Low frequency fluctuation study using a microwave interferometer and Hα line emission measurement systems in the Pilot-PSI device

A frequency multiplied microwave interferometer, a H α line emission measurement system, and a high speed camera system were installed on the Pilot-PSI device for low frequency fluctuation study in the detached plasma condition. The two dimensional H α line emission and its fluctuation were monitored with a fast visible camera with H α filter. The coherent low frequency fluctuations of frequency of approximately 13 kHz were measured by all measurement systems. The stronger fluctuation intensities were observed in the downstream of the ionization front region in the detached plasma condition. Moreover, we show the clear difference between the strong fluctuation regions of the electron line density and H α line emission for the first time. This means that the fluctuations of H α line emissions was caused by not only electrons but also by hydrogen ions.


I. INTRODUCTION
Detached plasma production is one of the most important objects for DEMO reactor for decreasing the heat flux to the divertor plate. The detached plasma conditions are successfully produced in the linear plasma devices with optimization of background gas pressures and plasma source parameters. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] In the detached plasma condition, the coherent fluctuation with the frequency of E × B drift is observed in many linear plasma devices. [10][11][12][13][14][15][16] The fluctuation study in the diverter plasma devices are carried out to understand the intermittent convective plasma transport across magnetic field lines, for example, nonlinear processes related to instabilities and thought to be responsible for the particle and energy loss to the wall, are commonly referred to as blobs. [1][2][3][4][5] Detailed studies of blobs and density fluctuations have been performed mainly by using Langmuir probes with measuring the ion saturation current and floating potential. [1][2][3][4][5] In the divertor simulation linear plasma devices, electron density fluctuations have not been clearly observed. Microwave interferometry systems are employed to measure the electron line integrated density and its fluctuations in divertor relevant plasmas. [11][12][13][14] In order to study the mechanisms of the fluctuations in the detached plasma condition, we installed the frequency multiplied microwave interferometer system, the Hα emission detector, and a high speed Hα camera system to study fluctuations in the Pilot-PSI device. Pilot-PSI is a magnetized linear plasma device designed for investigations of plasma-surface interactions at ITER relevant parameters. [16][17][18][19][20][21] The objective of the study is to know the fluctuation characteristics in the detached plasma condition. The strong fluctuations were observed around the recombination front region where the strong gradients of plasma parameter existed in the detached plasma conditions. [11][12][13][14][15] In the other researches except for pilot-PSI, there were no measurements of microwave interferometer system and Hα line emission system, simultaneously, while the Langmuir probe and the microwave interferometer system were mainly used for fluctuation study. They have an advantage to measure the plasma without disturbances. In this research, we simultaneously use the microwave interferometer system and Hα line emission measurement systems to study fluctuation characteristics in the detached plasma of Pilot-PSI device. The fluctuation frequency of approximately 13 kHz which caused by E × B drift was observed and the two dimensional fluctuation characteristics of Hα line emissions show the radial and axial fluctuations in the detached plasma conditions for the first time. We found that strong fluctuation region of the Hα line emission was observed at the upstream from the strong fluctuation region of the electron line density for the first time.

II. EXPERIMENTAL APPARATUS
Schematic view of Pilot-PSI is shown in Fig. 1. It consists of a 1 m long and 40 cm diameter stainless steel vacuum vessel placed inside five magnetic fields coils. The plasma is produced by a cascaded arc discharge operating in various gases (z = 0 mm). 16 An axial magnetic field of up to 1.6 T confines the plasma on the axis of the vessel in the form of a 0.5 m long beam of about 1-2 cm in diameter. In this study the arc was operated in hydrogen at a gas flow of 3.5 standard liter per minute (slm; 1 slm = 4.5 × 10 20 particles/s). A discharge current (I) and a magnetic field strength (B) were changed from 80 to 100 A and from 0.4 to 1.6 T shot-by-shot, respectively. The pressure in the vacuum vessel during arc operation is usually kept at a value of 1 -10 Pa. To facilitate the plasma detachment, we have varied the background pressure in the range of 5.5 -18.0 Pa by the means of changing the pumping speed. The electron density and temperature near the target is less than 1 × 10 11 cm -3 and 0.1 eV, respectively. In this plasma condition, the detached plasma is produced.
The frequency multiplied microwave interferometer system (Fig. 2), which was constructed in GAMMA 10, [11][12][13][14]22 was located at either about 290 mm downstream of the source nozzle (z = 290 mm, port #2) or at a distance of about 30 mm in front of the target surface (z = 550 mm, port #3). It is a heterodyne interferometer system equipped with a 17.5-GHz phase locked dielectric resonator oscillator, a 37.5-MHz temperature-compensated crystal oscillator, as well as frequency multipliers for obtaining 70 GHz probing beam and reference beam. The interferometer design achieves a frequency stable interferometer system. Horn antennas are set perpendicularly to the plasma beam. The cut off plasma density of the system is 6 × 10 13 cm -3 . Spatial resolution of the interferometer system is about 3 cm. Phase detection signals are recorded by an oscilloscope (Tektronix, DPO4034) with sampling rate of 1 MSa/s. Molecular Activated Recombination (MAR) and ion electron recombination are efficient production channels for excited neutrals. 20 Hα line emission fluctuation could depend on both the electron and hydrogen ion fluctuations. To investigate this, a Hα line emission detector of photodiode (Thorlabs, SM1PD1A) and a fast visible camera (Phantom V12.1, 55009 fps, 256 × 256 pixels, and 12 bit) equipped with an Hα band pass filter were used. The Hα line emission detector was set at the upstream port (z = 50 mm,

