Behavior of detached simulation plasma by suddenly changed particle flux in GAMMA 10/PDX

Mitigating the effect of high heat and particle fluxes on the divertor region through plasma detachment is one of the foremost challenges in nuclear fusion reactors. The occurrence of an edge-localized mode (ELM) in H-mode tokamak plasma significantly influences the state of the detached plasma.^1–6 The ELM burn-through correlated with the detachment in divertor plasma and ELM. The effect of small ELM burn-through showed the result of comparison between ionization and recombination in divertor plasma. It is essential to examine the impact of transiently intensified heat and particle loads resulting from ELM under the detached plasma conditions.

Linear devices have been performed the pioneering studies exploring the concept of plasma detachment. Linear devices have contributed to the basic understanding of plasma detachment in many phenomena.⁷ The ELM simulation in the linear plasma devices using Pilot-PSI and Magnum-PSI were performed in high flux plasma and transient heat and particle source experiments.^8,9 The effects of ELM-like high heat and particle flux to the divertor plate were mainly investigated to study the behavior of the plasma facing materials. In Magnum-PSI, additional power providing the plasma source was used to perform ELM simulation experiments in detached plasma. They produced higher density of up to 10²¹ m⁻³ and a temperature of over 5 eV plasma and injected it into the detached plasma condition. They observed the plasma change from a detached to an attached condition.^10–12 In the NAGDIS-II linear divertor simulation plasma device, rf-heating were used for studying the dynamic behavior of detached plasma during ELM-like plasma heat pulse.^7,13

We carried out the divertor simulation plasma experiments under the higher temperature plasma condition, such as tokamak scrape-off layer (SOL) relevant plasma.^14–23 The electron temperature reached several tens of eV, and ion temperature reached up to 100 eV under normal plasma conditions at the tandem mirror device GAMMA 10/PDX. In GAMMA 10/PDX, it is useful to study ELM simulation under detached plasma conditions because it can produce higher temperature plasma similar to SOL plasma by using additional heating methods such as electron cyclotron heating (ECH), ion cyclotron heating (ICH), and neutral bean injection (NBI). Thus, we can relatively easily change plasma density and temperature by changing adjusting the additional heating powers. Moreover, many detailed plasma diagnostics for divertor simulation plasma are set in GAMMA 10/PDX. Within the divertor simulation experimental module (D-module) of the GAMMA 10/PDX end cell (EC), we introduced radiator gases to generate detached plasma and investigate the mechanisms underlying detached plasma formation, which lead to reduced heat and particle fluxes on the divertor plate. GAMMA 10/PDX predominantly confines plasma within the central cell (CC), with plasma escaping toward the end and being introduced into the D-module (Fig. 1). In the D-module, electron temperature and density were measured using electrostatic probes (ESPs; Nos. 1–5) mounted on a V-shaped tungsten target plate. To comprehensively study the detached plasma upstream of the target plate, an EC Thomson scattering system (EC-TS) within a dual-path TS system, an EC microwave interferometer system (EC-MIF), and a movable ESP were deployed. Microwave interferometer systems were installed in each cell, while a multichannel microwave interferometer system (MMIF) is positioned in the CC. In addition, a high-speed camera system (HSCAM) was employed to monitor two-dimensional (2D) detached plasma profiles in the D-module.

To investigate the dynamic responses with the impact of sudden increases in particle flux density on the detached plasma condition, akin to an ELM simulation, we simultaneously introduced supersonic molecular beam injection (SMBI) and CC ECH (C-ECH) into the core plasma (central and anchor plasmas), thereby increasing the particle flux density in the detached plasma within the D-module. SMBI was directed into the eastern anchor plasma, whereas C-ECH was administered into the CC. Prior to the SMBI and C-ECH injections, the typical electron temperature and density in the CC and D-modules were 40 eV and 2 × 10¹⁸ m⁻³, and 1–30 eV and 0.01–1 × 10¹⁸ m⁻³, respectively.

