Optimization activities on design studies of LHD-type reactor FFHR

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-

mentioning

confidence: 99%

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

Masuzaki

Kobayashi

Shoji

et al. 2011

“…Here a brief review of the heliotron reactor design FFHR-2m2 [2] is given as an example. FFHR-2m2 is a LHD-type heliotron reactor based on the LHD high-beta discharges.…”

Section: Design Window Of a Lhd-type Heliotron Reactormentioning

confidence: 99%

“…diation shielding for the superconducting coils [2]. In the design of FFHR-2m2, the blanket thickness at the all positions including the minimum position is set to be ≥ 1.0 m. Figure 2 shows how these engineering constraints determine the design window on R c -B t,c plane (B t,c is the helical coil major radius and the toroidal field at the helical coil winding center averaged over one helical pitch).…”

Section: Design Window Of a Lhd-type Heliotron Reactormentioning

confidence: 99%

“…Therefore, in the case of 30 FPYs operation, the fast neutron flux at the front of the helical coil must be suppressed < 10 10 n/cm 2 /s. In the design study of FFHR-2m2, the reduction of the blanket thickness by thinning the radiation shield layer using tungsten carbide has already been proposed [2]. But the reflection of the fast neutrons from the magnet was not considered.…”

Section: Flexible Design Concept For Demomentioning

confidence: 99%

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A Robust Design Window for the Heliotron DEMO Reactors

Goto

Miyazawa

Tanaka

et al. 2011

According to the achievements in the Large Helical Device (LHD) experiments, a design of a DEMO reactor with a LHD-type heliotron system is foreseeable. On the other hand, a DEMO is the next step reactor and high reliability and feasibility are demanded in its design. In this study, a robust design window, i.e., the design window that is not sensitive to a change in uncertain physics parameters, was surveyed through parametric scans using a system design code. It was found that a difference in main design parameters (major radius, magnetic field strength, fusion output) gives only a small change in a dependence of physics requirements on the physics conditions. Therefore, it is important to find the design window with a lower requirement on the confinement improvement to assure the design robustness. In this respect, a reduction in the minimum inboard blanket space, one of the key parameters in a LHD-type heliotron reactor design, can effectively expand the design window and contributes to the design robustness. An acceptance of higher neutron wall load and an achievement of further high confinement improvement are also expected to make both a DEMO and a commercial reactor to be more feasible and economically attractive.

“…High-density, low-temperature operation reduces the divertor heat flux by increased bremsstrahlung radiation [2,3] and ameliorates the pellet penetration problem. Finding a method of controlling the thermally unstable plasma at this operation point has been a major problem.…”

Section: Introductionmentioning

confidence: 99%

Alpha Particle Slowing-Down Characteristics and the Effect on MHD Instability Excitation at High-Density Operation Points in FFHRs

Matsuura

Mitarai

Sagara

et al. 2011

Alpha-particle slowing-down behaviors at low-temperature, high-density operation points in force-free helical reactors (FFHRs) are examined on the basis of a Fokker-Planck (FP) simulation that simultaneously consider the balance among generation, slowing down, and loss from the plasma in parallel with the density dependence of the Alfvén speed. An accurate treatment of the boundary velocity region between thermal and non-thermal components is shown to be important in evaluating the alpha particle population that can induce instability. In a typical high-density, low-temperature operation point in an FFHR, this population is reduced.