I-mode operation in helium plasma with pure radio frequency wave heating and ITER-like tungsten divertor on EAST

In order to support future ITER non-nuclear operation phase, comprehensive experiments with helium (He) ion (refer to the helium-4 isotope) as the dominant ion species have been carried out in the superconducting tokamak EAST under the condition of pure radio-frequency (RF) wave heating and ITER-like tungsten divertor [1, 2]. The helium concentration (C_He), defined as the ratio of helium (He) to deuterium (D) content, was found to play a critical role in determining plasma performance [3]. Both energy confinement time and edge localized modes (ELMs) frequency correlate with the change of C_He. At very similar plasma parameters of magnetic field B_T = 2.4 T, plasma current I_p = 0.5 MA and line-averaged plasma electron density n_e ∼ 3 × 10¹⁹ m⁻³, the high-confinement mode (H-mode) power threshold (P_thr) increases linearly with C_He, consistent with the results from DIII-D experiments [4]. At the time of experiments, it was very hard to access H-mode when C_He exceeded 70%, which was mainly attributed to the limited available power source. However, an improved confinement mode (labelled I-mode) regime [5] has been achieved for the first time in He plasma on EAST in the case of high C_He. The I-mode operation regime features high confinement with a temperature transport barrier at the plasma edge region [6–10], and excellent core impurity exhaust capability, nearly identical to that of L-mode plasma due to non-existence of the particle edge transport barrier (ETB) [11]. In contrast to the H-mode regime, the I-mode plasma also has the merit of absence of ELM instability, which could cause critical damage to plasma facing components via transient high heat and particle fluxes.

The I-mode operation regime on EAST [12] has been widely achieved in D plasma in the unfavorable B_T divertor configuration ( B × ∇ B directed away from the active X-point), where the L–H transition threshold power is significantly higher and the energy confinement is much worse than those at the favorable configuration [13]. The access to the I-mode regime in D plasma on EAST was found to be independent from the heating method and wall material. The appearance of weakly coherent mode (WCM) with a frequency of 50–120 kHz generally accompanied the occurrence of L-I transition on EAST, in line with Alcator C-Mod and ASDEX Upgrade results. The WCM on EAST is located within a certain radial extent (2–3 cm) at the plasma edge. A novel microscopic physics mechanism that can sustain stationary I-mode in D plasma has been recently reported on EAST [14], which sheds a light on the development of a long pulse high confinement I-mode scenario foreseen for future fusion devices, such as ITER and CFETR.

In this paper, the characteristics of I-mode operation in He plasma with pure RF-heating and tungsten divertor has been investigated for the first time on EAST, under the favorable B_T configuration. Previous experiments on C-Mod have also realized stationary I-mode operation in D plasma under specific plasma shapes in the favorable B_T configuration, which is thought to be a key actuator in accessing I-mode [6]. It is common knowledge that I-mode plasma is usually achieved in the unfavorable B_T configuration condition where L-H transition threshold power is much higher. The much higher H-mode threshold power for He plasma extends the operation window to access the I-mode regime, providing a novel area to investigate and understand pedestal physics related to the formation of ETB. The paper is structured as follows. Introduction to I-mode in He plasma is given in the next section. Section 3 describes the impact of helium concentration on L–I transition threshold power and energy confinement in I-mode operation. Section 4 compares the I- and H-mode operation including energy confinement and divertor heat flux at similar helium concentrations, followed by a summary and conclusion.

In this paper, the main plasma parameters for helium I-mode regime are as follows, I_p = 0.5 MA, B_T = 2.4 T and n_e = 2.5–4 × 10¹⁹ m⁻³. All of the discharges are performed in the upper-biased divertor configuration with ion ∇ B drift towards the active X-point, commonly known as the favorable B_T direction. It should be noted that the I-mode regime in D plasma is mostly achieved at the unfavorable B_T configuration. RF wave including the lower hybrid (LH) wave of both 2.45 and 4.6 GHz and electron cyclotron (EC) resonant frequency of 140 GHz working at X2 mode [15] were used as heating and current drive methods where LH was dominant. The pure RF wave heating enables the plasma to come into ITER-like operation conditions of dominant electron heating and zero external torque.

A typical L–I transition process in He plasma can be found in figure 1, where time traces of several key parameters are given. At t ∼ 2.58 s, the second EC power source was injected into the plasma leading to a total RF heating power of ∼3.5 MW (P_LH = 2.6 MW plus P_EC = 0.9 MW), and both the plasma stored energy W_dia and the electron temperature T_e, represented by ECE signal, increased immediately while no obvious change of plasma density was found. After that, the W_dia and ECE signal kept increasing but with a slightly slower ramp rate compared to that induced by initial heating of EC power. This bifurcation point is indicted by the green dashed line in figure 1, which is ∼70 ms (nearly one energy confinement time) after the injection of EC power. Meanwhile, the WCM with frequency of 30–90 kHz along with the 13 kHz edge temperature ring oscillation (ETRO) were observed in the time-frequency spectrum of turbulence rotation velocity measured by Doppler reflectometry. As the n_e and He-I line emission count stayed nearly constant during the second increasing period of W_dia and T_e, as well as the appearance of WCM and ETRO, transition from L to I mode in He plasma can be identified for the first time on EAST.

