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Recent advances in the high power and steady state heating system and experiment results of the Large Helical Device (LHD) are reviewed in this paper. Plasma performance is extended largely through high power NBI, ECH and steady state ICRF heating devices, and improved operation techniques. The NBI of a 28 MW has extended the plasma parameter regime such as ion ITB plasmas, has a central ion temperature of more than 8 keV, and the extremely high-density plasmas ten times higher than the tokamak limit. An ECH system with seven gyrotrons (total power of 4.6MW) has been operated for pre-ionization and plasma heating. The high electron temperature regime was extended toward a higher density regime and a central electron temperature of 13.5 keV was achieved with a line-averaged electron density of ne= 1 x 1019m-3. Steady state operation plasma with ne= 1.2 x 1019m-3, ion and electron temperature of 2 keV, and plasma sustainment time of 48 min was achieved with ICH and ECH heating power of 1.2 MW for majority helium with minority hydrogen.

AbstractTrials of steady-state operation (SSO) in the Large Helical Device (LHD) were started when a continuous wave (cw) gyrotron with the output power up to 0.2 MW was introduced to the electron cyclotron heating (ECH) system on LHD in 2003. During the first trial of SSO in the seventh LHD experimental campaign in 2004, severe temperature increase on the waveguide transmission line and, as a result, intense pressure increase in the evacuated waveguide occurred, which terminated the operation at 756 s. Additional pumping sections and cooling structures on the transmission line worked well, allowing a 3900-s sustainment of plasma with ne= 0.15 × 1019m−3and Te0= 1.7 keV by 0.1 MW injection power in 2005. The improvement of the ECH system by introducing cw gyrotrons with higher power for further improvement of plasma performance in SSO is in progress.Investigations on electron cyclotron current drive (ECCD) physics have been advanced a few years after the proof of ECCD in LHD. By obliquely injecting second-harmonic X-mode EC waves in toroidal direction, stable current up to 5.5 kA is driven, which was evaluated as a difference in plasma currents of the co- and counter-ECCD discharges with 0.1-MW EC wave power. It takes a few seconds for the driven current to saturate. Change in profile of rotational transform by ECCD and profile of driven current density are investigated by use of motional Stark effect measurement. Peaked and localized driven current profile at the plasma core region was confirmed for on-axis second-harmonic ECCD discharges.

AbstractAs one of the main objectives of operation of the Large Helical Device (LHD), long-pulse plasma discharge experiments have been carried out using ion cyclotron range of frequency (ICRF) heating power (mainly using a minority heating method). Discharges with electron density ne< 1019m-3and Te0> 1 keV have been sustained with an ICRF heating power around ~1 MW and an electron cyclotron heating power of ~0.1 MW. The total injected heating energy exceeded 1.6 GJ. Many technological improvements were made before undertaking long-pulse plasma discharges, e.g., the installation of a steady-state high-rf power generator and a liquid stub tuner capable of real-time impedance matching. Over the past decade, the achieved pulse length has increased to 1 h. One of the keys to this success was dispersion of the local plasma heat load onto divertors, which was accomplished by cyclically sweeping the magnetic axis inward and outward. Eventually, the plasma terminated due to the penetration of impurities, which originated from the release of thin flakes on the divertor plates. The pulse length was extended by installing new divertor plates with better thermal conduction. A mode conversion heating scenario has been considered as an alternative to the minority ICRF heating scenario; the former may have advantages due to the lack of an ion cyclotron resonance layer in front of the antennas in the mode conversion case.

AbstractFor expanding applicable parameter ranges of electron cyclotron resonance heating (ECRH), various methods of ECRH have been studied with use of millimeter-wave sources of 77-, 82.7-, 84-, and 168-GHz gyrotrons in the Large Helical Device (LHD). The fundamental ordinary (O-) mode and the second-harmonic extraordinary (X-) mode are mainly used for starting up, sustaining, and controlling the plasma. Heating efficiencies of ECRH by launching of these modes have been investigated experimentally for wide range of the central electron density and compared with power absorption rates obtained by ray-tracing calculation. ECRH by the third-harmonic X-mode has been performed in each magnetic configuration Bax= 1 and 2 T with launching of 84-GHz range and 168-GHz millimeter waves. Increases of the electron temperature and the stored energy were observed in both cases. ECRH by the electrostatic electron Bernstein wave (EBW) has been expected to be a promising substitute in parameter ranges where the conventional methods of ECRH by the electromagnetic modes are not available. To perform ECRH by the EBW in LHD, extraordinary-EBW (X-B) and ordinary-extraordinary-EBW (O-X-B) mode conversion processes, the propagation of the wave, and the absorption have been investigated experimentally and theoretically.