In a previous study, the dynamic response to a particle flux using additional C-ECH in the divertor simulation plasma was studied in GAMMA 10/PDX.¹⁸ The electron temperature decreased, and a rollover of electron density was observed upon an additional supply of hydrogen gas, both with and without ECH, under the detached plasma conditions. Moreover, with an increasing gas supply, the plasma detached due to molecular activated recombination (MAR). In the case of additional gas supply, the ionization front moved to the upstream side, and the particle flux flowing into the D-module increased with ECH.

This study elucidated the behavior of the detached plasma following the injection of an abruptly heightened particle flux induced by SMBI and/or C-ECH in the core plasma. Notably, variations in electron densities and temperatures were observed within the dual-path multipass TS system in the D-module and CC after the SMBI and/or C-ECH injections. Furthermore, the monitoring of 2D detached plasma conditions using HSCAM showed the direct impact of sudden alterations in particle flux on the divertor simulation plasma.

The tandem mirror GAMMA 10/PDX consists of minimum-B anchors on both sides of the main confined region in the CC, with ECs set at the end regions. The divertor simulation experiments were conducted using the D-module on the west side of the EC. The x, y, and z axes represent the vertical and horizontal directions perpendicular to the magnetic field and parallel to the magnetic field, respectively. For hot-ion-mode plasma production, the wave deposition for the ion cyclotron range of frequencies (ICRF) was utilized for magnetohydrodynamic (MHD) stabilization at the anchor cell (AC) and ion heating at the CC. Figure 1(a) shows the schematic of GAMMA 10/PDX, showing the dual-path Thomson scattering (TS) system, which includes CC-TS (z = 0.60 m) and EC-TS (z = 10.825 m), for the simultaneous measurement of electron temperature and density at both the CC and EC. In addition, microwave interferometers (MIFs) were installed to measure the average line electron density in each cell. The D-module installed in the western EC is shown in Fig. 1(b). ESPs were positioned on a V-shaped tungsten target plate (Nos. 1–5 at z = 10.968, 10.903, 10.837, 10.773, and 10.708 m, and Nos. 17 and 18 at z = 10.573 and 10.438 m, respectively, set on the same flux tube as in ESP No. 5). A movable ESP (IP, z = 10.35 m) was installed at the inlet of D-module. Calorimeters were installed to y and z directions on the target plate (Nos. 1–5 same z positions as the ESPs Nos. 1–5). The spatial distribution of heat flux along target plate was measured by using calorimeters. The inner angle of the V-shaped target was set to 45°. A radiator gas-injection line was installed at the inlet of D-module. Hydrogen gas was used in this experiment to produce the detached plasma.

The details of the dual-path TS system installed in GAMMA 10/PDX are provided in Refs. ^19–22. The YAG laser was split into two paths using a polarization control system and introduced into both the CC and EC using the same number of laser shots during a single plasma shot. Both the CC-TS and EC-TS employed a multipass system. In this experiment, laser energies of 1.4 J/pulse and 0.3 J/pulse were used for the CC-TS and EC-TS, respectively. The CC-TS system comprises three spherical mirrors for collecting 90° TS light, seven five-channel polychromators, and high-speed oscilloscopes, allowing the measurement of electron temperatures and densities at seven radial positions. The EC-TS system utilizes 160° backscattered TS light and can measure the electron temperature and density at the center of the D-module plasma.

The MIFs were set up at various locations, including the CC (z = −0.6 m), AC (z = 5.2 m), barrier cell (BC, z = 8.6 m), plug cell (PC, z = 9.7 m), and EC-MIF (z = 10.786 m), to measure the electron line densities. The frequency of the microwaves in the CC-, AC-, BC-, and PC-MIFs was 70 GHz, whereas that in the EC-MIF was 64 GHz. The EC-MIF is a heterodyne interferometer system with a local integrated array (LIA) system²³ that employs only one LIA channel aligned in the z direction for this experiment. The length of interferometer chords across the plasma in the D-module is ∼0.26 m, and the average plasma density in the D-module is calculated by dividing the measured line density by 0.26 m.

