Neutral Gas Compression in the Helical Divertor with a Baffle Structure in the LHD Heliotron

Abstract

The helical divertor in the Large Helical Device (LHD) was partially modified before the experimental campaign in 2010 to demonstrate the ability of particle control by installing a baffle structure. The baffle structure consists of water cooled divertor plates combined with baffle plates and a dome in the private region. The neutral pressures in the modified and an existing unmodified helical divertors have been measured by using fast ion gauges during the campaign. The recycled neutral gas was successfully compressed in the modified divertor during discharges, and more than ten times higher pressure was observed there than in the unmodified divertor as expected from the neutral transport calculations. Keywords: LHD, helical divertor, baffle structure, particle control, neutral compression, fast ion gauge DOI: 10.1585/pfr.6.1202007The control of the neutral particles using a divertor is a crucial issue especially for next generation fusion devices, such as ITER, DEMO and the helical reactor FFHR [1]. Hydrogen isotopes, helium ash and impurities have to be pumped to sustain the fusion burning plasma steadily. In tokamaks, particle control experiments using closed divertors and divertor pumping have been conducted [2][3][4]. In helical devices, closed divertor experiments were also conducted in the Compact Helical System (CHS) [5], Wendelstein-7AS [6] and LHD [7] using a magnetic island. In LHD, the local island divertor (LID) experiment was conducted, and the effective particle control was demonstrated [8]. Furthermore, the super dense core (SDC) plasma operational regime caused by the formation of the internal diffusion barrier (IDB) was obtained in the high pumping operation using LID [9]. However, the wetted area on the divertor head in LID was so small that it cannot be utilized during long pulse operation with high performance plasma. The wetted area in the helical divertor which is naturally equipped in the heliotron magnetic configuration is larger than that in the LID. However, the neutral pressure in the helical divertor is up to 0.01 Pa order even during a high density discharge in which the line averaged density is higher than 10 20 m −3 , and it is considered that a ten times higher neutral pressure is necessary for effective particle control using divertor pumping [10]. The relatively low neutral pressure in the helical divertor was considered to be caused by the three dimensional plasma distribution in the divertor and the large volume of the vac-

First EMC3‐EIRENE Simulations with Divertor Legs of LHD in Realistic Device Geometry

“…Simulation without a divertor leg leads to different global transport of hydrogen and impurities near divertor plates and that can be significant in the case of the closed divertor configuration introduced recently in LHD to make efficient particle control by pumping of hydrogen gas. Although increase of neutral gas pressure in the closed configuration was confirmed by simulation [8] and experiment [9] in good agreement, plasma transport simulation was limited to open divertor configuration without the legs.…”

“…The most obvious difference measured in experiment between open and closed divertor configurations is the increase of neutral pressure under the dome structure [9]. In order to simulate that, we carried out a series of simulations with constant input power P = 8MW and different density n LCFS = 1, 2, 4, 6 and 8 × 10…”

Design of a Vacuum Pumping System for the Closed Helical Divertor for Steady State Operation in LHD

“…The measured neutral particle pressure behind the dome is in the order of 0.1 Pa (in the molecular flow regime), which ensures that a Monte Carlo based particle tracking approach is appropriate for the analysis of the neutral particle transport [1]. An example of the geometrical model of the CHD configuration and the pumping system for the simulation is shown in Fig.…”

“…Two test modules for investigating the performance of the closed helical divertor (CHD) were installed in the inboard side of the torus in the Large Helical Device (LHD) in the last experiment campaign (2010y) [1]. The CHD consists of three components: the slanted divertor plates, a dome with slanting sides, and the target plates.…”

Recent Fusion Research in the National Institute for Fusion Science

“…The DD experiments are planed so as to realize the confinement improvement and to realize higher ion temperature plasmas. The installation of a full closed divertor system is planned for impurity control, stable divertor detachment and so on [22]. 80% of the inboard divertor area will be modified to the closed configuration with in-vessel pumping by 2012-2013.…”