H. Igami

Formation of spherical tokamak equilibria by ECH in the LATE device

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.

A survey of mode-conversion transparency windows between external electromagnetic waves and electron Bernstein waves for various plasma slab boundaries

For the plasma slab boundary with monotonically increasing density profile along the x axis and the magnetic field along the z axis, both Nz and Ny components 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 Nz and Ny is 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–Ny plane, can be categorized for the pair of parameters of the density scale length normalized to the wavelength in vacuum Ln/λ0 and the frequency to the cyclotron frequency ω/Ω.A survey of the transparency windows for various parameter ranges of Ln/λ0 and ω/Ω is presented. The windows are categorized into four types. The frosted type at the steepest density gradient region has a broad transparency profile but even the peak is not completely transparent. The perpendicular-X type at the next steep density gradient region also has a broad transparency profile with a completely transparent peak by the perpendicularly propagating extraordinary waves. The OXB type at the gentle density gradient region has a pair of completely transparent sharp peaks by the obliquely propagating ordinary waves at the optimal propagation angles with Nz = ±N∥opt and Ny = 0. The fourth is the g1 type in the intermediate density gradient region between the above two cases, which has two completely transparent peaks in the window. Finally, a simulation to examine the applicability of the survey to experiments is made using a test density profile, which elucidates key points for the application of the survey.

Polarization adjustment of incident electromagnetic waves for optimal mode-conversion to electron Bernstein waves

Polarization adjustment of incident transverse electromagnetic waves for optimal mode-conversion to electron Bernstein waves is proposed and analysed. The optimized wave injection excites most efficiently the electrostatic waves at the upper hybrid resonance (UHR) layer. Polarization adjustment is effective for the intermediate range of density gradient near the UHR layer, which has not been covered by the previously proposed schemes of perpendicular injection of X mode for the extremely steep density gradient case and oblique injection of O mode at the optimal injection angle for the gentle gradient case. Various methods of experimental polarization adjustment without detailed information on the density profile are proposed and discussed.

Observation of sawtooth crashes by a multi-toroidally positioned soft x-ray computer tomography system in the WT-3 tokamak

Sawtooth crashes in an Ohmically heated plasma with a low safety factor of qa 2.7 in the WT-3 tokamak have been observed by using soft x-ray computer tomography (SXCT) systems attached at three different poloidal cross sections around the torus. With this multi-toroidally positioned (MTP) SXCT system we have examined how the core plasmas collapse at the three different toroidal locations in terms of the shift and the shape of hot cores in order to visualize the whole helical hot structure around the torus during the crash. A variety of crashes have been observed. In any case collapsing proceeds in two steps, called the first and second phases. In the first phase collapsing proceeds slowly keeping the helical hot structure of an m = 1/n = 1 mode and/or an m = 2/n = 2 mode around the torus. In the second phase, collapsing proceeds rapidly as the helical hot structure is strongly deformed, and reaches the end in such a manner that the hot core shifted to the low field side is deformed into a thin crescent shape aligned along the inversion circle while that shifted to the high field side fades away or is deformed into a broad, warm structure depending on the types of crashes. Thus, present MTP SXCT has revealed for the first time a remarkable deformation of the helical hot structure around the torus during the sawtooth crashes.

Start‐up of Spherical Tokamak by ECH on LATE

(1) Physical process of initial formation of closed flux surface has been studied under the condition of steady external vertical fields by using a 2.45 GHz magnetron (5kW). After the injection of microwave power the plasma current increases gradually until up to 0.4 kA. Once the current reaches this value it quickly increases up to 1 kA within 2–3 msec. The magnetic measurements suggests that a small initial closed flux is formed at Ip=0.4kA and expands quickly to full closed flux in contact with the inboard limiter in accordance with the quick increase of the plasma current. (2) Plasma current up to 2 kA has been started‐up by injecting microwaves in the range of 20–50 kW from a 2 GHz klystron and adjusting the external vertical field temporally. Magnetic measurement shows that last closed flux surface with a minor radius of a=15 cm and a major radius of R =25 cm (R/a=1.7) is formed. (3) A new ECH system using a 5 GHz klystron (200kW,100 msec) is under construction. With this system, better coupling to ...

Survey of EBW Mode-Conversion Characteristics for Various Boundary Conditions

A survey of linear mode‐conversion characteristics between external transverse electromagnetic (TEM) waves and electron Bernstein waves (EBW) for various plasma and wave parameters has been presented. It is shown that if the wave propagation angle and polarization are adjusted appropriately for each individual case of the plasma parameters, efficient mode conversion occur for wide range of plasma parameters where the conventional ‘XB’ and ‘OXB’ scheme cannot cover. It is confirmed that the plasma parameters just at the upper hybrid resonance (UHR) layer strongly affect the mode conversion process and the influence of the plasma profiles distant from the UHR layer is not so much. The results of this survey is useful enough to examine wave injection/detection condition for efficient ECH/ECCD or measurement of emissive TEM waves for each individual experimental condition of overdense plasmas.

Formation of ST Plasmas by ECH on LATE

Experiments on formation of spherical tokamak (ST) plasmas by electron cyclotron heating (ECH) alone without a central OH solenoid have been carried out on LATE (Low Aspect ratio Torus Experiments). By injecting a 2.45 GHz microwave pulse of 6.3 kW for 4 seconds, a plasma current is initiated and ramped up to Ip 3 kA, and finally maintained for 1 second by adjusting the external vertical field. Magnetic measurements show that closed flux surfaces are formed and the last closed flux surface has an aspect ratio of A ~ 1.3 and an elongation of ? ~ 1.3. The electron density measured by an interferometer is more than 1.5 ? 1011 cm-3 which exceeds twice of the plasma cut-off density, suggesting that electron cyclotron heating by mode-converted electron Bernstein waves may take place.

General Properties of Scattering Matrix for Mode Conversion Process between B Waves and External EM Waves and Their Consequence to Experiments

General properties of scattering matrix, which governs the mode conversion process between electron Bernstein (B) waves and external electromagnetic (EM) waves in the presence of steep density gradient, are theoretically analyzed. Based on the analysis, polarization adjustment of incident EM waves for optimal mode conversion to B waves is possible and effective for a range of density gradient near the upper hybrid resonance, which are not covered by the previously proposed schemes of perpendicular injection of X mode and oblique injection of O mode. Furthermore, the analysis shows that the polarization of the externally emitted EM waves from B waves is uniquely related to the optimized polarization of incident EM waves for B wave heating and that the mode conversion rate is the same for the both processes of emission and the injection with the optimized polarization.