Study of detached plasma profile in the divertor simulation experimental module of tandem mirror GAMMA 10/PDX

Detached plasma formation in fusion plasma devices is one of the most important methods for handling heat and particle fluxes to the plasma facing components. There are many linear plasma devices for studying the detached plasma conditions without confined core plasma.^1–4 In GAMMA 10/PDX, using the divertor simulation experimental module (D-module) [see Figs. 1(a) and 1(b)], the underlying mechanisms for reducing heat and particle fluxes to the divertor plate under detachment conditions relevant for ITER SOL and divertor plasma^5–8 are investigated. In GAMMA 10/PDX, the tandem mirror confined core plasma exhibits an electron and ion temperature of 100 and 5 keV, respectively. This enables us to produce ITER SOL relevant plasma heat flow to the divertor simulation region except for plasma particle flux. The detached plasma structure, which contains the radiation region, ionization front region, and recombination region along the magnetic field line toward the divertor plate, was not clearly observed in several divertor experiments.^1,4,9–12 GAMMA 10/PDX confines the main plasma in the central cell (CC), and the escaping plasma is led to the D-module in the end cell (EC) to perform divertor simulation plasma experiments. In the D-module, the electron temperature and density are normally measured using the electrostatic probes on the V-shaped target plate. In the previous studies, the upstream plasma parameters in the D-module could not be observed.^5–7 For direct measurements and detailed detached plasma studies inside the D-module, an EC-Thomson scattering (EC-TS) system, which is a part of a dual-path TS system containing a CC-TS system, and an EC-microwave interferometer (EC-MIF) system were prepared. In addition, the movable electrostatic probe was placed at the inlet of the D-module for measuring the inlet plasma density and temperature. Moreover, a high speed camera system with a wavelength filter was used to measure the behavior of the plasma radiation inside the D-module.

In this paper, we show the detached plasma structure and the entire density and temperature structure from the core plasma to the divertor plate determined by using all measuring systems. The dynamics of the ionization region and the Hα and Hβ emission images were recorded to study the recombination process during plasma detachment.

GAMMA 10/PDX is the largest tandem mirror machine with a minimum-B anchor and consists of CC and an EC with a divertor simulation module.⁵ The x axis and y axis are defined perpendicular to the magnetic field in the vertical and horizontal directions, respectively, and the z axis is parallel to the magnetic field. In standard hot-ion-mode of operation, the ion cyclotron range of frequency (ICRF) waves are injected for the MHD stabilization at the anchor cell (AC) and for ion heating at the CC, respectively. Figure 1(a) shows the schematics of the experimental setup of a part of GAMMA 10/PDX, indicating the dual-path Thomson scattering system, which contains CC-TS (z = 0.60 m) and EC-TS (z = 10.875 m), for measuring the electron temperature and density both at the central and end cells, simultaneously, as well as the MIFs for measuring the average electron density at each cell and in the D-module. Figure 1(b) shows the D-module installed at the end cell. The inner angle of the V-shaped target was set to 45°. Electrostatic probes (ESPs) are mounted on the 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 set on the same flux tube of ESP No. 5), and a movable ESP (IP, z = 10.35 m) is installed at the plasma inlet of the D-module. The typical electron temperature and density in the D-module are 1–20 eV and (0.01–1) × 10¹⁸ m⁻³, respectively. For enabling hydrogen gas puff into the D-module, a gas injection line is installed at the D-module inlet, as shown in Fig. 1(b).

Details of the CC-TS and EC-TS in the dual-path TS system are shown in Refs. 13 and 14. A 10 Hz YAG laser (Continuum, Powerlite 9010) is injected into both the CC and EC, simultaneously, in the same laser shot in a single plasma shot. In this experiment, we used laser energies of 1.4 J/pulse and 0.3 J/pulse for CC and EC-TS, respectively. The CC-TS consists of a split YAG laser beam, three spherical mirrors for collecting the TS light at 90°, a five channel polychromator, and high speed oscilloscopes. We can measure the radial electron temperature and density at seven radial positions. The EC-TS measures the 160° backscattered TS light for measuring the electron temperature and density in the D-module plasma center.

The MIFs are set at the CC (z = −0.6 m), anchor cell (AC, z = 5.2 m), barrier cell (BC, z = 8.6 m), plug cell (PC, z = 9.7 m), and EC-MIF (EC-MIF3 and EC-MIF4 at z = 10.786 and 10.766 m, respectively) for measuring the electron line densities. The CC, AC, BC, and PC-MIFs use a 70 GHz microwave, and EC-MIFs use a 64 GHz microwave. The EC-MIFs are the heterodyne interferometer system equipped with a local integrated array (LIA) system.¹⁵ In this experiment, only three channels of LIA aligned in the z direction are available. The length of the interferometer chords across the plasma in the D-module is ∼0.26 m. We used the average plasma density in the D-module as dividing the line density by 0.26 m.

To measure the behavior of the plasma radiation inside the D-module, a high speed camera system with a wavelength filter of Hα or Hβ is used.⁸ The signal level of the high speed camera with the wavelength filter is proportional to the product of neutral and electron densities; however, it is not absolutely calibrated. The frame rate and shutter speed were changed from 20 kHz and 10 µs to 10 kHz and 20 µs along with the radiation intensity. The amplitude resolution was 8 bits, and the frame size was 240 × 320 pixels.

