F. Shimpo

Steady-state operation and high energy particle production of MeV energy in the Large Helical Device

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. High temperatures of approximately 1?2?keV, and steady-state plasma sustainment operations have been reported. Recently the steady-state operation regime was greatly extended in the Large Helical Device (LHD). A high-temperature plasma was created and maintained for 54?min with 1.6?GJ in the 2005FY experimental programme. The three-dimensional heat-deposition profile of the LHD helical divertor was modified, and during long-pulse discharges it effectively dispersed the heat load using a magnetic axis swing technique developed at the LHD. A sweep of only 3?cm in the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies and partially by electron cyclotron of fundamental resonance frequency. By accumulating the small flux of charge-exchanged neutral particles during the long-pulse operation, a high energy ion tail which extended up to 1.6?MeV was observed. This is the first experimental evidence of high energetic ion confinement of MeV range in helical devices. The long-pulse operations lasted until a sudden increase in radiation loss occurred, presumably because of metal wall flakes dropping into the plasma. The sustained line-averaged electron density and temperature were approximately 0.8 ? 1019?m?3 and 2?keV, respectively, at a 1.3?GJ discharge (#53776) and 0.4 ? 1019?m?3 and 1?keV at a 1.6?GJ discharge (#66053). The average input power was 680?kW and 490?kW, and the plasma duration was 32?min and 54?min, respectively. These successful long operations show that the heliotron configuration has a high potential as a steady-state fusion reactor.

Non-inductive plasma current start-up by EC and RF power in the TST-2 spherical tokamak

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device. Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high βp spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed detailed information on three discharge phases: (i) in the initial current formation phase, the plasma current increases with the stored energy, and the current is in the same order as that predicted by theory. (ii) In the current jump phase, the current density profile, which is peaked near the outboard boundary, is not deformed but increases slowly and the initial closed flux surface appears when the current reaches a maximum. (iii) In the current sustained phase, equilibrium is characterized by the hollowness of the current density profile, and it determines the fraction of the current inside the last closed flux surface to the total current. Both EC and RF injections show a similar equilibrium. While MHD instabilities often terminate the RF sustained plasma, no such phenomenon was observed in the EC sustained plasma.

Long-pulse plasma discharge on the Large Helical Device

A long-pulse plasma discharge of more than 30 min duration was achieved on the Large Helical Device (LHD). A plasma of ne = 0.8 × 1019 m−3 and Ti0 = 2.0 keV was sustained with PICH = 0.52 MW, PECH = 0.1 MW and averaged PNBI = 0.067 MW. The total injected heating energy was 1.3 GJ. One of the keys to the success of the experiment was a dispersion of the local plasma heat load to divertors, accomplished by sweeping the magnetic axis inward and outward. Causes limiting the long pulse plasma discharge are discussed. An ion impurity penetration limited further long-pulse discharge in the 8th experimental campaign (2004).

Steady-State Amplifier at Megawatt Level for LHD ICRF Heating

Abstract A high-power, wide-band, steady-state amplifier was developed as a part of research and development for ion cyclotron range of frequency (ICRF) heating in the Large Helical Device at the National Institute for Fusion Science. A double coaxial cavity was adopted to cover the wide frequency range of 25 to 100 MHz. An analysis of this cavity is compared with results of static tests, and good agreement is shown. In a high-power test, long-pulse operation of 5000 s at an output power of 1.6 MW, which is a world record for steady-state operation of an ICRF amplifier, has been achieved as a low-impedance-mode operation is adopted. Cooling of various elements of the amplifier is important in the steady-state operation. This paper reports how the steady-state operation is obtained through cooling. An analysis of heat removal in response to the temperature rise of a coaxial cable is also reported.

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

A degradation of heating efficiency was observed during high harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) occurred. Suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high ? plasmas. In order to understand PDI, measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), arrays of RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry and fast optical diagnostics in TST-2. The frequency spectrum usually exhibits ion-cyclotron harmonic sidebands f0 ? nfci and low-frequency ion-cyclotron quasi-modes (ICQMs) nfci. PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna. The lower sideband power was found to increase quadratically with the local pump wave power. The lower sideband power relative to the local pump wave power was larger for reflectometer compared with either electrostatic or magnetic probes. The radial decay of the pump wave amplitude in the SOL was much faster for the ion saturation current than for the floating potential. These results are consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the low-frequency (ICQM). Two additional peaks were discovered between the fundamental lower sideband and the pump wave in hydrogen plasmas. The frequency differences of these peaks from the pump wave increase with the magnetic field. These decay modes may involve molecular ions or partially ionized impurity ions.

