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

Effects of α particles on plasma confinement and the removal of helium ash inthe burning plasmas
Weixin Guo1, Lu Wang1, M S Hussain2, Mingzhu Zhang1, Guangting Zhu1
Huazhong University of Science and Technology, Wuhan, China
2
University of Science and Technology of China, Hefei, Chinae-mail: wxguo@hust.edu.cn
1

We have firstly studied effects of alpha (α)
particles on CTEM- ZF system [1, 2]. Firstly, thedispersion relationship with α particles has been
derived based on the linear gyrokinetic andbounce kinetic theories, and it is thus found thatthe density gradient driven CTEM instability inlong wavelength regime can be destabilized by αparticles mainly due to the downshift of real
frequency by dilution effects, as compared to thecase without α particles. This is essentiallyattributed to more precession resonant electrons
around smaller phase velocity of CTEM and theconsequent stronger excitation of CTEM
instability. Besides, through the combination of
α particles’ effects on CTEM instability andpolarization shielding, it is finally found that αparticles can enhance the ZF growth rate driven
by CTEM turbulence, and also broaden the rangeof radial wavenumber of ZF for positive ZF
growth rate. This is mainly resulted from thedestabilization of CTEM instability as well as thereduction of polarization shielding by thepresence of α particles.
Meanwhile, effects of α particles on thetransport of helium ash driven by CTEM
turbulence are also analytically studied using
quasi-linear theory [3]. It is found that the ratio
between helium ash diffusivity and effective
electron thermal conductivity 𝐷!" /𝜒#$$ driven
by CTEM turbulence, which is a proper
normalized parameter for quantifying theefficiency of helium ash removal, is smaller thanunity. This indicates the less efficient removal ofhelium ash through CTEM turbulence ascompared with ion temperature gradient (ITG)
turbulence in [4]. However, the efficiency ofhelium ash removal is increased 55% by thepresence of 3% α particles with their density
gradient being twice that of electrons, and thisenhancement can be further strengthened bysteeper profile of α particles. This is mainly
because the enhancement of helium ash
diffusivity by α particles is stronger than that of
the effective electron thermal conductivity.
Besides, Increasing the fraction of T ions as wellas the temperature ratio and flattening the density

profile of electrons as shown in Figs. 1 and 2, arealso beneficial to the removal of helium ash, andα particles further enhance the efficiency ofhelium ash removal under these favorable
parameter regimes.
All these theoretical results are toward toprovide the possible reference for improving theplasma confinement and expelling helium ash inthe future reactors such as ITER.
References[1] Hussain, M.S., W. Guo, and L. Wang. Plasma
Physics and Controlled Fusion, 2021. 63(7):075010.
[2] Hussain, M.S., W. Guo, and L. Wang. Nuclear
Fusion, 2022. 62(5): 056013.
[3] Zhu, G., L. Wang and W. Guo, et al. Nuclear
Fusion, 2022. 62(12): 126011.
[4] Angioni, C., et al. Nuclear Fusion, 2009.
49(5): 055013.

Fig. 1. Normalized diffusivity Ds/χeff of helium ash, D–T ionsand α particles as a function of T ions.

Fig. 2. Normalized diffusivity of helium ash DHe/χeff as
functions of Te (Ti = THe = 10 keV is fixed) and R/Lne