In order to study the formation of the detached plasma structure inside the D-module, the electron density and temperature were measured from the core to edge region using the dual-path TS system, EC-MIF, and all probes. The plasma is heated and maintained while applying ICRF waves from t = 51 to 440 ms. The additional hydrogen gas puffing began at t = 50 ms, and the pulse duration was 400 ms for a plenum pressure of 750 mbar in the D-module to produce a detached plasma condition from t > 200 ms. The gas pressure by additional H₂ gas is thought to produce the detached plasma condition.^5–8 

Time evolutions of diamagnetism in CC and line average densities in each cell are shown in Figs. 2(a) and 2(b), respectively. The red line, blue line, green line, black line, and brown line show the line average densities of central cell (NECC), west anchor cell (NEWA), west barrier cell (NEWB), west plug cell (NEWP), and end cell (EC-MIF3), respectively. In Fig. 2(c), time evolution of pressure in the D-module was measured by the ASDEX gauge in the same plasma sequence. The diamagnetism gradually decreased with injection of additional gas puffing in the D-module. The electron line density in the CC was kept constant. In the west anchor, west barrier, and west plug cells, the electron line densities were increased by the additional gas puffing. The line density in EC showed the role over behavior.

We show the time evolutions of electron temperature and densities measured by ESPs of Nos. 1–5 on the V-shaped target plate in Figs. 3(a)–3(e) and ESP No. 18 in Fig. 3(f) by red circles and blue squares, respectively. The electron temperatures are less than 10 eV from t > 200 ms on the V-shaped target plate. The electron densities show the rollover characteristics observed by the ESP Nos. 1–4 on the V-shaped target plate. The temporal evolution of the electron density of ESP No. 5 shows the gradual decrease after the electron density peak at t = 150 ms. The time evolution of the electron density of ESP No. 18 has the density increasing profile, and the increasing rate decreases after t = 220 ms. The peak electron density time at ESP No. 5 is faster than that at ESP No. 1.

The time evolutions of electron temperatures (a) and densities (b) measured by using the CC-TS, IP, EC-TS, and No. 1 probe, respectively, are shown in Fig. 4; the averaged electron densities measured by EC-MIF3 and EC-MIF4 are also shown in Fig. 4(b).

Figures 5(a)–5(i) show two-dimensional images of Hα radiation [(a), (d), and (g)], Hβ radiation [(b), (e), and (h)], and Hα/Hβ intensity ratio [(c), (f), and (i)] measured by using the high speed camera system in the D-module at t = 100, 220, and 380 ms, respectively. The intensities of Hα and Hβ increase from t = 100 ms to t = 200 ms. The higher intensity regions of Hα and Hβ move to the upstream region. The intensity ratio of Hα/Hβ increases at t = 380 ms, and the maximum intensity ratio region is observed at the central region in the D-module.

In this detached plasma experiment, the radiator hydrogen gas was injected into the D-module from t = 50 ms and the electron temperature decrease and density increase were observed after t = 100 ms. The electron temperatures obtained by IP before t = 220 ms are almost the same as that measured by CC-TS, while the electron densities are very low. Around t = 220 ms, electron density peaks were observed inside the D-module by using EC-MIFs. After that, the electron densities moved to the decreasing phase. It is the rollover characteristics. The plasma of the downstream region was in the detached plasma condition from t > 220 ms. At t = 380 ms, the region in the detached plasma condition was axially expanded in the D-module.

Figure 6 shows the axial electron temperature (a) and density (b) profiles around the D-module and CC at t = 100, 220, and 380 ms. The electron temperatures decreased along with the z axis toward the corner of the V-shaped target plate. The electron density at the IP and the averaged density at the EC-MIFs are almost identical at t = 100 ms before the effect of the additional gas puffing. At t = 220 ms, the line average electron densities measured by EC-MIFs are much higher than that measured by both IP and EC-TS. This indicates that the ionization front region was located around the measuring region by EC-MIFs (around the center of the D-module) at t = 220 ms. The electron temperature and density of the D-module are ∼7 eV and 1 × 10¹⁸ m⁻³, respectively, at t = 220 ms and Z = 10.8 m. The molecular activated recombination (MAR) process is the dominant process in the downstream region of the D-module around the No. 1 probe, while in the upstream region of Z < 10.8 m, the ionization process is dominant at t = 220 ms.⁶ The MAR-dominant region seems to extend in the axial direction from t = 220 ms. At t = 380 ms, the line average density decreased and the electron density at the IP position slightly increased. The electron temperature was gradually decreased from ∼7 eV at IP to ∼2 eV at the No. 1 probe. It shows that the ionization front region moved to the upstream of the D-module. It is considered that the ionization front region moves upstream because there is no higher density region in the downstream area.

In Fig. 5, the higher intensity regions of Hα and Hβ emissions move to the upstream region. The intensity ratio of Hα/Hβ increases at t = 380 ms, and the high intensity ratio region is observed in the central region in the D-module. The increase of the intensity ratio of Hα/Hβ shows that MAR occurred in the downstream region and the ionization region moved to the upstream of the D-module at t = 380 ms.^6,8

The ionization front region in the detached plasma experiments is clearly observed for the first time, and it moved to the upstream region of the D-module along with the increase of the gas pressure in GAMMA 10/PDX.

We used the dual-path TS, which includes the CC-TS and EC-TS, the EC-MIF, the IP, and the high speed camera system for measuring upstream plasma on the detached plasma experiments in GAMMA 10/PDX. We successfully measured the core (CC) and edge (EC) plasma electron temperature and density simultaneously and obtained the axial density and temperature profiles in the detached plasma experiments. We clearly observed the ionization region along the central axis in the detached plasma for the first time in GAMMA 10/PDX. It is useful to study more details of the ionization region behavior in the detached plasma condition.

The authors would like to thank the members of the GAMMA 10 group of the University of Tsukuba for their collaboration. This work is performed with the support and under the auspices of the NIFS Collaboration Research program (Grant No. NIFS17KUGM120, NIFS14KUGM086).

The authors have no conflicts to disclose.

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