ICRF Heating and Ion Tail Formation in LHD

Abstract Various ion cyclotron range of frequencies (ICRF) heating experiments have been conducted in the Large Helical Device (LHD) by changing the magnetic field strength and the wave frequency using hydrogen and helium. When the resonance layer of hydrogen was located in the peripheral region on the lower-magnetic field side, efficient electron heating, i.e., mode conversion heating, was realized. When the ion cyclotron resonance layer was located near the “saddle point” of magnetic field strength, where the gradient of the magnetic field strength is zero, hydrogen ions were efficiently heated by the minority ion heating. The second-harmonic ion cyclotron heating experiments were also conducted by decreasing the magnetic field strength, and the plasma was successfully sustained for 1 s. Ion tails were observed in the ion heating modes. High-energy ions were well confined by the inward-shifted magnetic configuration. The ion tail formed by the second-harmonic heating was enhanced by the injection of a perpendicular neutral beam.

ICRF Heating System in LHD

Abstract A heating system for the Large Helical Device (LHD) based on the ion cyclotron range of frequencies (ICRF) heating is reviewed. Various physical and engineering issues were studied and solved to construct an effective and stable system for high-power, steady-state experiments in LHD. Successful results were achieved using six loop antennas. The physical design of the ICRF antenna was an important subject during the research and development phase. A single current strap antenna was adopted to maintain high coupling resistance. The antenna designed to conform to the LHD plasma shape provided effective plasma heating. Steady-state operation is one of the most important mission items of superconducting LHD device. Many ICRF components, including the transmitter, transmission line, impedance matching tuner, feedthrough ceramics, and antenna launcher, were developed and applied in long-pulse experiments. All components are water cooled to remove the heat loss during the operation. Especially, a liquid stub impedance tuner using dielectric liquid was developed and implemented for the first time in a plasma experiment. An antenna launcher was also designed with the ability to change its position during the steady-state operation. Steady-state operation for 54 min with an input energy of 1.6 GJ was achieved, the largest input energy on record for a toroidal plasma device.

Thirty-minute plasma sustainment by real-time magnetic-axis swing for effective divertor-load-dispersion in the Large Helical Device

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. A high temperature of approximately 2keV, and steady-state plasma-sustainment operation of the Large Helical Device (LHD) [O. Motojima, K. Akaishi, H. Chikaraishi et al., Nucl. Fusion 40, 599 (2000)] is reported. High-temperature plasmas were created and maintained for more than 30min with a world record injected heating power of 1.3GJ. The three-dimensional heat-deposition profile of the LHD helical divertor was modified and during long-pulse discharges it effectively dispersed the heat load using a magnetic-axis swing technique developed at the LHD. A sweep of only 3cm of the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The modification of the heat-load profile was explained well by field-line tracing. The steady-state plasma was heated and sustained mainly by hydrogen minority i...

Long Pulse Plasma Heating Experiment by Ion Cyclotron Heating in LHD

It is very important to demonstrate the ability to sustain the plasma in a steady state on the Large Helical Device (LHD), which has external helical magnetic coils and is a superconducting device. The long pulse discharge experiment was carried out using the ion cyclotron range of frequencies (ICRF) heating mainly. The plasma discharge of 31 minutes and 45 seconds was achieved by a total injected heating energy of 1.3GJ. Swing of the magnetic axis to scatter the local heat load on the divertor plate was one of the key methods for the steady state operation. The repetitive hydrogen pellet injection was tried successfully to fuel the minority hydrogen ions for long pulse operation.

Applications of non-resonant RF forces for the improvement of tokamak reactor performance

The application of radiofrequency (RF) forces to a tokamak divertor plasma for the improvement of its relevance to reactors is proposed and studied. An RF field is applied to the divertor region of a tokamak by use of wave guide launchers in a way that is suitable for a reactor environment. Since the ponderomotive force is dependent on the charge to mass ratio of the ions, various useful applications are considered. They cover some of the key issues in recent research towards the development of nuclear fusion: reduction of the heat load on the divertor plate, improvement of the tritium inventory, impurity control and helium ash removal