L. M. Kovrizhnykh

High power density electron cyclotron experiments in the L2M stellarator

The results of electron cyclotron heating (ECH) experiments in the L-2M stellarator are presented. The main goal of the experiments is to investigate the physics of ECH and of plasma confinement at high values of the volume heating power density. A current free plasma is produced and heated by extraordinary waves at the second harmonic of the electron cyclotron frequency ( omega 0=2 omega ce). The experimental results are compared with the empirical LHD scaling and with numerical simulations of plasma confinement and heating processes based on neoclassical theory using the full matrix of transport coefficients, including some additional anomalous corrections

Backscattering of gyrotron radiation and short-wavelength turbulence during electron cyclotron resonance plasma heating in the L-2M stellarator

Backscattering of gyrotron radiation (θ = π) by short-wavelength density fluctuations (k⊥ = 30 cm−1) in the plasma of the L-2M stellarator was studied under conditions of electron cyclotron resonance (ECR) plasma heating at the second harmonic of the electron gyrofrequency (75 GHz). The scattering of the O-wave emerging due to the splitting of the linearly polarized gyrotron radiation into the X- and O-waves was analyzed. The signal obtained after homodyne detection of scattered radiation is a result of interference of the reference signal, the quasi-steady component, and the fast oscillating component. The coefficients of reflection of the quasi-steady component, R=2(Y), and fast oscillating component, R∼2(Y), of scattered radiation are estimated. The growth of the R∼2(Y) coefficient from 3.7 × 10−4 to 5.2 × 10−4 with increasing ECR heating power from 190 to 430 kW is found to correlate with the decrease in the energy lifetime from 1.9 to 1.46 ms. The relative density of short-wavelength fluctuations is estimated to be 〈n∼2〉/〈ne2〉 = 3 × 10−7. It is shown that the frequencies of short-wavelength fluctuations are in the range 10–150 kHz. The recorded short-wavelength fluctuations can be interpreted as structural turbulence, the energy of which comprises ∼10% of the total fluctuations energy. Simulations of transport processes show that neoclassical heat fluxes are much smaller than anomalous ones. It is suggested that short-wavelength turbulence plays a decisive role in the anomalous heat transport.

Effect of ECRH regime on characteristics of short-wave turbulence in plasma of the L-2M stellarator

This paper reports on studies of short-wave turbulence in the plasma of the L-2M stellarator under markedly different conditions: with doubling the ECR heating power (100 and 200 kW) and with restricting the plasma radius by a sector limiter. The role of such short-wave turbulence in anomalous transport can appear important for conditions of a thermonuclear reactor. Experiments were carried out in a basic magnetic configuration of the L-2M stellarator during ECRH at the second harmonic of the electron gyrofrequency (75.3 GHz) at average electron densities of (1.5–1.7) × 1013 cm−3. The energy confinement time was ~3.5 ms at P0 = 100 kW and was reduced to ~2 ms at P0 = 200 kW. When the limiter was introduced inside the plasma to a depth of 2 cm from the last closed flux surface, τE decreased by a factor of 1.3–1.4. Plasma density fluctuations were measured from the scattering of gyrotron radiation at the second harmonic of operating frequency (~150 GHz). A quasioptical receiving system allowed measurements of scattered radiation from plasma regions r/a ≤ 0.6 at scattering angles π/4 ≤ Θ ≤ π/2 (24 cm−1 ≤ k⊥ ≤ 44 cm−1). The short-wave turbulence was studied for two radial positions of the scattering region: r/a = 0.3–0.4 and r/a = 0.5–0.6. Short-wave turbulence exhibits features of strong plasma turbulence. It is experimentally established that a change in the energy confinement time in the L-2M stellarator correlates with the level of short-wave turbulence.

