T. OISHI et al.

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EFFECT OF DEUTERIUM PLASMAS ON CARBON IMPURITY TRANSPORT IN THE EDGE STOCHASTIC MAGNETIC FIELD LAYER OF LARGE HELICAL DEVICE

T. Oishi1,2 1National Institute for Fusion Science, National Institutes of Natural Sciences Toki, Japan 2Department of Fusion Science, SOKENDAI (Graduate University for Advanced Studies) Toki, Japan Email: oishi@nifs.ac.jp

S. Morita1,2, M. Kobayashi1,2, G. Kawamura1,2, Y. Liu2, M. Goto1,2 and the LHD Experiment Group1 1National Institute for Fusion Science, National Institutes of Natural Sciences Toki, Japan 2Department of Fusion Science, SOKENDAI (Graduate University for Advanced Studies) Toki, Japan

Abstract

The ergodic layer of Large Helical Device (LHD) consists of stochastic magnetic fields with three-dimensional structure intrinsically formed by helical coils. Reduction of the parallel impurity transport in the ergodic layer, so called “impurity screening”, has been studied in LHD. In the paper, the space-resolved vacuum ultraviolet (VUV) spectroscopy for carbon impurities is applied to deuterium (D) plasmas to clarify the effect of the bulk ion mass on the impurity transport in the ergodic layer. Doppler profile measurement at the second order of CIV line emission (2 × 1548.20 Å) is attempted using a 3 m normal incidence VUV spectrometer in the edge plasma at a horizontally-elongated plasma position. The flow velocity becomes the maximum value at the position close to the outermost region of the ergodic layer. The direction of the observed flow is same as the friction force in the parallel momentum balance calculated with a three-dimensional simulation code, EMC3-EIRENE. The flow velocity increases with the electron density in the hydrogen (H) plasmas. The result supports a prediction from the simulation that the friction force becomes more dominant in the force balance in higher density regime. It leads to the increase in the impurity flow which can develop the impurity screening. On the other hand, the flow velocity in the D plasma is smaller than that in the H plasma. The mass dependence of the thermal velocity of the bulk ions could be one of the reasons for the difference of the flow values between D and H plasmas when the friction force term is dominant in the momentum balance.

1. INTRODUCTION

Stochastization of edge magnetic fields has been extensively studied not only for the ELM mitigation but also for the plasma detachment and the impurity transport [1-3]. A thick stochastic magnetic field layer called “ergodic layer” of the large helical device (LHD) consists of stochastic magnetic fields with three-dimensional structure intrinsically formed by helical coils, while well-defined magnetic surfaces exist inside the last closed flux surface [4]. It is therefore extremely important to study the impurity behaviour and transport in the ergodic layer and to compare with those in the scrape-off layer of tokamaks. In LHD, it is found that carbon impurities are screened by the presence of the ergodic layer [5] and iron impurities are more effectively screened. As a result, the iron density in core plasmas of LHD is found to be extremely low despite the stainless steel vacuum vessel [6]. The effect of “impurity screening” has been also compared between the scrape-off layer of a tokamak and the ergodic layer of a helical device [7]. The precise measurement of impurity behavior is very important to develop a deeper understanding of the impurity transport in the edge ergodic layer.

A transport model for the impurity behavior in the ergodic layer has been proposed considering the parallel momentum balance on impurity ions along a magnetic field line connecting the core plasma and the divertor plate based on the following equation;

,6.271.01 22////

////

s

TZ

s

TZ

VVmZeE

s

nT

nt

Vm ie

s

impZi

ZZZ

Z

ZZ

(1)

where five terms in the right-hand side are contributions of impurity ion pressure gradient, parallel electric field, friction force between bulk ions and impurity ions, electron thermal force, and ion thermal force, in the order [8]. Among these terms, the friction force terms and the ion thermal force term are the dominant terms. When the ion density gradient increases, the friction force increase resulting the impurity flow is directed toward divertor plates, which means the impurity screening. On the other hand, when the ion temperature gradient increases,

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the ion thermal force increases resulting that the impurity flow is directed toward the core plasmas, which means the impurity accumulation. Therefore, a precise profile measurement of the impurity flow is truly required to examine the validity of the theoretical modelling on the impurity transport in stochastic magnetic field layer.

Recently, the carbon flow in the ergodic layer was measured in hydrogen (H) plasmas with vacuum ultraviolet (VUV) spectroscopy and a close relation between the impurity flow and the impurity screening was experimentally verified for the first time by the comparison between the spectroscopic observations and the impurity transport simulation based on a three-dimensional simulation code, EMC3-EIRENE [9-11]. In the paper, the VUV spectroscopy for carbon impurities is applied to deuterium (D) plasmas to clarify the effect of the bulk ion mass on the impurity transport in the ergodic layer.