The SMBI is a high-speed and highly directive gas injection system capable of injecting neutral particles deeper into the core plasma using a simple setup.²⁴ In the SMBI, high-pressure hydrogen gas was injected through a fast solenoid valve with a laval nozzle. In GAMMA 10/PDX, the SMBI system was installed in the upside port of the east AC at z = −3.70 m. In the experiment, the SMBI was set at a plenum pressure of 2.0 MPa and a pulse width of 0.5 ms.

The C-ECH, generated by a 28 GHz, 500 kW gyrotron, was installed at z = −2.45 m in the east CC for effective electron heating. The C-ECH employs a mirror and two polarizers to change the injection position and polarization to improve the heating performance and transport effect.

The HSCAM with a wavelength filter of Hα or Hβ was used to observe the 2D behavior of plasma emissions inside the D-module.^16–19 The output of HSCAM is proportional to the neutral hydrogen and electron densities. The frame rate and shutter speed were adjusted from 20 kHz to 10 µs to 10 kHz and 20 µs, respectively, depending on the emission intensity. The intensity resolution and frame size were 8 bits and 240 × 320 pixels, respectively.

Hydrogen plasma was produced and heated via ICRF waves from t = 51–440 ms, coupled with additional hydrogen gas puffing, which served as a radiator gas in the D-module from t = 50–450 ms, employing a plenum pressure of 1200 mbar for the detached plasma experiments. SMBI, with a plenum pressure of 2 MPa, was introduced into the east AC at t = 301–301.5 ms. Time evolution plots of the diamagnetism and line density with SMBIs in the CC (a) and the electron temperature and density obtained by the probe in the D-module (b) are shown in Fig. 2. In the EC, following the injection of hydrogen radiator gas, a rollover behavior was observed in the electron density, accompanied by a decrease in the electron temperature. Detached plasma was obtained after t = 250 ms. The effect of SMBI exhibited a delay of ∼2 ms in the electron line density. During the SMBI period, the electron line density distinctly increased, whereas the diamagnetism decreased.

Plots illustrating the time evolution of the electron temperatures (red circles) and densities (blue squares) obtained by CC-TS and CC-MIF (green line) are shown in Fig. 3(a). The time width of the graph was expanded from t = 295 to t = 315 ms, as shown in panel (b). The electron densities and temperatures before and during SMBI were ∼30 eV and 1.5 × 10¹⁸ m⁻³ and 15 eV and 1.8 × 10¹⁸ m⁻³, respectively. Following the application of the SMBI, the electron temperature decreased, whereas the density increased. Figures 4(a) and 4(b) show the time evolution of the electron temperatures and densities obtained by EC-TS and EC-MIF, respectively, with the time width expanded from t = 295 to t = 315 ms. In the D-module, the electron temperature slightly decreased, while the density clearly increased with the SMBI application. Figure 5 shows the time evolution of ion flux density measured by movable ESP. With the effect of SMBI, the ion flux was decreased.

Figure 6 shows the Hα and Hβ emission images, along with images of the I_Hα/I_Hβ ratio measured by the HSCAM. Images depicting the Hα and Hβ emissions and I_Hα/I_Hβ ratio measured before SMBI at t = 300.0 ms are shown in panels (a)–(c); those during SMBI at t = 303.95 ms are shown in panels (d)–(f); and those after SMBI at t = 306.0 ms are shown in panels (g)–(i), respectively. SMBI enhanced the detached plasma conditions in the D-module.

C-ECH was initiated at t = 300–310 ms with a power of 100 kW, alongside SMBI with a 2 MPa plenum pressure introduced into the east AC at t = 301–301.5 ms. Time evolution plots of diamagnetism and line density with both C-ECH and SMBI in the CC (a) and the electron temperature and density obtained by the probe in the D-module (b) are shown in Fig. 7, indicating C-ECH and SMBI time. The SMBI exhibited a delay of ∼2 ms in the electron line density. During the C-ECH period, the electron line density increased notably, whereas the diamagnetism decreased. Following the SMBI, they recovered during the C-ECH period. The electron line density and diamagnetism decreased with the SMBI during C-ECH.

