•The use of diode pumping in 2 J × 50 Hz 3 ns Nd:YAG laser improve beam quality.•Cavity dumping approach in 946 nm Nd:YAG laser allow to obtain 3 ns pulse duration.•Zero diffraction length ...Regenerative Amplifier allow to reach 1 J in 946 nm Nd:YAG.•Image-rotation in Regenerative Power Amplifier significantly improves beam quality.
We developed high energy Q-switched 1064 nm and 946 nm Nd:YAG lasers for ITER Divertor Thomson Scattering (DTS). The 1064 nm Nd:YAG laser is designed to be used as the main DTS probing laser and features output energy of 2 J at 3 ns pulse duration. MOPA configuration with phase conjugation mirror provides divergence of 1.3 times the diffraction limit and high stability of the output beam parameters with repetition rate varied from 1 to 50 Hz. The 946 nm Nd:YAG laser designed for calibration by an auxiliary Thomson spectrum at the neighboring wavelength. The 946 nm laser consists of MO based on cavity dumping, high gain regenerative preamplifier and zero diffraction length ring cavity regenerative power amplifier with image-rotation providing 1 J × 50 Hz at 3 ns pulse duration with divergence of 1.5 the diffraction limit. As far as we know this is the highest value of the output energy obtained at 946 nm with nanosecond pulse duration until now. Both lasers use unified pump modules providing high maintainability of the system. The paper describes laser design approaches, state of the art and outlook of further developments.
The ITER design has highlighted the fundamental need to monitor the machine operation in more detail. The mission of the Thomson scattering diagnostics in the ITER divertor research/operation is ...discussed with due attention paid to challenges and capabilities of the existing diagnostic design.
One of the main challenges of the implementation of divertor Thomson scattering system on ITER is weak laser scattering signal to be detected against intense background plasma radiation. The paper ...review briefly the line and continuum radiation data from present magnetic fusion devices in the spectral range of interest to TS diagnostics. The results will form the basis of design and development of the TS diagnostics for the ITER divertor.
The metal mirrors for deflecting laser radiation in the divertor Thomson scattering system of ITER are found to be inapplicable due to high density of laser radiation on a mirror. This is caused by ...the short distance from the laser focus to the mirror surface. We report on investigations of the possibility to use dielectric mirrors and protective sapphire plates in a rotary unit. We study the laser damage thresholds of optical elements, which are supposed to be used in the laser input channel. These optical elements are the dielectric mirrors deposited on different substrates (quartz, sapphire, and single-crystal molybdenum) and the protecting sapphire plate. A number of sapphire samples were irradiated by a neutron flux up to 10
19
n/cm
2
(E > 100 keV) and annealed. Laser damage thresholds of the elements were measured at room temperature and at the ITER operating temperature of 150°C.
A YAG:Nd laser operating with 10-Hz repetition rate was used in the experiments. The laser pulse parameters were 1064-nm wavelength, 16-ns duration, and 250 to 300 mJ of energy.
The experiments have not identified any dependence of optical element damage threshold versus the number of laser pulses. No damage was observed after 10
5
laser pulses with energy density just 10% below the damage threshold level.
The applicability of these optical elements in the divertor Thomson scattering system of ITER had been proven.
Because of the requirement of eliminating carbon deposition on mirrors that facilitate optical methods of plasma diagnostics in thermonuclear synthesis devices, the experimental study has been ...carried out of gasodynamic conditions and the relationship between the partial flows of hydrogen and methane, which are supplied to the direct current glow discharge. The formation of hydrocarbon deposits would be completely suppressed under these conditions. The processes of methane conversion in the discharge into heavier volatile hydrocarbons have been analyzed by mass spectroscopy and conversion into polymer-like a-C:H films have been analyzing by weighing and electron probe microanalysis. The degree of methane conversion was continuously decreasing upon the dilution of methane with hydrogen. The deposition was suppressed completely when methane concentration in the laminar flow of the mixture was approximately 1 mol %. The process of deposition of a-C:H films changed into the process of film erosion when methane content in the mixture was below 0.5 mol %, the Knudsen number
Kn
≈ 0.1, and the Reynolds number
Re
≈ 2. In order to provide the removal of plasmolysis products from the surface of the mirror located in a special diagnostic port of ITER, the specified gasodynamic conditions created by the carrier gas (hydrogen or deuterium) should be ensured, with the component of the velocity vector being directed at an angle to the surface under protection.
•· The paper presents studies of deposits produced by QSPA-Be machine. · Characterization of the deposits revealed PC-BeO stricture of the films. · The deposits were found to be ...similar to those obtained in JET-ILW outer divertor.· The one pulse of QSPA-Be provided deposition of ∼7 nm contaminant layer. · Test plasma cleaning of DTS protective window mock-ups is discussed.
Studies of contaminants obtained by spraying ITER-grade Be on a quasi-stationary plasma gun facility QSPA-Be are presented. Contaminating films, consisting mainly of Be and O in approximately equal proportions, were deposited on substrates of quartz, sapphire, single crystalline silicon (SC-Si) and NaCl crystal. Characterization of the deposits was performed using SEM, XPS, EBS, AFM, SE, TEM&SAED and micro-interferometry showing polycrystalline BeO films similar as found in JET-ILW. Films on SC-Si and NaCl were used to characterize their composition and morphology. The cleaning rates in the 81.36 MHz RF discharges in He or D2 at 2 Pa were measured on the SC-Si target. The measured etching rate of the deposited films was several times higher than the rate calculated from the theoretical value of beryllium oxide sputtering yield. Test cleaning of these contaminants was carried out in a capacitively coupled RF discharge (CCRF) from the surface of sapphire and quartz plates, which were considered as a mock-ups of the protective window of the first mirror unit (FMU) designed for ITER divertor Thomson scattering diagnostic system (DTS).
Plasma parameters in the Globus-M spherical tokamak have been measured upon injection of a deuteron beam with energy about 30 keV and a power of up to 700 kW. Dependences of the ion temperature and ...neutron yield on the plasma current and density and the gap width between the wall and plasma are considered. Possible characteristics of fast-ion confinement in the modified Globus-M2 tokamak are discussed.
Fast degradation of in-vessel optics is one of the most serious problems for all optical diagnostics in ITER. To provide the resistance to mechanical and thermal stresses along with a high stability ...of optical characteristics under deposition-dominated conditions we suggest using high-reflective metallic (Ag or Al) film mirrors coated on silicon substrate and protected with thin oxide film in the divertor Thomson Scattering (TS) diagnostics. The mirrors coated with Al
2O
3 and ZrO
2 films were tested under irradiation by deuterium ions. The experimental results on the oxide films sputtering are discussed in the context of their applicability for the first mirror protection in ITER.
In the experiments carried out on the Globus-M tokamak in regimes with injection of 26-keV neutral beams with a power of 0.75–0.85 MW, two branches of instabilities excited by fast ions were observed ...in the early stage of a discharge: a low-frequency energetic particle mode (EPM) in the frequency range of 5–30 kHz and a high-frequency mode in the range of 50–200 kHz, identified as a toroidal Alfvén eigenmode (TAE). The TAE developed in the initial phase of the discharge at
q
(0) > 1 and terminated when sawtooth oscillations were excited at
q
(0) < 1. The spectrum and spatial localization of the mode agree with predictions of the linear theory. The modes observed in the Globus-M tokamak possess both properties common to other tokamaks and their own specific features.
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