Realization of T_e0 > 10 keV long pulse operation over 100 s on EAST

1.   Introduction

The development of a high core electron temperature $T_{\mathrm{e}0}$ of over 8.6 keV (~100 million degrees Celsius) has been set up as one of the EAST main goals since the campaign in 2018, and the plan is to extend the duration of the scenario to the thousand-second timescale in the future [1–3]. This is a platform for investigating electron thermal transport under weak collisionality, which is close to ITER dimensionless parameters [4]. The study of electron-dominant heating is a crucial issue because the sustaining of fusion power at a steady output requires controlling the electron thermal transport in low collisionality, and thus heating fueling ions for self-consistent burning [5–7]. The collisionality parameter ${\nu }_{\mathrm{e}*}~0.02$ in EAST in the high-$T_{\mathrm{e}0}$ scenario is close to the dimensionless value in ITER [1, 3]. This is also a platform to investigate the particle balance [8–11], which is an integrated issue related to plasma-wall interaction and radio frequency (RF) wave heating.

In 2018, EAST realized this scenario of a normalized collisionality parameter ${\nu }_{\mathrm{e}*}~0.02$ under pure RF heating with a pulse length of $t=10\mathrm{s}$ [1]. In the 2019 campaign, the EAST group made great progress in developing the scenario and the underlying physics, especially in studying multiscale instabilities and interactions. The discharge pulse was extended to 20 s, which is ten times as long as the plasma current diffusion timescale [3]. In the 2021 campaign, the RF-driven fully noninductive current and $T_{\mathrm{e}0}>10\mathrm{keV}$ discharge have been extended to 105 s with several engineering upgrades and novel techniques. First, a newly upgraded lower divertor is equipped with an excellent water-cooling system for heat flux and particle exhaust control. Second, a novel technique of injecting a lithium (Li) aerosol into the edge plasma is developed for density control [8].

2.   Experiment setup

EAST is a medium-size non-circular tokamak with a major radius $R_0=1.80\pm 0.05\mathrm{m}$ and a typical minor radius $a=0.45\mathrm{m}$ [12]. The maximum torodial axial magnetic field is $B_0=2.5\mathrm{T}.$ In 2021, the electron cyclotron resonance heating (ECRH) was upgraded, allowing a total power of up to 1.4 MW for the central electron heating. The infrared radiation (IR) camera system was also upgraded in 2021, which covers all RF antennas and over 70% of divertor targets, to monitor the surface temperature of key plasma-facing materials. The integrated optical system provides an identical view of IR and visible light to ensure safe operation. An algorithm processing method is used in deducting and correcting linear drift of the integrator for magnetic diagnostic data in the EAST plasma control system.

3.   Results

3.1   T_e0 > 10 keV over 100 s

Repeatable plasmas with $T_{\mathrm{e}0}>10\mathrm{keV}$ have been achieved by a combination of ECRH and a low hybrid current drive (LHCD) in EAST. Figure 1 shows the primary measurements in a typical steady-state long pulse with high $T_{\mathrm{e}0}.$ The plasma current $I_{\mathrm{p}}=0.5\mathrm{M},$ and $V_{\mathrm{loop}}~0\mathrm{V}$ in figure 1(a) indicate the Ohmic current $J_{\mathrm{oh}}~0\mathrm{kA}{\mathrm{m}}^{-2}$ and a fully non-inductive current driven by pure RF powers in plasma. Figure 1(b) shows the measurements of line-averaged electron density ${\overline{n}}_{\mathrm{e}0}=1.8\times {10}^{19}{\mathrm{m}}^{-3}$ and the density feedback by supersonic molecular beam injection (SMBI). Figure 1(c) shows that the values of $T_{\mathrm{e}0}$ in the current flat top are over 10 keV and the ${\beta }_{\mathrm{p}}~0.8,H_{98}~0.9.$ The duration of high-$T_{\mathrm{e}0}$ plasmas in EAST can reach 100 s. Figure 1(d) shows the RF wave power $P_{\mathrm{LH}}=2.5\mathrm{MW}$ and $P_{\mathrm{EC}}=1.4\mathrm{MW},$ and the maximum temperature of the tungsten lower divertor measured by the tangential IR camera [13] is less than 250 ℃ and approaches saturation in 2 s, keeping nearly flat during current flat-top, because the lower divertor has been newly upgraded with an excellent water-cooling system.

