Stability Of MHD Modes Research Articles

Electrical insulation properties in a cold plasma of alumina coating for the in-vessel stabilizing shell of the RFX-mod2 fusion device

This paper presents the results of experimental tests on samples made of copper coated with alumina layer, performed to assess the reliability of its dielectric properties for applications in the low temperature plasma at the edge of a fusion device. The cue of the study was related to the plasma facing components of the RFX-mod2 fusion device (Marrelli et al., 2019, Peruzzo et al., 2023, Peruzzo et al., 2019), devoted to the experimental study of the magnetic confinement of fusion plasmas in a variety of configurations, including the reversed-field pinch and the tokamak. In RFX-mod2 an in-vacuum copper shell for the passive stabilization of MHD modes will surround the plasma. To avoid potentially harmful electrical discharges, which could be induced by rapid transients of the plasma current, this structure must be covered with an electrically insulating layer. For RFX-mod2 an alumina coating was chosen, whose dielectric properties have been tested both in air and in the presence of weakly ionized plasma. Electrical tests, conducted on copper samples with alumina deposits of about 100μm thickness, revealed that the ceramic layer has a high electrical resistance value in air (>1GΩ), but electrical discharges can occur in presence of a weakly ionized plasma, depending on compactness and porosity of the alumina layer, causing local melting of the alumina and expulsion of copper droplets from the substrate. Scanning Electron Microscope (SEM) analyses revealed that in the failed samples the ceramic layer was irregular and rough, with interconnected cavities and cracks, which could reduce its effective thickness and explain the dielectric breakdown at relatively low voltages (<400V). The analyses also showed that samples with a more compact layer present a higher dielectric strength in the presence of the plasma, highlighting that compactness and porosity play crucial roles in ensuring good insulation for materials in a plasma. This study led the definition of the requirements for the insulating coating of the plasma facing components of the RFX-mod2 fusion machine, however the results can be useful for other fusion and non-fusion plasma applications requiring electrical insulation, which can span from industrial devices to spacecrafts.

Global plasma oscillations in electron internal transport barriers in TCV

In the Tokamak à Configuration Variable (TCV) (Hofmann F et al1994 Plasma Phys. Control. Fusion 36 B277), global plasma oscillations have been discovered in fully non-inductively driven plasmas featuring electron internal transport barriers (ITB) with strong ECRH/ECCD. These oscillations are linked to the destabilization and stabilization of MHD modes near the foot of the ITB and can lead to large oscillations of the total plasma current and line-averaged density, among others. They are intrinsically related to the fact that ITBs have large pressure gradients in a region of low magnetic shear. Therefore, the ideal MHD limit is relatively low and infernal modes can be unstable. Depending on the proximity to the ideal limit, small crashes or resistive modes can appear which affect the time evolution of the discharge. Being near marginal stability, the modes can self-stabilize due to the modification of the pressure gradient and local q-profile. The plasma recovers good confinement, reverses shear and the ITB builds up, until a new MHD mode is destabilized. TCV results show that this cycling behaviour can be controlled by modifying the current density or the pressure profiles, either with Ohmic current density perturbation or by modifying the ECH/ECCD power. It is demonstrated that many observations such as q ⩾ 2 sawteeth, beta collapses, minor disruptions and oscillation regimes in ITBs can be assigned to the same physics origin: the proximity to the infernal mode stability limit.

Disruption Avoidance in the Frascati Tokamak Upgrade by Means of Magnetohydrodynamic Mode Stabilization Using Electron-Cyclotron-Resonance Heating

Disruption avoidance by stabilization of MHD modes through injection of ECRH at different radial locations is reported. Disruptions have been induced in the FTU (Frascati Tokamak Upgrade) deuterium plasmas by Mo injection or by exceeding the density limit (D gas puffing). ECRH is triggered when the V(loop) exceeds a preset threshold value. Coupling between MHD modes (m/n=3/2, 2/1, 3/1) occurs before disruption. Direct heating of one coupled mode is sufficient to avoid disruptions, while heating close to the mode leads to disruption delay. These results could be relevant for the International Thermonuclear Experimental Reactor tokamak operation.

Kinetic theory of geomagnetic pulsations: 4. Hybrid gyrokinetic simulation of drift‐bounce resonant excitation of shear Alfvén waves

A one‐dimensional linear hybrid gyrokinetic‐magnetohydrodynamic δf particle in cell simulation code is developed to study the detailed mechanisms of energetic particle drift‐bounce resonant destabilization of Alfvén modes in the ring‐current region of the magnetosphere. The model plasma is composed of a cold (10–100eV) component which provides inertia plus a tenuous energetic (∼10 keV) “ring‐current” component which provides both resonant destabilization and compressional stabilization of MHD modes. Full kinetic effects such as finite Larmor radii and particle magnetic bounce and precessional drift motions are retained nonperturbatively. A simple finite β dipolar equilibrium model is assumed (β is the ratio between plasma and magnetic pressures). Simulations show excellent agreement with earlier perturbative analyses. Results show that when the energetic ion thermal velocity is super‐Alfvénic, the ions destabilize both odd and even parity shear Alfvén MHD modes via the drift‐bounce resonances. The growth rates of the resulting modes scale linearly with plasma β. The most unstable of these modes are found to be drift‐bounce resonance destabilized modes with odd parity, with wave numbers such that k⟂ρ ≈ 0.5 at the equator (ρ is the energetic ion Larmor radius). The destabilization typically occurs at a critical wave number k⟂ρ ≈ 0.3. When the wave number is close to this critical value and the plasma β is close to the ideal MHD critical value, the mode frequency is determined by the energetic particle dynamics, similar to the energetic particle modes (EPMs) observed in laboratory fusion plasma experiments. When the energetic particles have Alfvénic or sub‐Alfvénic thermal velocity, they contribute to damping of the MHD modes via the bounce resonance.

