The Large Helical Device (LHD) now under construction is a heliotron/torsatron device with a closed divertor system. The edge LHD magnetic structure has been studied in detail. A peculiar feature of the configuration is the existence of edge surface layers, a complicated three dimensional magnetic structure which does not, however, seem to hamper the expected divertor functions. Two divertor operational modes are being considered for the LHD experimenthigh density, cold radiative divertor operation as a safe heat removal scheme and high temperature divertor plasma operation. In the latter operation, a divertor plasma with a temperature of a few keV, generated by efficient pumping, is expected to lead to a significant improvement in core plasma confinement. Conceptual designs of the LHD divertor components are under way.

Divertor plasma characteristics in the Large Helical Device (LHD) have been investigated mainly by using Langmuir probes. The three-dimensional structure of the helical divertor, which is naturally produced in the heliotron-type magnetic configuration, is clearly seen in the measured particle and power deposition profiles on the divertor plates. These observations are consistent with the numerical results of field line tracing. The particle flux to the divertor plates increases almost linearly with the line averaged density. The high-recycling regime and divertor detachment, which are observed in tokamaks, have not been observed even during high density discharges with low input power. Both electron density and temperature decrease with increasing radius in the stochastic layer with open field lines, and at the divertor plate they become fairly low compared with those at the last closed flux surface. This means the reduction of pressure along the magnetic field lines occurs in the open field line region in LHD.

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In reduced recycling discharges in the Large Helical Device, a super dense core plasma develops when a series of pellets are injected. A core region with density as high as 4:5 10 20 m ÿ3 and temperature of 0.85 keV is maintained by an internal diffusion barrier with very high-density gradient. These results may extrapolate to a scenario for fusion ignition at very high density and relatively low temperature in helical devices. DOI: 10.1103/PhysRevLett.97.055002 PACS numbers: 52.55.HcImprovement of plasma particle and energy confinement is a major challenge for toroidal magnetic fusion research, and will be important in igniting burning plasmas in ITER [1]. Various confinement improvement modes have been discovered including edge transport barriers (ETBs, or H mode) [2] and internal transport barriers (ITBs) [3][4][5]. In this Letter, we describe improved confinement in super dense core (SDC) plasmas, in diverted discharges in the Large Helical Device (LHD), a heliotron configuration in which the rotational transform is provided by external magnetic coils. This operational regime may extrapolate to a high-density, relatively low temperature ignition scenario for these devices.LHD has an external helical field with poloidal winding number l 2 and M 10 toroidal field periods. The major radius of the magnetic axis, R ax 3:5-3:9 m, average plasma minor radius a 0:6 m, and toroidal magnetic field B 3:0 T [6]. Depending on the relative currents in the helical and auxiliary poloidal coils, the rotational transform on axis, 0 =2 0:3-0:6 and the edge transform, a =2 1-1:5. One of the major goals of the LHD program is the demonstration of a reactor-relevant, diverted helical plasma. Two different divertor systems are available in LHD: the Helical Divertor (HD) [7] and the Local Island Divertor (LID) [8][9][10]. The HD is an intrinsic helical double-null divertor with an open divertor geometry, essentially like a helically twisting double-null tokamak poloidal divertor. The LID uses an m 1, n 1 resonant magnetic island (poloidal and toroidal mode numbers m and n, respectively) to guide particle and heat fluxes to divertor plates.A SDC plasma develops spontaneously in LHD as a highly peaked density profile is created by injection of multiple pellets from the outside midplane as illustrated in Fig. 1(a). The density and temperature profiles are depicted for the standard (R ax 3:75 m, B 2:64 T, P 10 MW) discharge diverted by the LID in Fig. 1(b). These profiles are measured using a Thomson scattering diagnostic along R horiz , the major radius in the poloidal plane where the plasma is horizontally elongated [ Fig. 1(a)]. A core region with electron density 4:5 10 20 m ÿ3 and temperatures 0:85 keV is maintained by an internal diffusion barrier (IDB) located at normalized minor radius 0:5. The radial width of the IDB is 0:10 m ( 0:2). The density gradient at the IDB is extremely high (rn 2:5 10 21 m ÿ4 ). Inside the SDC region, the density and temperature gradients are nearly zero. The density gradient outside the IDB is we...

OVERVIEW OF THE LARGE HELICAL DEVICE PROJECT. The Large Helical Device (LHD) has successfully started running plasma confinement experiments after a long construction period of eight years. During the construction and machine commissioning phases, a variety of milestones were attained in fusion engineering which successfully led to the first operation, and the first plasma was ignited on 31 March 1998. Two experimental campaigns are planned in 1998. In the first campaign, the magnetic flux mapping clearly demonstrated a nested structure of magnetic surfaces. The first plasma experiments were conducted with second harmonic 84 and 82.6 GHz ECH at a heating power input of 0.35 MW. The magnetic field was set at 1.5 T in these campaigns so as to accumulate operational experience with the superconducting coils. In the second campaign, auxiliary heating with NBI at 3 MW has been carried out. Averaged electron densities of up to 6 × 10 19 m-3 , central temperatures ranging from 1.4 IAEA-F1-CN-69/OV1/4 2 to 1.5 keV and stored energies of up to 0.22 MJ have been attained despite the fact that the impurity level has not yet been minimized. The obtained scarling of energy confinement time has been found to be consistent with the ISS95 scaling law with some enhancement.

Abstract. In the Large Helical Device (LHD), the volume averaged beta value <β dia > of 5 %, which is the highest value in all of heliotron/stellarators and relevant to the reactor requirement, was achieved by optimizing the magnetic configuration from the viewpoint of magneto-hydrodynamic (MHD) characteristics, transport and heating efficiency of the neutral beam. This beta value was instantaneously obtained by pellet injection and maintained for more than 10τ E , whereas the steady state plasma with a maximum <β dia > of 4.8 % was sustained for 85τ E by the gas-puff fueling. While it is predicted theoretically that stochastization of the peripheral magnetic field structure develops with an increment of <β dia >, no serious degradation of the global confinement has been observed in the present <β dia > range. The several low-order MHD activities located in the periphery were enhanced with the beta value and sometimes affect the local profiles. The amplitude of the mode in the periphery strongly depends on the magnetic Reynolds number, which is close to that of the growth rate and/or the radial mode width of the resistive interchange instability.

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In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (>10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.