M. Yu. Kantor

HIGH REPETITION RATE PROBING LASER FOR THOMSON SCATTERING DIAGNOSTICS

A new laser based on a multipass intracavity probing system for Thomson scattering diagnostics is presented in the article. The laser system provides a considerable gain in both repetition rate of laser pulses and the diagnostics sensitivity. These capabilities have their origin in a highly effective pumping-to-probing energy conversion which is intrinsic to the system suggested. Main factors affecting the performance of the system are considered. The application of the system in high temperature plasma diagnostics is tested experimentally, and the results are discussed in this article.

Anomalous transport and multi-scale drift turbulence dynamics in tokamak ohmic discharge as measured by high resolution diagnostics and modeled by full-f gyrokinetic code

Quantitative reproduction of selected micro-, meso- and macro-scale transport phenomena as measured in the FT-2 tokamak is reached by Elmfire global full-f nonlinear gyrokinetic particle-in-cell simulation predictions. A detailed agreement with mean equilibrium flows, oscillating fine-scale zonal flows and turbulence radial correlation length observed by a set of sophisticated microwave backscattering techniques, as well as a good fit of the thermal diffusivity data in the central and gradient region of discharge are demonstrated. Both the shift and the broadening of the power spectrum of synthetic and experimental Doppler reflectometry diagnostics have been found to overlap perfectly at various radial positions, indicating similar rotation and spreading of the selected density fluctuations. At the same time similar radial electric field dynamics, spatial structure and outward geodesic acoustic mode (GAM) propagation have been observed by comparisons of the probability distribution function, the dominant frequency, the coherence and the cross-phase of the simulated and experimentally measured radial electric field fluctuations, identifying the turbulent driven GAM as a key contributor to the observed strong temporal variation of the radial electric field affected by impurity ions.

10 kHz repetitive high-resolution TV Thomson scattering on TEXTOR: Design and performance "invited…

In late 2003 a 10kHz multiposition Thomson scattering diagnostic with high spatial resolution became operational on the TEXTOR tokamak. In the initial phase of operation, one burst of 18 pulses of 12J each with a repetition rate of 5kHz could be extracted from the laser system. The installation of a low-dope ruby rod (spring 2005) resulted in a system, which can deliver higher pulse energy and moreover a divergence of better than 0.7mrad, leading to a big improvement in the detection of Thomson scattering photons. Furthermore, the number of laser pulses in one burst could be extended to even more than 30. The achieved laser energy of more than 15J∕pulse makes it possible to measure electron temperature and density profiles with an observational error of 8% on the electron temperature (Te) and 4% on the electron density (ne) at ne=2.5×1019m−3, per spatial element of 7.5mm. The viewing optics enables sampling of either the full plasma diameter of 900mm with 120 spatial channels of 7.5mm each or a 160mm long...

10 kHz repetitive high-resolution TV Thomson scattering on TEXTOR

In December 2003 a new 10u2002kHz multiposition Thomson scattering diagnostic with high spatial resolution has become operational on the TEXTOR tokamak. The system is the follow up of the high-resolution double-pulse Thomson scattering diagnostic. The conventional ruby laser has been replaced by a 10u2002kHz intracavity laser system and the spectrometer detector has been upgraded with two ultrafast complementary metal–oxide–semiconductor cameras combined with a special image intensifier stage. In the initial phase of operation, a burst of 18 pulses decaying from 17u2002tou20028u2002J, with a repetition rate of 5u2002kHz, could be extracted from the laser. At a laser energy up to 12u2002J per pulse, ten electron temperature and density profiles were measured with an observational error of 10% on the electron temperature (Te) and 5% on the electron density (ne) at ne=2.5×1019u2002m−3 per spatial element of 12u2002mm. The resolution of the detection optics enables to sample either the full plasma diameter of 900u2002mm with 120 spatial channels of...

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value nLH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures Te). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(Nz), were calculated. Simulations using the FRTC code with allowance for the P(Nz) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.

Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field Er and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field Er is proposed to explain the formation of a transport barrier.

Test of a periodic multipass-intracavity laser system for the TEXTOR multiposition Thomson scattering diagnostics

A multipass intracavity laser probing system operating in a burst mode has been tested for the Torus Experiment for Technology Oriented Research Thomson scattering diagnostics. The parameters tested were the probing pulse energy and power as well as pulse repetition frequency. The system is to be applied for the dynamic study of fast plasma phenomena (e.g., transport barrier formation and filaments) requiring both high time and spatial resolutions of the electron temperature measurements.

Possibility of quasicontinuous measurement of the evolution of the electron temperature of a plasma by the Thomson scattering method

A method is proposed for increasing the frequency of Thomson scattering measurements of the electron temperature of a plasma to 100 kHz. The approach makes it possible to raise the diagnostic sensitivity under stationary discharge conditions by three orders of magnitude. The possibilities of the method are confirmed experimentally.

Use of additional helium puffing for the diagnostics of plasma parameters at the FT-2 tokamak

The experiments carried out at the FT-2 tokamak in which additional pulsed puffing of helium into the hydrogen plasma was used for diagnostic purposes are considered. To estimate the necessary content of helium ions in the experiments on studying short-scale plasma oscillations, the ionization-recombination balance was simulated numerically under the assumption of a toroidally homogeneous influx of the working gas onto the boundary of the plasma column. In these simulations, the effective density of the neutral gas incident on the plasma boundary was determined by the iteration method, which made it possible to provide agreement between the obtained solution and the experimental discharge conditions. In particular, the correspondence of the determined admixture content to both the plasma quasineutrality condition and the value of the effective charge Zeff, as well as agreement between the calculated and measured plasma density profiles, was ensured. The simulations were performed under the assumption of anomalous diffusion coefficients for all plasma components. The temporal variations of the ionization-recombination balance were checked by comparing them with the measured spectra of radiation in the HeI, HeII, and Hα lines. In the current drive experiments, variations in ne(r) at the discharge periphery were examined by the method based on the proportionality of the intensity ratio of the helium spectral lines, HeI(668 nm)/HeI(728 nm), to the plasma density. In these calculations, the factors relating the intensity ratio of these lines to the plasma density were taken from the literature on spectral diagnostics.

Stability of a laser cavity with a two-mirror multipass system

An investigation is made of the stability of a laser cavity in which one of the nontransmitting mirrors is a multipass system consisting of two spherical mirrors. The stability of the cavity depends strongly on the number of passes of the beam in the multipass system, the configuration of the mirrors, and the constriction of the beam entering the system. A stable cavity configuration ensures low diffraction losses in the system and efficient lasing.