Eiichi Yatsuka

Chevron beam dump for ITER edge Thomson scattering system

This paper contains the design of the beam dump for the ITER edge Thomson scattering system and mainly concerns its lifetime under the harsh thermal and electromagnetic loads as well as tight space allocation. The lifetime was estimated from the multi-pulse laser-induced damage threshold. In order to extend its lifetime, the structure of the beam dump was optimized. A number of bent sheets aligned parallel in the beam dump form a shape called a chevron which enables it to avoid the concentration of the incident laser pulse energy. The chevron beam dump is expected to withstand thermal loads due to nuclear heating, radiation from the plasma, and numerous incident laser pulses throughout the entire ITER project with a reasonable margin for the peak factor of the beam profile. Structural analysis was also carried out in case of electromagnetic loads during a disruption. Moreover, detailed issues for more accurate assessments of the beam dumps lifetime are clarified. Variation of the bi-directional reflection distribution function (BRDF) due to erosion by or contamination of neutral particles derived from the plasma is one of the most critical issues that needs to be resolved. In this paper, the BRDF was assumed, and the total amount of stray light and the absorbed laser energy profile on the beam dump were evaluated.

Conceptual design of the collection optics for the edge Thomson scattering system in ITER.

Neutron and gamma-ray irradiation complicates the design of the edge Thomson scattering (TS) system in ITER. The TS light is relayed through the relaying optics with labyrinth and fiber coupling optics. Electron density of 2×10(19) m(-3) is sufficient to measure T(e) and n(e) within a 10% and 5% margin of error, respectively, with a spatial resolution of 5 mm. This system can cover from 0.85 to 1 of the normalized minor radius. The time resolution is 10 ms, which is determined by the repetition rate of the laser device. A super-Gaussian is the ideal laser profile for the laser injection optics to avoid a breakdown of the filling gas used in density calibration through Raman scattering.

Thomson scattering diagnostic systems in ITER

Thomson scattering (TS) is a proven diagnostic technique that will be implemented in ITER in three independent systems. The Edge TS will measure electron temperature Te and electron density ne profiles at high resolution in the region with r/a>0.8 (with a the minor radius). The Core TS will cover the region r/a<0.85 and shall be able to measure electron temperatures up to 40 keV . The Divertor TS will observe a segment of the divertor plasma more than 700 mm long and is designed to detect Te as low as 0.3 eV . The Edge and Core systems are primary contributors to Te and ne profiles. Both are installed in equatorial port 10 and very close together with the toroidal distance between the two laser beams of less than 600 mm at the first wall (~ 6° toroidal separation), a characteristic that should allow to reliably match the two profiles in the region 0.8<r/a<0.85. Today almost every existing fusion machine has one or more TS systems installed, therefore substantial experience has been accumulated worldwide on practical methods for the optimization of the technique. However the ITER environment is imposing specific loads (e.g. gamma and neutron radiation, temperatures, disruption-induced stresses) and also access and reliability constraints that require new designs for many of the sub-systems. The challenges and the proposed solutions for all three TS systems are presented.

Progresses in development of the ITER edge Thomson scattering system

This paper includes discussions of spatial resolution and accuracy of the edge Thomson scattering system in ITER (ITER ETS). In the present design, the dominant factor for spatial resolution degradation relative to the scattering length is aberrations of the collection optics. A scattering length of approximately 4 mm is acceptable to obtain a spatial resolution of 5 mm. Statistical errors were evaluated according to measurement accuracy. Since the background light during ITER plasma discharge is much stronger than the Thomson scattering, the laser pulse duration is one of the most crucial specifications to obtain accurate measurements. The impact of fast sampling relative to current integration was also investigated. It is expected that the measurement accuracy improves when the waveform of the scattered light is sampled directly particularly for low density measurement.

Principles for local measurement of anisotropic electron temperature of plasma using incoherent Thomson scattering

Anisotropic electron temperature can be measured by utilizing two Thomson scattering spectra from different scattering angles. The ideal conditions for measurement are as follows: the sum of the two scattering angles is 180?, and the magnetic field is parallel to the bisector of the angle formed by the direction of laser propagation and the line of sight or is perpendicular to this bisector. When these conditions are satisfied, one of the spectra predominantly reflects the parallel electron temperature, and the other spectrum predominantly reflects the perpendicular electron temperature. A deviation in the direction of magnetic field of the order of approximately a few tens of degrees from the ideal direction is acceptable in terms of the deviation obtained by both rotating parallel to and tilting from the scattering plane.

