Thomson Scattering Measurement of Two Electron Temperature Components in Transition to Detached Plasmas

Abstract

We have performed laser Thomson scattering (LTS) measurements during the transition between attached and detached helium plasmas in the linear divertor simulator NAGDIS-II. In the detached plasma, the LTS spectrum shows a discrepancy with a single Gaussian function. The discrepancy is resolved by the spectrum fitting with a sum of two Gaussian functions, indicating that the electron energy distribution contains two different temperature components.Detached plasma plays an essential role for magnetically confined fusion devices to prevent divertor plates from being seriously damaged. In the detached plasmas, volume plasma recombination is one of the most important processes, strongly depending on electron temperature T e and density n e . In order to design DEMO and future devices, reliable plasma simulations should be established to predict the particle and heat loads on the divertor plate. A recent SOLPS modeling of DIII-D detached plasmas indicated that atomic and molecular processes and kinetic effects were important for reliable modeling particularly in low-temperature recombining plasmas [1]. Therefore, the precise measurements of T e and n e in the detached plasmas are an essential issue.Laser Thomson scattering (LTS) is known as a reliable technique for the measurements of T e and n e . LTS measurements of detached plasmas have been conducted in linear devices. In the MAP-II device, T e below 0.1 eV was acquired and compared with T e measured with Boltzmann plot and collisional-radiative (CR) model [2]. In the Magnum-PSI device, single-laser-pulse LTS measurements were performed with low observational errors [3]. LTS techniques for measurements of detached plasmas have also been used in the DIII-D device. A 2D divertor characterization was performed using the divertor Thomson scattering system through the transition from attached to detached plasmas [4].In this study, we applied LTS techniques to helium (He) plasma in the linear plasma device NAGIDIS-II (NAGoya DIvertor Simulator) and measured T e and n e in between attached and detached plasma states. In this LTS system, the second harmonics (wavelength 532 nm) of an Nd:YAG laser (Continuum: Surelite II-10: pulse width 5 -6 ns, pulse energy −0.3 J, repetition rate 10 Hz) was used. The collected light is transferred through the optical fiber (23 channels for observation) to the spectrometer, in which a volume phase holographic grating (2600 l/mm) was used. The signals were measured with the Gen-III ICCD camera (Andor: iStar) and accumulated for 300 s (3000 laser pulses). In order to reduce the stray light, baffles are equipped on the laser path, and a viewing dump is placed at the end of the field of view (detailed setup was described in Ref. [5]). Detachment was induced by controlling the neutral gas pressure, P, via the amount of injected gas near the end target.A typical LTS spectrum and fitting curve in attached He plasma (P = 5 mTorr) are shown in Fig. 1. The spec- Fig. 1 Typical LTS spectrum and fitting curve in attached plasma (P = 5.1 mTorr).

Development of Thomson Scattering Measurement System for Upstream Plasmas in the NAGDIS-II Device

“…A comparison of plasma parameters in downstream shows the reduction rate of the static plasma pressure P s from upstream to downstream varies by 14 % to 0.45 % as ionizing plasma is changed to recombining plasma. We also found that two different temperature components [10] exists only in the downstream region of recombining plasmas.…”

“…In the previous study, it was mentioned that two electron temperature components existed when the plasma state was in the transition from ionizing to recombining plasmas [10], suggesting that the two temperature components were attributed to fluctuations of plasma parameters. Figure 6 shows LTS spectrum and EEDF of downstream at P end = 6.5 mTorr, like the plasma state mentioned above.…”

“…Laser Thomson scattering (LTS) is known as a reliable method to measure electron density (n e ) and electron temperature (T e ) even in recombining plasmas. The LTS method has been used in various plasma devices, e.g., Pilot-PSI [7], Magnum-PSI [8], PSI-2 [9], NAGDIS-II [10], and other devices [11,12].…”

“…In this study, we have established an LTS measurement system in the upstream region of the linear plasma device NAGDIS-II, in addition to another LTS system developed in the downstream region [10]. These systems enable us to obtain reliable plasma parameters in upstream and downstream and to discuss the scale length of plasma parameters along the magnetic field without the disturbance.…”

Double‐probe measurement in recombining plasma using NAGDIS‐II

“…The tendency that T ed and T e_LTS slightly increased at P n > 2.2 Pa was observed in Figure a. The recent spectroscopic and LTS study using NAGDIS‐II showed that two different temperature components could independently appear in time . The slight increase in T e might indicate that the high temperature component was dominant when P n > 2.2 Pa.…”