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

Validation of the plasma-wall self-organization model fordensity limit in ECRH-assisted start-up of Ohmic discharges
on J-TEXT

Jiaxing Liu1, Ping Zhu1,2*, Dominique Franck Escande3, Junli Zhang1, Donghui Xia1*,Yuhan Wang1, Jiaming Wang1, Qinghu Yang1, Jiangang Fang1, Xiaoqing Zhang, Li Gao1, Zhifeng
Cheng1, Zhipeng Chen1, Zhoujun Yang1, Zhongyong Chen1, Yonghua Ding1, Yuan Pan1 and theJ-TEXT team1
School of Electrical and Electronic Engineering, Huazhong University of Science and Technology
2Department of Engineering Physics, University of Wisconsin-Madison3
Aix-Marseille Université
e-mail (speaker): liu_jiaxing@hust.edu.cnFor decades, people used the empirical scaling law
for the tokamak density limit nG[m-3] =Ip[MA]/(πa[m]2)×1020, where Ip is the plasma current and a is the minor
radius. Recently, a power balance model consideringradiation introduced a modified scaling for the densitylimit, (IpP/a4)4/9, where P is the heating power [1]. This
radiative scaling is in better agreement with the tokamak
experimental databases as indicated in Fig. 4 of [1]. The
primary factors influencing these power balance limits
stem from impurity radiation, which is largely
controlled by plasma-wall interactions [2]. This radiation
affects the amount of heat reaching the limiter/divertor
targets, subsequently determining the plasma
temperature in the target region. The target region
temperature also significantly impacts impurity
production, which in turn influences impurity radiation.
This feedback mechanism forms the foundation of therecently proposed plasma-wall self-organization(PWSO)
model [2].
The PWSO model predicts a significantly enhanced
density limit, which may be attainable in tokamaks withECRH-assisted ohmic startup and sufficiently high initia
neutral density. Experiments have been conducted on
J-TEXT to validate such a density limit scenario basedon this model. Although the wall conditions have someinfluence on the experiment due to the J-TEXT limiter
material is mainly composed of carbon, the experimental
results demonstrate that increasing the pre-filled gas
pressure or ECRH power during the start-up phase canhelp enhance plasma purity and decrease the CIII
radiation level as well as increase the target region
plasma temparature. And this generally leads to a higher
density limit at the flat-top, as shown in Fig. 1. Besides,
the PWSO 0D and 1D model are applied to calculate thedensity limit of J-TEXT. Both 0D and 1D models predict
the existence of a density-limit basin at higher plasma
temparature around target and a desity-free basin atlower plasma temparature around target. As shown in Fig.
2, The J-TEXT experimental data quatitatively agreewith the PWSO model’s predictions under somepapameter assumptions and are located in the density
limit basin of the PWSO prediction.
In general, the density limits predicted from the 0D
and 1D PWSO models are compared and validated withthe J-TEXT experimental data, which are located in thedensity-limit basin and demonstrate quantitative
agreement with the PWSO model predictions.
This work is supported by the National MCF Energy
R&D Program of China under Grant Nos.
2019YFE03050004.
References[1] P. Zanca et al, Plasma Phys. Control. Fusion. 64,054006(2022)[2] D.F. Escande et al, Nucl. Fusion.62, 026001(2022)

Figure 1. The relationship between (left) plasma
temparature around limiter target as well as (right)
density limit and the corresponding radiation power RCIII
measured from experiments for varying ECRH power
and pre-filled gas pressure in the start-up phase.

Figure 2. The density limits as functions of the target
region plasma temperature Tt using (a): linear and (b):logarithmic coordinates as predicted from the PWSO 0D(solid line) and 1D (dashed line) models in comparison
with the experimental data (circular symbol).