Plots depicting the time evolution of the electron temperatures (red circles) and densities (blue squares) obtained by CC-TS and CC-MIF (green line) are shown in Fig. 8(a), with the expanded time width of the graph from t = 295 to t = 315 ms displayed in panel (b). The electron densities and temperatures before and during SMBI and C-ECH were ∼40 eV and 1.8 × 10¹⁸ m⁻³, and 60 eV and 2.0 × 10¹⁸ m⁻³, respectively. With the C-ECH application, the electron temperature and density increased, whereas with the SMBI application, both decreased slightly. Figures 9(a) and 9(b) show the time evolution of the electron temperatures and densities obtained by EC-TS and EC-MIF, respectively, with the time width expanded from t = 295 to t = 315 ms. In the D-module, the electron temperature and density increased with C-ECH application from 3 to 7 eV and 1.0 × 10¹⁸ to 1.6 × 10¹⁸ m⁻³, respectively, and slightly decreased owing to the SMBI effect. Figure 10 shows the time evolution of ion flux measured by movable ESP. With the effect of C-ECH, the ion flux was clearly increased. Unfortunately, the time resolution of the ion flux measurement was low, and the effect of the SMBI was not observed.

Figure 11 shows the Hα and Hβ emission images, along with the intensity ratio of I_Hα/I_Hβ images. Images depicting the Hα and Hβ emissions and I_Hα/I_Hβ before C-ECH at t = 300.0 ms are shown in panels (a)–(c), those during C-ECH at t = 302.0 ms are shown in panels (d)–(f), those during C-ECH and SMBI at t = 304.25 ms are shown in panels (g)–(i), those at the C-ECH decay period at t = 310.0 ms are shown in panels (j)–(l), and those after C-ECH at t = 311.75 ms are shown in panels (m)–(o), respectively. C-ECH transitioned the plasma from detached to attached conditions, whereas SMBI weakened the effect of C-ECH on the D-module. Following SMBI, the plasma returned to a completely attached state. Immediately after C-ECH, the plasma reverted to a detached state.

Additional particle injection via SMBI under detached plasma conditions led to an increase in the electron density and a decrease in the electron temperature in the central plasma. Conversely, both the electron density and temperature decreased in the EC. This strengthened the detached state and boosted the MAR process, as indicated by the I_Hα/I_Hβ ratio. The decreased electron density in the EC was recovered to the previous density in ∼2 ms. The intensified detachment condition transitions back to the original state in a few ms. In Figs. 6(d) and 6(e), higher intensity regions appear narrower compared to images at other times. Although the cause of this effect is not apparent, it is believed that the radial profile of particle beams flowing toward the end was altered by the SMBI injection.

Under detached plasma conditions, additional heating from the C-ECH and SMBI in the core plasma shifted the plasma from detached to attached. In the CC, the C-ECH increased the electron temperature, enhancing ionization in the D-module and transitioning the plasma from detached to attached in ∼2 ms. This resembles the behavior observed during ELM, such as ELM burn-through. The particles injected via SMBI weakened the ionization in the D-module, causing a decrease in the electron temperature in the higher-temperature plasma generated by the C-ECH. Figure 11(i) shows that the MAR process was not clearly discernible during the SMBI-affected period, likely because the high electron temperature hindered the MAR process induced by C-ECH in the D-module. After the effect of the C-ECH, the plasma returned to the detached plasma condition again in less than 2 ms.

The total heat fluxes measured by using a calorimeter at the corner were ∼0.067 and 0.072 MW/m² with SMBI and with C-ECH and SMBI, respectively. The heat flux was increased with both SMBI and C-ECH injections. The heat flux of 0.005 MW/m² is the effect of C-ECH. If the heat flux at the movable ESP is about 70 times larger than that at the corner, the heat flux before the D-module is ∼0.35 MW/m² with C-ECH injection.