AbstractThe electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) has been in stable operation for ~11 yr in numerous plasma experiments. During this time, many upgrades to the system have been made, such as reinforcement of the gyrotron tubes, modification of the power supply depending on gyrotron type, and increase in the number of transmission lines and antennas. These efforts allow the stable injection of millimeter-wave power in excess of 2 MW. In parallel, various transmission components were evaluated, and antenna performance was confirmed at a high power level. The coupling efficiency of the millimeter wave from the gyrotron to the transmission line and the transmission efficiency through the waveguide were further improved in recent years. The feedback control of the wave polarization has also been tried to maximize the efficiency of wave absorption. The gyrotron oscillation frequency was reconsidered in order to extend the flexibility of the magnetic configuration in plasma experiments. The development of 77-GHz gyrotrons with the output of 1 MW per few seconds in a single tube is currently taking place in collaboration with the University of Tsukuba. Two such gyrotron tubes already have been installed and were used for plasma experiments recently. An ECRH system with a capability of the steady operation is required, because the LHD can continuously generate confinement magnetic fields using superconducting magnets. Not only the gyrotron but also the transmission system and components must withstand continuous power operation. Further acceleration of both the power reinforcement and a steady-state capability will allow the sustainment of high-performance plasmas.

AbstractSecond-harmonic electron cyclotron (EC) current drive experiments have been performed in the Compact Helical System (CHS). The driven current changes its direction according to the change of the EC-wave beam direction in agreement with an expectation from the Fisch and Boozer theory in the case of low-field-side injection of EC waves. The EC-driven current varies as a function of the magnetic axis position of CHS plasmas. The cause of the variation was experimentally investigated by a magnetic field scan. Setting the second-harmonic resonance layer near the magnetic axis was favorable to maximize the total EC-driven current. The main cause of the dependence of the driven current on the magnetic axis position is attributed to the change of distribution of the magnetic field along the beam path due to the change of the beam direction to aim at the magnetic axis in the three-dimensional helical magnetic field of the CHS.

Evident increases in the plasma stored energy by applying 54.5-GHz electron cyclotron (EC) waves have been observed in overdense plasmas sustained by neutral beam injection in the Compact Helical System. The heating effect was seen even for a high density of 8 × 1019m-3, that is, more than twice the cutoff density of 3.8 × 1019m-3of the 54.5-GHz waves. The 54.5-GHz EC wave beams were obliquely injected into high-density plasmas. Dependences of the heating effect on the experimental conditions such as the polarization and the injection power of the EC waves, and the magnetic field were investigated. A higher left-hand circular polarization fraction and higher injection power resulted in a longer plasma duration time and a higher increment of the plasma stored energy. Variation of the electron temperature profile in the magnetic field scan experiment shows the power deposition in the plasma core region inside the plasma cutoff layer. These experimental results show that the main cause for this heating mechanism is electron Bernstein wave heating via an Ordinary-eXtraordinary-Bernstein (O-X-B) mode conversion process.

AbstractThe long-pulse plasma discharge experiment is an important experiment in the Large Helical Device, which has a superconducting coil system and the capability of steady-state operation. The experiment of long-pulse plasma discharge was carried out using mainly ion cyclotron range of frequencies heating. The maximum plasma duration is 31 min and 45 s, and the total injected heating energy reached 1.3 GJ. Swing of the magnetic axis is adopted as an effective method to scatter the local heat load on the divertor plate during the discharge. The plasma was terminated abruptly by the influx of metallic impurities accompanied by a spark in the vacuum vessel.

AbstractRemarkable progress to access the reactor-relevant regime has been made in a recent experiment in the Large Helical Device. Optimizing the rotational transform, the average beta value of 4.3%, which is the highest record among helical devices, was achieved. The high-performance plasma with a fusion triple product up to ~2.2 × 1019m−3·keV·s was sustained for >7 s by repetitive hydrogen pellet injection. With regard to steady-state operation, which is one of the key issues to realize a fusion reactor, discharges for >30 min were successfully sustained by ion cyclotron range of frequency heating with the aid of the magnetic axis swing technique to reduce the heat load to the plasma-facing component. In the discharge, the total input energy to the plasma reached 1.3 GJ, which also established a new record.

In Large Helical Device, a power transmission line consists of corrugated waveguides with inner diameter of 88.9 mm for electron cyclotron resonance heating is evacuated in order to transmit higher power by reducing a possibility of electrical breakdown in the line. Some characteristics of such transmission lines, vacuum pumping system and comparison of pressure distribution simulated and gauged along the transmission line are described. An effect of vacuum pumping for higher power transmission is demonstrated.