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Different from the I-mode operation in D plasma reported on ASDEX Upgrade, which usually gradually evolves into H-mode at constant heating power level [16], the I-mode regime in He plasma on EAST could reach a stationary state, as seen in figure 2. The stationary I-mode lasts from 2.65 s to 8 s, which is only limited by the EC heating power duration. The power spectra of turbulence rotation velocity show the ETRO mode of ∼13 kHz at all three radial locations of ρ ∼ 0.89 (red), ρ ∼ 0.95 (blue) and ρ ∼ 0.97 (black) in figure 2(c), indicating identical microscopic physics mechanism sustaining stationary I-mode for both He and D plasmas [14]. However, the 30–90 kHz mode only appears in the power spectra at ρ ∼ 0.89 and ρ ∼ 0.95, confirming itself as the WCM.

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Figure 3 illustrates the temporal evolutions of turbulence spectrum measured by Doppler reflectometry, along with the intensity of ion diamagnetic drift turbulence (IT) and electron diamagnetic drift turbulence (ET). From [14], the ETRO is found to accompany by alternating turbulence transitions between ET and IT, which can be also clearly observed from figure 3(a). The dominant turbulence mode was found to alternate between ET and IT, as IT intensity (blue) valley well corresponded to the peak of ET intensity (red). This phenomenon is consistent with that identified in D plasma on EAST.

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The L–H threshold power against plasma density in He plasma has been intensively investigated at various operation parameters of I_p = 0.4–0.5 MA and B_T = 1.8–2.4 T, see [3]. Figure 4 gives the results of L–I threshold power (diamond symbols) against plasma density at I_p = 0.5 MA and B_T = 2.4 T, along with the L–H statistic results (circle symbols) for reference. Here, in this paper, the experimental threshold power is defined as P_loss = P_ohm + P_aux − dW_dia/dt at the occurrence of L–I and L–H transition, where P_ohm is the ohmic power, P_aux is the total absorbed RF power and W_dia is the plasma stored energy. As can be clearly found from figure 4, both threshold powers of L–I and L–H in He are higher than the scaling law results for D plasma [17]. Due to limited data points for L–I case, it is not reliable to determine the dependence of I-mode threshold power on density now. The appearance of a relatively lower P_loss value (red diamond) that is heated by combination of RF and neutral beam (NB) compared to those with RF-only heating gives a hint that additional NB power seems more efficient in triggering transition from L to I mode, in agreement with the ASDEX Upgrade investigation that ion heat flux plays a key role in pedestal formation [16]. As is well-known, NB power can direct heat main ions, which is the main difference compared to dominant electron heating via RF power. Besides, the four blue diamond points at n_e ∼ 4 × 10¹⁹ m⁻³ own even higher P_loss values compared to the corresponding H-mode threshold power in He plasma, which is mainly attributed to the higher helium concentration (C_He > 70%), which should play a critical role in pedestal physics [3].

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In order to evaluate the helium concentration (C_He) impact on plasma performance at very similar conditions, experiments were carried out at various C_He levels under I_p = 0.5 MA and B_T = 2.4 T. The basic parameters of two typical discharges are shown in figure 5, where the red color represents the lower C_He ∼ 55% case and black curves refer to the higher C_He ∼ 74% one. The lower C_He plasma entered into H-mode (t ∼ 3 s) with total RF power of ∼3.7 MW, while only I-mode achieved for high C_He with optimized plasma density at even higher heating power level by adding extra 0.5 MW of LH. Note that running at n_e ∼ 3.6 × 10¹⁹ m⁻³ was expected to facilitate H-mode access, corresponding to the minimum L–H transition power threshold [3]. Due to the helium concentration effect on enhancing heating power needs for accessing H-mode, the higher C_He plasma for shot #94369 could only go into the I-mode regime with such an insufficient power source.