Stability and variations of plasma parameters in the L-2M stellarator during excitation of the induction current in the regime of ECR plasma heating

Results are presented from experimental studies of variations in the plasma parameters during the excitation of a multiaxis magnetic configuration by the induction current (up to 17 kA) in the basic magnetic configuration of the L-2M stellarator in the regime of ECR heating at a microwave power of ∼200 kW (∼1 MW m−3) and an average plasma density of (1–2) × 1019 m−3. The current direction was chosen to reduce the net rotational transform (the so-called “negative“ current). The current was high enough for the rotational transform to change its sign inside the plasma column. Computer simulations of the L-2M magnetic structure showed that the surface with a zero rotational transform is topologically unstable and gives rise to magnetic islands, i.e., to a multiaxis magnetic configuration. Magnetic measurements showed that, at negative currents above 10 kA, intense bursts of MHD oscillations with a clearly defined toroidal mode number n = 0 were observed in the frequency range of several kilohertz. Unfortunately, the experimental data are insufficient to draw the final conclusion on the transverse structure of these oscillations. The radial temperature profiles along the stellarator major radius in the equatorial plane were studied. It is found that the electron temperature decreases by a factor of 1.3 in the plasma core (r/a ≤ 0.6) and that the temperature jump is retained near the boundary. A change in turbulent fluctuations of the plasma density during the excitation of a negative current was studied using wave scattering diagnostics. It is found that the probability density function of the increments of fluctuations in the plasma core differs from a Gaussian distribution. The measured distribution is heavy-tailed and broadens in the presence of the current. It is found that the spectrum of turbulent fluctuations and their Doppler shift near the plasma boundary are nonuniform in the radial direction. This may be attributed to the shear of the poloidal velocity. The experimental results indicate that the formation of regions with a zero rotational transform in the plasma core somewhat intensifies plasma transport.

Effect of the transverse magnetic field on turbulence and parameters of a plasma column in the L-2M stellarator

The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5−2)×10−, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (Bv/B0=5×10−3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (Bv/B0∼3×10−3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.

Dependence of the energy lifetime on the type of anomalous losses in stellarators

A relatively simple model of transport process in stellarators that was proposed earlier by the author on the basis of neoclassical theory makes it possible to determine the density and temperature profiles of the plasma components, the ambipolar electric field profile, and the particle and energy lifetimes from the given device parameters and given particle and energy sources with allowance for anomalous losses. The results of numerical simulations carried out with this model for the L-2M, ATF, CHS, and LHD stellarators over broad ranges of plasma densities and absorbed powers showed that the plasma energy lifetimes in these devices coincide to within factors on the order of two with those found from empirical scalings. A specific model of anomalous losses was chosen for calculations. Results are presented from simulations with a more general form of the anomalous thermal conductivity. Namely, the thermal conductivity is chosen to be Kj(a) ≈ NαTjβB0−γ, where N(r) is the plasma density and Tj(r) is the temperature of the jth plasma component (j = e, i). The parameters α, β, and γ are set equal to α = 1, β = 2, and γ = 1; α = 0.5, β = 2.5, and γ = 1; α = 1.5, β = 2, and γ = 2; α = 1, β = 2.5, and γ = 2; and α = 1.5, β = 2.5, and γ = 2. The simulations have been done for the L-2M and LHD stellarators. It is found that, in all the five models, the calculated energy lifetimes τc are essentially independent of the functional form of the anomalous thermal conductivity and coincide to within a factor on the order of two with those following from the LHD scaling.

Plasma energy balance in the L-2M stellarator

AbstractResults are presented from studies of the effect of the discharge parameters (in particular, plasma density and heating power) and the characteristics of the magnetic configuration (e.g., rotational transform) on the confinement of a low-pressure plasma during electron-cyclotron resonance heating in the L-2M stellarator. An analysis shows that the plasma energy in the steady-state phase of a discharge is fairly well described by the product of power functions of the plasma density, heating power, and rotational transform:

Current status and prospects of research on Plasma Physics and Controlled Fusion in 2009 in Russia

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