The transition of a radial electric field from a negative to a positive value is observed in the compact helical system when the electron loss is sufficiently enhanced by the superposition of the off-axis second harmonic electron cyclotron heating on the neutral beam heated plasmas. Existence of the threshold for the enhanced particle flux required to cause the transition is experimentally certified. The observed threshold is compared with a theoretical prediction. PACS numbers: 52.55.Hc, 52.50.Gj A radial electric field near the plasma periphery has been found to play an important role in the improved confinement such as in the //-mode plasmas [1][2][3]. Theoretical models of the L/H transition in tokamaks have been proposed. It is predicted that the change in the radial electric field or in the plasma rotation has a strong influence on the transition [4][5][6][7][8]. In stellarator devices, the neoclassical theory suggests that the electric field reduces the helical ripple loss, and consequently improves the plasma confinement [9][10][11]. Multiple solutions of the electric field that satisfy the ambipolar constraint often arise when particle fluxes have a nonlinear dependence on the electric field. There are generally two stable states in the stellarator plasmas which are called the ion and the electron roots [11]. In a stellarator reactor, it is an important scenario to attain the electron root with higher energy confinement time through heating electrons in the start-up phase [9]. In a Heliotron-E device, the radial electric field at r~0.7-0.9a is found to be positive (the electron root) for the low density plasma (n e < lxlO 13 cm -3 ) and negative (the ion root) for the high density plasma (n e >2x\0 u cm " 3 ) [12]. In the Wendelstein VII-A stellarator, the observed electric field in the plasma with electron cyclotron heating (ECH) (n e~~5 xl0 13 cm -3 ) is consistent with a theoretical prediction [13]. In the advanced toroidal facility, the positive electric field is observed for the low density plasma with ECH (n e -5 xlO 12 cm -3 ) [14]. In the compact helical system (CHS) [15], the observed radial electric field is negative in the typical neutral beam (NB) heated plasmas [16]. The electric field becomes more negative near the plasma edge for the higher electron density.It is generally observed that ECH has an effect of density pump-out both in tokamaks [17] and in stellarators [18,19]. In CHS, it is observed that the particle confinement becomes worse in the plasma with second harmonic ECH at low field side resonance than at high field side resonance [20]. One of the candidates to explain the mechanism of the density pump-out is the outward flux due to the poor confinement of perpendicularly accelerated electrons by ECH [21]. In this Letter, we present the transition of the radial electric field from the ion root to the electron root triggered by enhancing the electron particle flux with ECH.CHS is a heliotron/torsatron device with a pole number of /=2, a toroidal period number m=8, and an aspect r...

Comparative studies of environmental burden have been made for fusion reactors based on diŠerent conˆnement systems and blanket modules by calculating CO 2 emission amount. The conˆnement systems of fusion reactor considered in this paper are the Tokamak reactor (TR), helical reactor (HR), and spherical Tokamak reactor (ST). Several blanket modules such as Li/V, Flibe/FS, and LiPb/SiC blankets are evaluated under the condition of 1 GW electric power output and speciˆc beta values. The calculated amounts of CO 2 emission from fusion reactors are 9.2 11.3 g CO 2 /kWh. This range is the same as that of emission from hydraulic power and atomic power plants which are regarded as clean energy sources now. A substantial amount of CO 2 is emitted from superconducting magnet systems. TR and HR, which use large superconducting coil systems, emit much CO 2 as a whole. If we adopt a higher beta design, the demand on coil systems is relaxed and better fusion reactors emitting less CO 2 can be constructed.

High beta plasmas with a volume averaged equilibrium beta value of 2.1% were produced in CHS using tangential neutral beam injection. This beta value was achieved with the confinement improvement (reheat mode) observed after turning off strong gas puffing. Wall conditioning with titanium gettering was used to make high density operation (ne ⩽ 8 × 1019 m-3) possible for low magnetic fields (Bt = 0.6 T). The discharges start with the magnetic hill configuration (in vacuum) and finally achieve Mercier stable equilibrium owing to the self-stabilization effect given by the magnetic well which is produced by the plasma pressure. The Shafranov shift was about 40% of a plasma minor radius. Magnetic fluctuations did not increase with increasing plasma pressure when the beta value exceeded 1%. Dynamic poloidal field control was applied to suppress the outward plasma shift with increasing plasma pressure. Such operation gave an additional increase of beta value compared with the constant poloidal field operation

The MHD equilibrium properties of neutral beam heated plasmas have been experimentally investigated in the Compact Helical System (CHS)a low aspect ratio (AP -5) heliotrodtorsatron. This configuration is characterized by a strong breaking of helical symmetry. The radial profiles measured by various diagnostics have shown a significant Shafranov shift due to the plasma pressure. The deviation of the magnetic axis from its vacuum position has become as large as 50% of the minor radius. When the three-dimensional equilibrium code VMEC is used to reconstmct the equilibrium from the experimental data, the result is in good agreement with the experimentally observed Shafranov shift as well as with the diamagnetic pressure in plasmas with ( p ) I 1.2% and Po I 3.3%. This value corresponds to half of the conventional equilibrium p limit defined by the Shafranov shift reaching a value of half of the minor radius. Although tangential neutral beam injection causes pressure anisotropies, pH/pI 5 3, the description of the equilibrium assuming isotropic pressure is consistent with the experiment.

Reference 1-GWe D-T reactors; tokamak TR-1, spherical tokamak ST-1 and helical HR-1 reactors, are designed using PEC (Physics Engineering Cost) code, and their plasma behaviors with Internal Transport Barrier (ITB) operations are analyzed using TOTAL (Toroidal Transport Analysis Linkage) code, which clarifies the requirement of deep penetration of pellet fueling to realize steady-state advanced burning operation. In addition, economical and environmental assessments were performed using extended PEC code, which shows the advantage of high beta tokamak reactors in COE and the advantage of compact spherical tokamak in lifetime CO 2 emission reduction. Comparing with other electric power generation system, the cost of fusion reactor is higher than that of fission reactor, but on the same level of oil thermal power system. The CO 2 reduction can be achieved in fusion reactors same as in the fission reactor. The EPR of high-beta tokamak reactor TR-1 could be higher than that of other systems including fission reactor. These systematic design and comparative simulation analyses on both tokamak and helical reactors can be done by the help of the above two codes.