Figure  1.  Time histories of EAST high-$T_{\mathrm{e}0}$ long-pulse discharge #98958. (a) Plasma current $I_{\mathrm{p}}$ and loop voltage $V_{\mathrm{loop}},$ (b) line-integrated electron density ${\overline{n}}_{\mathrm{e}0}$ and the feedback signal of SMBI, (c) central electron temperature $T_{\mathrm{e}0},$$H_{98}$ and ${\beta }_{\mathrm{p}},$ (d) radio frequency wave power $P_{\mathrm{LH}}$ and $P_{\mathrm{EC}}$ and lower-outer flat type divertor temperature measured with an infrared (IR) camera.

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3.2   Key parameter control in long duration

Figure 2(a) shows the new ITER-like lower tungsten divertor with flat structure, which enhanced particle and power exhaust capability during 100 s high $T_{\mathrm{e}0}$ operation. A careful operational optimization between the plasma shape (e.g., X-point) and the outer gap in lower single null (LSN) configuration was carried out for the maintenance of the high-RF power coupling and for the avoidance of the formation of hot spots on the 4.6 GHz LHCD antenna with a power up to 2 MW. The LSN magnetic configuration uses the iso-flux control [14] scheme based on the high performance of the newly upgraded ITER-like tungsten lower divertor in heat flux control. Figure 2(b) shows the LSN magnetic configuration and excellent shaping control (which matches very well with the shape control points at all of 20 s, 40 s and 60 s) during long-pulse operation.

Figure  2.  (a) The new lower divertor picture with flat structure, (b) LSN magnetic configuration and red star control points, (c) clear green color in the CCD frame at $t=40\mathrm{s},$ showing the injection of the Li aerosol into the plasma edge, traces of the position of X-point (d) and recycling parameter (e).

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Because the RF coupling is very sensitive to the plasma density, the density control in long-pulse plasma operation is very important [15]. The EAST group made a big effort to achieve plasma density control with a timescale over 100 s. By means of the novel technique of injecting a Li aerosol into the edge plasma continuously and in real time combined with SMBI feedback [9, 16], the plasma density is well controlled in the long-pulse operation, as shown in figure 1. The green color in figure 2(c) indicates that a Li aerosol is injected into the edge plasma.

An excellent plasma shape control within a few millimeters is important not only to the RF coupling but also to the constant density control. Figure 2(d) indicates that the major radius of X-point is controlled within a range of a few millimeters. The recycling parameter below unity (figure 2(e)) during 100 s reflects a good maintenance of the plasma wall condition. The global recycling coefficient R_global is evaluated by equation [17]:

where $N_{\mathrm{e}}$ is the electron density, ${\tau }_{\mathrm{p}}$ is particle confinement time, f is the fueling efficiency of external gas injection and $q{Q}_{\mathrm{injection}}$ is the external gas injection rate.

