionality andbecome sources of kinetic instabilities. Burst of highfrequency kinetic modes have been recently observed inseveral tokamaks. Fast electrons in tokamaks and STs are
attractive in terms of excellent absorber of electron
cyclotron waves that are injected for plasma current drive
but have a concern that they may lead to so-calledrunaway electrons (reaching relativistic speed withoutbraking) and damage the plasma facing components. As amethod of controlling such fast electrons to mild energies,
pitch-angle scattering by kinetic modes are expected. If
sufficient knowledge is obtained, there is a possibility ofactive control of fast electrons by externally exciting
appropriate modes to exploit benefits of fast electrons
while increasing the critical toroidal electric field for thegeneration of runaway electron (RE) beam. This function
is especially important during a disruption (thermal andcurrent quench). Several tokamaks and STs have been
conducting research on it mainly in terms of RE issues.
Although some experiments and simulations suggest that
their effects are remarkable for RE beam physics, theirunderstanding is still insufficient and more dedicatedexperimental studies are required. The difficulty of therelevant studies is in measuring a detailed structure ofhigh-frequency kinetic modes, typically whistler waves,and dynamics of fast electrons in a denser hot plasma than
the geomagnetic plasma.
So far, studies of the kinetic modes during
disruptions are all based on the measurement ofcorresponding oscillations of the electromagnetic fields.
On QUEST, along with various magnetic measurements,
by developing a fast-response detector of hard X-rays, it
was firstly found that the fast electrons confined in themagnetic bottle exhibited fluctuations with the samefrequencies with the kinetic modes that were destabilizedby high anisotropic electron velocity distribution (seeFigure 1. Fastly founded oscillations of fast electrons ina magnetically confined laboratory plasma on QUEST.
The frequencies of high intensities correspond to thoseof the high-frequency kinetic modes, which aredestabilized when the fast electrons are strongly
accelerated by the inductive toroidal electric field.
Figure 1). This is the clear evidence of the interaction
between the excited kinetic modes and driving fastelectrons. In addition, we are close to diagnosing electron
velocity distribution in a ST for the first time bydeveloping a pitch-angle resolved fast electron probe; adirect particle measurement in a hot plasma, which has notbeen commonly performed in fusion plasma unlike inspace plasma, was achieved using a novel high-speed
particle measurement with canceling X-ray contribution.
This new measurement and various results shown together
will accelerate the understanding of the dynamics of fastelectrons with accompanying kinetic modes and lead toattractive control of fast electrons in a magneticallyconfined laboratory plasma for fusion development.
This work was partly supported by JSPS KAKENHI
Grant Number 21K03511 and partly by NIFS bilateralcollaboration research program under grant no.
NIFS22KUTR172.