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It was found that the stored energy W_dia in the case of C_He ∼ 55% was ∼14% higher than that of C_He ∼ 74% at identical plasma density (time slice indicated by two dashed lines in figure 5(a)). To investigate the physics causes, plasma profiles of T_e and n_e are given in figure 6, where the T_e was measured by Thomson scattering and n_e was measured by the density profile reflectometry. For both cases, the pedestal structure of electron temperature was clearly observed, while that of density was only found for the H-mode case (red, C_He ∼ 55%), which confirmed again that the high C_He ∼ 74% plasma stayed in the I-mode regime. The slight deviation in the electron density profile at the plasma edge is not adequate to account for the ∼14% difference in W_dia, indicating that ion channel might play a dominant role. Limited to diagnostics technical issues, it is not possible to obtain experimental information on the T_i and n_i profiles at present.

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A statistic comparison of normalized energy confinement, H_98,y2, against C_He is plotted for three sets of database in figure 7, where blue diamonds are I-mode operation in He plasma running at favorable B_T, black circles represent H-mode He plasma also with favorable B_T and green diamonds are D plasma in I-mode regime at unfavorable B_T. All data were collected from the upper-biased divertor configuration at I_p = 0.5 MA and B_T = 2.4 T, with variables of n_e = 2–4.5 × 10¹⁹ m⁻³ and P_loss = 1.8–4.5 MW. The C_He was found to play a dominant role in determining the energy confinement, as all cases were well aligned to decrease with increasing C_He. It should also be convincible to conclude that for He plasma the energy confinement of I-mode is comparable to that H-mode at similar C_He. The physics understanding on the impact of C_He on transition from L to I/H mode and energy confinement is still ongoing by analyzing the edge E_r profile and its time evolution.

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In the helium experiments with relatively high concentration, the repetitive I–H–I transition was frequently observed at constant power injection. The time trace of one typical discharge is given in figure 8, where the I–H–I transition occurred several times and finally evolved into stationary I-mode. The rapid increase of n_e and W_dia, along with the sudden reduction in He-I signal at the same time revealed that the plasma entered into H-mode regime, which sustained for ∼40 ms and then transited into I-mode (judging by the appearance of WCM, which is not shown here). The time evolution of normalized energy confinement followed well with the I–H–I transition, with H_98,y2 reaching ∼1.02 during H-mode and slightly reduced to ∼0.98 at I-mode, in good agreement with the conclusion that for He plasma the I-mode confinement is comparable to H-mode at similar C_He.

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In the aspect of divertor power load control, the I-mode regime is more preferable for long-pulse high performance operation compared to H-mode, since the extremely high heat flux induced by ELMs is eliminated. A quantitative comparison between the ELM high heat flux accompanied with H-mode and the stationary heat flux at I-mode can be found in figure 9. The density profile shown in the left panel again confirmed the realization of I-mode as the particle transport barrier was not formed. In the right panel of figure 9, the blue curve represents the heat flux profile on upper-outer tungsten divertor, which was obtained by the heat flux computation code recently developed on EAST [18]. Note that the sampling rate of infra-red thermography diagnostics is only 100 Hz [19], which is much larger than the ELM characteristic time (typically ∼ 1 ms), resulting in underestimation of the peak heat flux induced by ELMs. Thus, the wording of ELM-averaged was used in this paper. Compared to the stationary heat flux at I-mode, ELM-averaged H-mode heat flux was much higher, increased by a factor of 1.5. In addition, the affected area of ELM-averaged heat flux was also wider than that of I-mode.

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The I-mode regime in helium plasma at favorable B_T (ion ∇ B drift towards active X-point) is accompanied by the typical 30–90 kHz WCM and a ∼13 kHz ETRO. The longest stationary I-mode in helium has been obtained at I_p = 0.5 MA, B_T = 2.4 T with a total injected power of 3.5 MW, lasting for ∼5.4 s without transitioning to H-mode. The physics mechanism sustaining the stationary I-mode in He plasma is found to be identical to that of D plasma, where the ETRO accompanied by the alternating between the ion diamagnetic drift turbulence and the electron diamagnetic drift turbulence is the key. It should be noted that the pulse length is only limited by the heating power duration. The L–I transition power threshold is lower than that of L–H at similar helium concentration (C_He). With increasing C_He, the required auxiliary heat power to access I-mode increases obviously and the energy confinement degrades linearly. Comparison of electron channel profile of n_e and T_e for low and high C_He cases indicates that reduction of energy confinement at high C_He would mainly come from the ion channel, which needs further investigation. The I-mode regime in helium plasma is comparable to H-mode with respect to energy confinement, and naturally owns the advantages of ELM-stability to eliminate high heat flux induced by ELMs and non-existence of particle transport barrier to facilitate impurity exhaust, which would make itself become one of the most promising operation scenarios for future fusion devices.

This work was supported by the National MCF Energy R&D Program (2019YFE03040000), the National Natural Science Foundation of China under Contract No. 12005262, and the Users with Excellence Program of Hefei Science Center CAS under Grant Nos. 2020HSC-UE009 and 2021HSC-UE018.