3.3   Power balance analysis

The plasma current density consists of the Ohmic part ($J_{\mathrm{oh}}$), the low hybrid wave-driven part ($J_{\mathrm{LH}}$), the electron cyclotron-driven one $J_{\mathrm{EC}}$ and the gradient-driven bootstrap one ($J_{\mathrm{bs}}$) [18–20]. The Ohmic part is given by $J_{\mathrm{oh}}=V_{\mathrm{loop}}/{\eta }_{\mathrm{sp}},$ where $V_{\mathrm{loop}}$ is the surface loop voltage and ${\eta }_{\mathrm{sp}}$ is the Spitzer resistivity; $J_{\mathrm{oh}}=0{\mathrm{kA}\mathrm{m}}^{-2}$ for our fully-noninductive case because $V_{\mathrm{loop}}=0\mathrm{V},$ as shown in figure 1. The electron cyclotron wave in high-$T_{\mathrm{e}0}$ discharges is mostly manifested in the role of heating, and $J_{\mathrm{EC}}$ is very low. The current components are modeled using the code ONETWO [2, 21] with experimental inputs such as the $T_{\mathrm{e}}$ profile, ion temperature $T_{\mathrm{i}}$ profile and electron density $N_{\mathrm{e}}$ profile, as shown in figure 3. The $q_0$ is about 0.75 in the selected time as shown in figure 3(a). In this case, $J_{\mathrm{RF}}=0.75J_{\mathrm{total}},$ and $J_{\mathrm{BS}}=0.25J_{\mathrm{total}}$ at the location of $\rho =0.25,$ where the gradient of pressure peaks. The region of enhanced temperature profile and broadened current profile only limits within the $q=1$ surface. The off-axis peak of the current profile is evidenced by the hard x-ray (HXR) profile (figure 3(c)).

Figure  3.  (a) $T_{\mathrm{e}},$ $T_{\mathrm{i}}$ and $q$ profiles at 6 s, (b) $N_{\mathrm{e}}$ profile at 6 s, (c) the HXR profile with energy range of 20–40 keV, (d) modeled current profiles. The vertical dashed line indicates the location of the $q=1$ surface.

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3.4   Multiscale interaction and discussion

Previous observations with EAST suggest that the existence of the m/n = 1/1 mode plays a key role in sustaining stationary high-$T_{\mathrm{e}0}$ long-pulse plasmas [1]. The sawtooth replaced by a saturated MHD mode [22] is important to high performance plasma with high $T_{\mathrm{e}0}.$ Because the presence of sawtooth instabilities will reduce performance and might trigger deleterious instabilities. In agreement with the observations in 2018 and 2019, the helical m/n = 1/1 mode is also present in the plasma core in long-pulse high-$T_{\mathrm{e}0}$ discharge #98958 in 2021, as shown in figure 4. As shown in figure 4(b), the m/n = 1/1 mode can interact with small-scale turbulence, which is driven by electron temperature gradient, hence a decrease the turbulence intensity. In [3], a turbulence driven current has been confirmed, and it could be a candidate explanation of multiscale interaction between turbulence and m/n = 1/1 mode. Saturated interchange mode around the magnetic axis of plasma drives a near-helical flow pattern [22] and then mitigates the core turbulence level, which could be another reason for multiscale interaction. Multiscale interaction provided a new mechanism for turbulence mitigation under low torque.

Figure  4.  An m/n = 1/1 helical mode is observed in the core of a high-$T_{\mathrm{e}0}$ discharge with a duration over 100 s. (a) Windowed frequency spectrum of soft x-ray measurement, (b) competition of m/n = 1/1 mode and small-scale turbulence measured by Doppler backscattered diagnostic. The growing of m/n = 1/1 mode decreases the intensity of turbulence.

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4.   Conclusion and future plan

A steady-state long-pulse discharge with high-$T_{\mathrm{e}0}$, lasting longer than 100 s, which is aided by an ITER-like tungsten divertor, has been obtained with a fully noninductive plasma current driven by LHCD and ECRH. This newly achieved steady-state long-pulse high-$T_{\mathrm{e}0}$ scenario demonstrates the progress of physics related to multiscale instabilities under weak collisionality. EAST plans to demonstrate the steady-state high confinement (H mode) operation with $T_{\mathrm{e}0}>8.5\mathrm{keV}$ and maintain it over 1000 s (wall-plasma equilibrium time) in the future. Furthermore, a newly developed novel $V_{\mathrm{loop}}$ feedback control technology will be applied for MHD control in future.