Plasma Production by Electron Cyclotron Heating on the Internal Coil Device Mini-RT

The internal coil device Mini-RT (Ring Trap) has been constructed in order to study the extremely high beta plasma confinement predicted in two fluid relaxation theory. Plasma is produced by electron cyclotron heating with a 2.45 GHz, 2.8 kW microwave. Plasma experiments were carried out for two conditions: with a mechanically supported coil, and with a magnetically levitated one. In the case in which the internal coil was mechanically supported, plasma densities up to ne=7×1016 m-3 were produced with a filling gas pressure of pn=4×10-2 Pa, and plasma could not be available at a gas pressure less than 10-2 Pa due to the loss of high energy electrons by the supporting structure. It has been clearly demonstrated that the levitation of the internal coil, which was followed by the removal of the supporting structure from the plasma confinement region, improves plasma parameters markedly. The levitation of the internal coil enables plasma production at a lower filling pressure ( pn=1.5×10-3 Pa). It also enables over-dense plasma production (ne=1.5×1017 m-3, while cutoff density is 7.6×1016 m-3 for 2.45 GHz microwave). Since the calculation shows that the efficiency of mode conversion from fast X-mode wave to electron Bernstein wave (FX–SX–B conversion) is sufficiently high (~87%) in the Mini-RT device, the over-dense plasma production is thought to be caused by heating with electron Bernstein wave (EBW).

Development of laser beam injection system for the Edge Thomson Scattering (ETS) in ITER

This paper focuses on the design and development of the laser injection system for the ITER Edge Thomson Scattering system (ETS). The ITER ETS achieves a temporal resolution of 100 Hz by firing two 50 Hz laser beams alternatively. The use of dual lasers enables us to perform the Thomson scattering measurements at a temporal resolution of 50 Hz in case that one of the laser systems stops functioning. A new type of beam combiner was developed to obtain a single beam that is collinear and fixed linearly polarized from two laser beams using a motor-driven rotating half-wave plate. The rotating half-wave plate method does not induce misalignment even if the rotating mechanism malfunctions. The combined beam is relayed from the diagnostic hall to the plasma using mirror optics and is absorbed at the beam dump integrated on the inner blanket. The beam alignment system was designed to direct the laser beam onto the center of the beam dump head. The beam position at the beam dump is monitored by four alignment laser beams which propagate parallel to the diagnostic Nd:YAG laser beam and imaging systems installed outside the diagnostic port.

Radio-frequency electromagnetic field measurements for direct detection of electron Bernstein waves in a torus plasma

To identify the mode-converted electron Bernstein wave (EBW) in a torus plasma directly, we have developed an interferometry system, in which a diagnostic microwave injected outside of the plasma column was directly detected with the probing antenna inserted into the plasma. In this work, plasma production and heating are achieved with 2.45 GHz, 2.5 kW electron cyclotron heating (ECH), whereas diagnostics are carried out with a lower power (10 W) separate frequency (1-2.1 GHz) microwave. Three components, i.e., two electromagnetic (toroidal and poloidal directions) and an electrostatic (if refractive index is sufficiently higher than unity, it corresponds to radial component), of ECRF electric field are simultaneously measured with three probing antennas, which are inserted into plasma. Selectivities of each component signal were checked experimentally. Excitation antennas have quite high selectivity of direction of linear polarization. As probing antennas for detecting electromagnetic components, we employed a monopole antenna with a length of 35 mm, and the separation of the poloidal (O-wave) and toroidal (X-wave) components of ECRF electric field could be available with this antenna. To detect EBW, which is an electrostatic wave, a small tip (1 mm) antenna was used. As the preliminary results, we detected signals that have three characteristics of EBW, i.e., short wavelength, backward propagation, and electrostatic.

Note: Lossless laser beam combiner employing a high-speed rotating half-wave plate

We have developed a laser beam combiner employing a high-speed rotating half-wave plate based on the specific requirements of the Thomson scattering measurement systems in the ITER. The polarization extinction ratio of the output beam may exceed 1000 and was maintained for more than 1 h via feedback control of the half-wave plate rotation speed. The pointing fluctuations introduced by rotating the half-wave plate were in the order of microradians. The high-speed rotating half-wave plate provides a lossless means of combining laser beams together with stable beam pointing.

Anisotropic electron temperature measurements without knowing the spectral transmissivity for a JT-60SA Thomson scattering diagnostic

This paper focuses on a method for measuring the electron temperature (T(e)) without knowing the transmissivity using Thomson scattering diagnostic with a double-pass scattering system. Application of this method for measuring the anisotropic T(e), i.e., the T(e) in the directions parallel (T(eparallel)) and perpendicular (T(eperpendicular)) to the magnetic field, is proposed. Simulations based on the designed parameters for a JT-60SA indicate the feasibility of the measurements except in certain T(e) ranges, e.g., T(eparallel) ~ 3.5T(eperpendicular) at 120° of the scattering angle.