The dynamic response of the higher density flux in the diverter simulation detachment plasma was studied. The effects of the heat and particle flux by the electron heating using C-ECH in the CC were investigated. The effect of C-ECH made the diverter simulation plasma from the detached condition to attached condition. In addition to the C-ECH, the SMBI was injected in the AC. In the ELM-like condition in tokamak, the electron and ion temperatures are ∼100 and 300 eV. Moreover, the electron density is 1 × 10¹⁹ m⁻³. In the experiments, the electron temperature and density were 40 eV and 2 × 10¹⁸ m⁻³, respectively, before additional heating. The detached plasma to attached plasma condition by the higher temperature plasma using C-ECH and re-detached by lower higher density plasma flow by SMBI.

In a previous study, the plasma dynamic response by injection of ECH in the CC was observed in the detached plasma.¹⁸ The gas incident in the D-module was injected before plasma generation and maintained at a constant pressure. In contrast, in our study, the gas enters and is injected after plasma generation, with the amount of gas changing over time, transitioning the plasma from attached to detached conditions. This allows us to observe changings in the plasma state within a single plasma shot. Moreover, in the previous study, there was no change in the D-module plasma electron temperature when ECH was applied. However, in our study, both the electron temperature and electron density increased during C-ECH application. In the previous paper, the pressure inside the D-module was high because the injection gas applied before plasma discharge. It is thought that the C-ECH effect was blocked upstream of the D-module, causing ionization upstream and maintaining a detached state downstream, thereby reducing the C-ECH effect. In comparison, this study was able to test the injection of higher electron temperature and electron density flow into the detached plasma by C-ECH. It is the first study to directly investigate the behavior from the detached plasma to the attached state.

The base electron density, in the order of 10¹⁸ m⁻³, is low compared to the SOL plasma in tokamaks and other divertor simulation linear plasma devices, which are in the order of 10¹⁹ m⁻³. Therefore, we are developing the SMBI and pellet injection study to produce higher density plasma conditions. The maximum plasma duration is 0.4 s in GAMMA 10/PDX, which is insufficient to observe steady-state phenomena. Then, we focused on studying the plasma behavior from the SOL-like higher temperature plasma to the divertor plate and the detached plasma production under the higher temperature plasma condition.

We continue to optimize the plasma density and temperature to simulate ELM through C-ECH and SMBI. This marks the initial observation of detached plasma behavior following the injection of C-ECH and SMBI into the core plasma, introducing higher electron temperatures and density particles into the detached plasma conditions.

To investigate the impact of sudden variations in particle density and temperature fluxes on the detached simulation plasma, we conducted experiments involving SMBI and C-ECH in the core plasma of GAMMA 10/PDX and introduced a higher particle flux into the D-module. Through these experiments, we unveiled the effects of abrupt changes in particle density and temperature fluxes on the detached simulation plasma within the D-module. Notably, these changes resulted in the transition from detached plasma to attached plasma in the D-module for the first time.

The authors thank the members of the GAMMA 10/PDX group at the University of Tsukuba for their collaboration. This study was conducted with the support of and under the auspices of a bidirectional collaboration research program at the University of Tsukuba (Grant Nos. NIFS20KUGM148, NIFS20KUGM159, and NIFS21KUGM165).

The authors have no conflicts to disclose.

Masayuki Yoshikawa: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Funding acquisition (equal); Investigation (lead); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Junko Kohagura: Data curation (equal); Validation (equal). Naomichi Ezumi: Data curation (equal); Investigation (equal). Tsuyoshi Kariya: Data curation (equal). Ryutaro Minami: Data curation (equal); Investigation (equal). Tomoharu Numakura: Data curation (equal). Mafumi Hirata: Data curation (equal). Satoshi Togo: Investigation (equal); Software (equal). Mizuki Sakamoto: Investigation (equal); Project administration (equal). Yousuke Nakashima: Data curation (equal). Yoriko Shima: Data curation (equal); Software (equal). Takuma Okamoto: Data curation (equal). Satoshi Takahashi: Data curation (equal). Ryo Yasuhara: Methodology (equal). Ichihiro Yamada: Methodology (equal). Hisamichi Funaba: Methodology (equal). Naoki Kenmochi: Methodology (equal). Shinji Kobayashi: Methodology (equal). Takashi Minami: Methodology (equal). Daisuke Kuwahara: Methodology (equal). Hennie V. D. Meiden: Methodology (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.