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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.

AbstractIn the system of electron cyclotron heating, highly overmoded, corrugated circular waveguides are used. To analyze propagating mode content in the waveguide, burn patterns of the thermal paper placed on the waveguide aperture are observed at several positions. Theoretical burn patterns are obtained by taking into account a nonlinear grayscale response of the thermal paper to the calculated power profiles. We have developed a new method of mode analysis by nonlinear optimization, which is based on an iterative error reduction of differences between observed and theoretical patterns. To examine the status of polarization, the transformation between hybrid modes and linearly polarized (LP) modes is derived. The method is applied to the 82.7-GHz transmission line connected with the gyrotron. The propagating wave is linear polarized and consists of [approximately]4% of the LP11odd mode, [approximately]95% of the LP01mode, and [approximately]1% of other modes. The calculated burn pattern is similar to the observed one, like a plateau. By using both center of power and weighted averages of the perpendicular wavenumber in these profiles, offset and tilting angles of an injecting electromagnetic beam to the waveguide entrance are inferred. These are verified to be consistent with the results by the coupling code of a Gaussian beam with hybrid modes.

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.

AbstractA 77-GHz, 1-MW gyrotron is being newly installed in the Large Helical Device not only to enhance the total heating power but also to increase the possibility of controlling the local plasma parameters. Our progress in installing the new gyrotron and evaluating its properties is discussed. We have already finished the installation of the peripheral components, including the transmission line, and conducted a test at 1 MW for a short pulse. Our plan is to operate this gyrotron at a power of up to 1 MW for 5 s. The conditioning of the gyrotron has been smoothly conducted, and a gyrotron output power up to 810 kW for 3.6 s has been achieved so far. The total injected power of electron cyclotron resonance heating to the plasma reached a value of [approximately]2.5 MW.

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.

For the plasma slab boundary with monotonically increasing density profile along the xaxis and the magnetic field along the zaxis, both Nzand Nycomponents of the refractive index are parallel to the plasma slab and are conserved in the mode-conversion process between the vacuum transverse electromagnetic (TEM) waves and the electron Bernstein (B) waves. Information of Nzand Nyis sufficient to identify the waves uniquely both for TEM waves and B waves coupled by mode conversion. Furthermore, the wave differential equation which governs the mode-conversion process can be written in the normalized form with a few numbers of the normalized parameters and variables for the linear density profile. Thus, the mode-conversion transparency window, which is presented as a contour plot of the mode-conversion rate versus the Nz-Nyplane, can be categorized for the pair of parameters of the density scale length normalized to the wavelength in vacuum Ln/?0and the frequency to the cyclotron frequency ?/?.

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?3and 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).

The main objective of the Low Aspect Ratio Torus Experiment (LATE) device is to demonstrate the formation of spherical torus (ST) plasmas by electron cyclotron heating (ECH) alone without a centre solenoid and establish its physical bases. By injecting a 2.45 GHz microwave pulse for 4 s, a plasma current of 1.2 kA is spontaneously initiated by P = 5 kW under a weak steady vertical field of Bv = 12 G and then ramped up slowly with a slow ramp-up of Bv for the equilibrium of the plasma loop and finally reaches 6.3 kA by P = 30 kW at Bv = 70 G. This current amounts to 10% of the total coil currents of 60 kA flowing through the centre post for the toroidal field. Magnetic measurements show that an ST equilibrium, having the last closed flux surface with an aspect ratio of R0/a ≃ 20.4 cm/14.5 cm ≃ 1.4, an elongation of κ ≃ 1.5 and qedge ≃ 37, has been produced and maintained for 0.5 s at the final stage of discharge. Spontaneous formation of ST equilibria under steady Bv fields, where plasma current increases rapidly in the time scale of a few milliseconds, is also effective and a plasma current of 6.8 kA is spontaneously generated and maintained at Bv = 85 G by a 5 GHz microwave pulse (130 kW, 60 ms). In both cases, the plasma centre locates near the second or third harmonic EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the harmonic EC heating by the mode-converted electron Bernstein waves supports the plasma.

The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.

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.