Analysis of quasistatic MHD perturbations and stray magnetic fields in a tokamak plasma

Mechanisms for the development of quasistatic MHD perturbations in a viscous rotating tokamak plasma are considered. The influence of stray magnetic fields on the stability of MHD modes in the plasma of the TFTR tokamak is analyzed.

Physics of Open-Field Line Plasmas. 5. ICRF Heating of Mirror-Confined Plasmas.

Radio Frequency (RF) waves in Ion Cyclotron Range of Frequencies (ICRF) are widely used in both heating and controlling mirror-confined plasmas. These include slow wave heating, plasma production by fast waves, heating of ions and electrons via mode conversion of fast waves. RF ponderomotive stabilization of MHD modes, and plug potential formation. Described here, are the investigations performed on simple mirrors and tandem mirrors (TMX, GAMMA10, Phaedrus, TARA, and HIEI) concerning ICRF heating and control.

MHD Stability in Axisymmetric Multiple Mirror Geometry

The marginal stability of MHD modes is discussed in application for high beta multiple mirror experiments planned at Budker Institute of Nuclear Physics. Flute modes arc dangerous in axisymmetric systems with β < 1. In the case of “wall confined” plasmas, (β ≫ 1), pressure slightly varies along the radius providing less radial gradient and more stability against MHD modes. Effect of ion-ion viscosity becomes important in corrugated magnetic field. It results in the reduction of the growth rate by a factor β1/2. In the process of start up and plasma heating β < 1. If flute modes are stabilized during this period by the line-tying mechanizm ballooning modes are still unstable when β > βcr. A very low ballooning margin is predicted in multiple mirror with the large number of cells: βcr < π2 /N2. For the number of cells N ≃ 10: βcr ≃ 5%. Results of the calculations are discussed in the context of old and new multiple mirror experiments.

Alpha Particle Physics in Toroidal Plasmas.

Alpha particle physics of toroidal (or tokamak) plasmas is discussed based on the reports for the survey of physics related to fast ions and alpha particles published by the Japan Society of Plasma Science and Nuclear Fusion Research.Main topics are alpha particle confinement, destabilization or stabilization of MHD modes by alpha particles, alpha particle driven bootstrap current, alpha heating driven thermal instability, and alpha ash exhaust.Diagnostics for alpha particles confined in toroidal plasma and for neutrons and gamma rays are explained briefly. Finally research subjects in future D-T reacting plasmas are discussed.

Stabilization of MHD modes in an axisymmetric plasma column through the use of a magnetic divertor

The introduction of a magnetic limiter is proposed for stabilization of MHD interchange modes in the central cell of a tandem mirror. The magnetic limiter creates a ring null in the magnetic field, and electrons which enter the null can stream azimuthally and thereby ‘short-circuit’ m = 1 fluctuations. This disallows a rigid m = 1 response and introduces finite Larmor radius stabilization effects in much the same way as they appear for higher azimuthal modes. Some pressure gradient can be maintained on the separatrix flux surface by locating the null on a local magnetic maximum. This scheme introduces the possibility of a fully axisymmetric tandem mirror.

Mode coupling in dynamic stabilization of MhD modes

Existing theory of dynamic stabilization of MHD modes completely disagrees with experimental results obtained for the m=1 mode in a linear screw pinch with superimposed HF axial fields (Bz). Theory fails to describe the experimental stabilizing condition and the magnitude and structure of the dynamic force (Fexp) derived from it. There is some evidence that the discrepancy results from the fact that existing theory is linear in the sense that anharmonic coupling terms for different modes are neglected. Some arguments for a non-linearized treatment od dynamic stabilization of MHD modes are given. In the special case of dynamic stabilization of m=1 mode by Bz a model calculation shows that m=0 and m=1 mode coupling should be taken into account in the case omega s>> omega 1( omega s stabilizing frequency, omega 1 growth rate of the m=1 mode'. One anharmonic term in the m=1 equation of motion is approximately xi 0 xi 1, which agrees with the order of magnitude and the structure of Fexp.

To the theory of nonlinear mode coupling in dynamic stabilization of MHD modes

The nonlinear interaction of oscillation modes is investigated on the basis of Lagrangian formalism. Equations describing the changes of the bound mode amplitudes versus time, are obtained. It is shown that the energy transformation between different modes is of a periodic nature: if in the initial moment of time an appreciable part of the energy is contained, for example, in them-th mode, then after a period of timeTt (called a time of nonlinear interaction) the energy will be transformed to then-th mode. Expressions forTt for cases with the interaction of two and three modes are obtained. As a particular case the process of nonlinear interaction of the electron “transverse” and “longitudinal” oscillations in the highfrequency hybrid resonance region of a “weakly” inhomogeneous plasma was investigated.