2. EXPERIMENTAL SETUP

The LHD coil system consists of a set of two continuous superconducting helical coils with poloidal pitch number of 2 and toroidal pitch number of 10 and three pairs of superconducting poloidal coils. Figure 1 shows schematic drawings of toroidal plasma with helical coils and horizontally-elongated poloidal cross section of LHD. Poincare plot of stochastic magnetic fields in the ergodic layer under vacuum condition are shown for poloidal cross sections at horizontally-elongated plasma position of LHD for the position of the magnetic axis, Rax, of 3.6 m. The stochastic magnetic fields in the edge ergodic layer are plotted with color scale indicating the magnetic field connection length in addition to the magnetic surfaces. The ergodic layer mainly consists of stochastic magnetic field lines with connection lengths from 10 to 2000 m, which correspond to 0.5 - 100 toroidal turns in the LHD. Radial thickness of the ergodic layer varies toroidally and poloidally. When the magnetic axis is shifted outwardly, the ergodic layer is wider and the plasma size within the LCFS is smaller.

A space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer is utilized to measure impurity emission profile in the edge and divertor plasmas of LHD in wavelength range of 300 - 3200 Å [12]. A 3m normal incidence VUV spectrometer (McPherson model 2253) is installed on an outboard midplane diagnostic port as shown in Fig. 2. A back-illuminated CCD detector (Andor model DO435-BN: 1024 × 1024 pixels) is used for measuring a focal image of VUV line emissions at the exit slit of the spectrometer. A high wavelength dispersion of 0.037 Å/pixel enables Doppler profile measurement of the impurity lines basically over the whole wavelength range. The viewing angle can be switched between “ full profile measurement ” mode to cover an entire vertical region of the elliptical LHD plasma at horizontally-elongated poloidal plasma cross section to measure the top-to-bottom vertical profile and “edge profile measurement” to focus the viewing angle on the bottom edge with a high spatial resolution for observations of vertical impurity profile in the ergodic layer as shown in Fig. 2.

3. MEASUREMENTS OF FLOW VELOCITY OF THE CARBON IMPURITY

Figure 3 shows a typical waveform of a discharge with a magnetic configuration with Rax = 3.6 m and the toroidal magnetic field, Bt, of 2.75 T. Power of

FIG. 1. Schematic drawings of toroidal plasma with helical

coils and horizontally-elongated poloidal cross section of

LHD. Z and R mean vertical and horizontal coordinates

with the torus center as the origin of the coordinate system,

respectively. Poincare plot of stochastic magnetic fields

at ergodic layer in vacuum condition is shown in poloidal

cross sections at horizontally-elongated plasma position of

LHD for a magnetic axis configuration of Rax = 3.6 m.

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the electron cyclotron heating (ECH) and the neutral beam injection based on the negative ion sources (n-NBI) central electron temperature and central electron density, and the plasma stored energy are shown together. The discharge initiated by ECH is grown by three n-NBI beams with total port-through power of 14 MW as shown in Fig. 3. The working gas was D2 and the beam particle of the neutral beam was hydrogen in this shot (denoted as “D plasma”). A space-resolved VUV spectroscopy was attempted in the flat-top phase of the discharges, i.e. 3.8 ~ 4.4 s in the discharge show in Fig. 3, by using a 3m normal incidence VUV spectrometer. The edge flow profile is investigated with high spatial resolution by using the viewing angle of the edge profile measurement of the VUV spectroscopy from Doppler shift of the second order of CIV line emission (2 × 1548.20 Å) at a horizontally-elongated plasma position of LHD. Figure 4 shows a wavelength spectrum in the full wavelength range covered in a single discharge including CIV lines with wavelengths of 1548.20 × 2 and 1550.77 × 2Å. There are some spike noises because of the neutrons and the -rays only in the spectrum obtained for the D plasmas [13]. However, the effect of the noises is not so serious in the discharges analysed in the paper. Therefore, we can obtain CIV line intensity, ion temperature, and flow velocity derived from the Doppler profile of CIV line with the wavelength of 1548.20 × 2 Å for both H plasmas and D plasmas.

Figure 5 shows vertical profiles at the bottom edge of the ergodic layer of (a) CIV line intensity, (b) ion temperature, and (c) flow velocity derived from the CIV line emission measured by VUV spectroscopy for an H plasma and a D plasma with a magnetic configuration with Rax = 3.6 m and Bt = 2.75 T. The observation range of the edge profile measurement of the VUV spectroscopy is shown in Fig. 2. The flow velocity

FIG. 3. Typical waveform of a discharge in which the

space-resolved VUV spectroscopy is attempted. (a)

ECH and n-NBI power, (b) central electron

temperature and central electron density, and (c)

plasma stored energy are shown together.