ARTICLE
scitation.org/journal/adv port #1) with the mirror to view the same observation cord with the interferometer system and the output signal is observed by the same oscilloscope as the interferometer system. The fast camera system was located at the same observation port as the interferometer system after removing the interferometer.
Thomson scattering system was also installed at two ports (z = 50 mm at port #1 and 500 mm at port #3) for measuring electron temperature and density radial profiles. It can measure them in a 1 mm spatial resolution and 10 Hz time resolution. Details of the system were shown in elsewhere. 21 Target plate is divided in three circle plates to investigate the radial potential profile and is set at z = 550 mm. The output signal of the core plate potential is measured by the same oscilloscope as the interferometer system.

III. FLUCTUATION MEASUREMENTS
Hydrogen plasma was produced with changing the magnetic field and discharge current of B = 0.4, 0.8, 1.0, 1.2, and 1.6 T, and I = 80, 100, 110, and 120 A, respectively, shot-by-shot. The plasma duration was 7 s. In the typical Pilot-PSI plasma, increase of the magnetic field strength shows the increase of electron density, and increase of the discharge current also shows the increase of electron density. The electron temperature slightly decreases along with the increase of the magnetic field strength and discharge current. Figure 3 shows the electron line density, Hα line emission, and target plate voltage measured by the interferometer system and Hα line emission detector, and target plate, respectively, measured at port #2 with magnetic field of B = 0.8 T and discharge current of I = 80 A. The line integrated density is about 2 × 10 13 cm -2 . The averaged electron density is about 1 × 10 13 cm -3 . Electron density and temperature measured by the Thomson scattering system are about 4 × 10 14 cm -3 and 3 eV at port #1 and less than 1 × 10 13 cm -3 and less than 0.2 eV at port #3, respectively, which showed that the plasma was in the detached plasma condition. The FFT analysed spectra of the electron line integrated density, Hα line emission, and the target plate potential are shown in Fig. 4(a), 4(b), and 4(c), respectively. They show the same strong coherent low frequency fluctuation of about 13 kHz and the higher harmonics. In Fig. 5(a) In Fig. 5(b), the fluctuation intensity is stronger at the plasma core region and upstream of the plasma. The fluctuation is clearly observed outside of the main plasma column compared to the Hα line emission image shown in Fig. 5(a). The 2D phase angle image shows that the phase angle of the fluctuation is maintained axially from the upstream to downstream and the radially rotating characteristics were observed. In Fig. 8, we show the phase angle averaged over axial direction radial dependence. The phase angle difference at the frequency of 13 kHz between the upper region of X = 150 and the We have found that four independent diagnostics measure the same coherent low frequency fluctuation in Pilot-PSI. In these plasma condition which we measured the low frequency fluctuation, the MAR and ion electron recombination (IER) are dominant effect to produce the excited neutrals. 20 The fluctuations were mainly produced by the rotating electrons. However, ions must be also rotating in the frequency of E × B drift rotation and affect the Hα emission fluctuation. In this experiment, we could not identify the efficiency of the ion rotation to the fluctuation.
As increasing the magnetic field strength, the electron density increased and as increasing the discharge current, the electron density mainly increased. The electron temperature slightly decreased along with the electron density increase. In Fig. 9, we show the electron density and temperature against the discharge current at the magnetic field strength of 1.2 T measured by the Thomson scattering system settled at #1 port. Unfortunately, we could not obtain the electron density and temperature dependences against the magnetic field strength. However, we show those dependences against the magnetic field strength at discharge current of 200 A on the another experiments in Fig. 9(b). It was hard to measure the electron In the detached plasma condition, the electron density and temperature decreased along the magnetic field to the target plate. 1 In front of the ionization front region where is upstream of the recombination front region, the electron density and Hα line emission intensity are larger and higher than that in the downstream plasma. The strong fluctuation intensities were observed in the detached plasma condition and it seems to be in the ionization region. Moreover, the strong fluctuation region of Hα line emission was in the upstream of that of the electron line density. The fluctuations of Hα line emissions were caused by not only the electrons by also by the hydrogen ions.

V. CONCLUSION
We measured the coherent low frequency fluctuations by using the microwave interferometer system and Hα emission measurements by photodiode and high speed camera systems without plasma disturbance, and the target plate, simultaneously, for the first time, in the Pilot-PSI device. The coherent fluctuation frequency of approximately 13 kHz which caused by E × B drift was obtained in all measurement systems. The strong fluctuation intensity was observed in the downstream of the ionization front region and that is in front of the recombination front. The strong fluctuation region of the Hα line emission was observed at the upstream from the strong fluctuation region of the electron line density for the first time. This means that the fluctuations of Hα line emissions were caused by both the electron and hydrogen ions.