FIG. 2. Schematic drawings of space-resolved VUV spectroscopy system using 3 m normal incidence VUV spectrometer

in LHD.

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FIG. 5. Vertical profiles at the bottom edge of the ergodic layer of (b) CIV line intensity, (c) ion temperature, and (d)

flow velocity derived from the Doppler profile of the second order of CIV line emission (2 1548.20 Å) for inward-

shifted magnetic configuration with Rax = 3.6 m. Open and closed circles show the results from D and H plasmas,

respectively.

along the sightline, vR, is given by vR = c ( / ), where c is the light speed, the Doppler-shift and the wavelength of line emission. The measured flow velocity is projection of the flow along the observation chord which can be approximately considered to be the direction of the plasma major radius. Therefore, a variable of vR is used to indicate the measured flow value. Positive and negative sign in the horizontal axis of Fig. 5(c) corresponds to the outboard an inboard direction along the plasma major radius, respectively. As shown in the figure, the emission intensity is larger in the D plasma than that in the H plasma because the sputtering rate of carbon atoms from the divertor plates is larger in the D plasma. The ion temperature has no clear change between the H plasma and the D plasma. The flow velocity toward the outboard direction develops clearly with the maximum value at Z = -480 mm, which is a location close to the outermost region of the ergodic layer in the H plasma. This direction is same as the friction force in the parallel momentum balance for Rax = 3.6 m calculated with a three-dimensional simulation code, EMC3-EIRENE [9]. On the other hand, the maximum value of the flow velocity in the D plasma is clearly smaller than that in the H plasma.

Figure 6 shows the maximum value of the flow observed at Z = -480 mm plotted against the line-averaged electron density. The flow increases with the electron density in the H plasmas. The result supports a prediction from the simulation that the friction force becomes more dominant in the force balance in higher density regime. It leads to the increase in the impurity flow which can develop the impurity screening. In the case of the D plasma, the flow has a smaller value. If we assume a steady state and we neglect terms except for the friction force ant the ion thermal force in the momentum balance, the parallel impurity flow observed in the experimemt, VZ//,obs, can be estimated with the follwing relationship

FIG. 4. Wavelength spectrum of CIV lines in the full wavelength range

covered in a single discharge for (a) D plasma and (b) H plasma.

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,cos6.2 2////,

s

T

mZVV i

Z

siobsZ

(2)

where Vi// is the thermal velocity of the bulk ions, Z is the charge of the impurity ion, s is the impurity-ion collision time, mZ is the mass of the impurity ion, ∂Ti/∂s (< 0 in this case) is the ion temperature gradiet along the magnetic field toward the divertor plate, and is the angle between the magnetic field line and the observation chord of the VUV spectroscopy. When ne increases, s becomes small and the second term on the right-hand side of Eq. (1) becomes negligible. Then, the maximum value of VZ//,obs is limited by Vi//, which can be a reason why VZ//,obs is saturated against the incsease of ne as shown in Fig. 6. In high density operations, i.e. ne ~ 5×1013 cm−3 on the horizontal axis in Fig. 6, which means the friction force is dominant, using parameters of the plasmas at the outermost regoin of the ergodic layer and the spectroscopic geometry of Ti,edge = 20 eV, ni,edge = 2×1013 cm−3, and = 75° for C+3 ions along the mangetic field line with the connection length of 100 m, VZ//,obs is 11.3 km/s for H plasmas and 8.0 km/s for D plasmas. The difference of the estimated values between D and H plasmas is caused by mi

1/2 dependence of Vi// when the friction force term is dominant in the force balance. Thus, the experimental observation with smaller flow values in the D plasmas as shown in Fig. 6 qualitatively agrees with the mass dependence of Vi//. However, quantitative discussion requiers more detailed comparison of edge plasmas parameters between H and D plasmas to explain the reason why the carbon flow in the H plasmas is approximately twice larger than that in the D plasmas at high density region.

4. SUMMARY

Carbon impurity transport in a thick stochastic magnetic field layer called “ergodic layer” located at the edge plasma of LHD is studied by space-resolved VUV spectroscopy for H and D plasmas. The directions of the carbon flows observed in both H and D plasmas are the same as those of the friction force in the parallel momentum balance on the impurity ions as a driving force of impurity screening effects. The maximum value of the carbon flow in the D plasma is smaller than that in the H plasma, demonstrating an isotope effect on the impurity transport. The mass dependence of the thermal velocity of the bulk ions could be one of the reasons for the difference of the flow values between D and H plasmas when the friction force term is dominant in the momentum balance.

ACKNOWLEDGEMENTS

The authors thank all the members of the LHD team for their cooperation with the LHD operation. This work is partially supported by the LHD project financial support (NIFS14ULPP010) and Grant-in-Aid for Young Scientists (B) (17K14426).

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FIG. 6. Observed C3+ flow at the bottom edge of the

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