External Helical Field Research Articles

Excitation of high wavenumber fluctuations by externally-imposed helical fields in edge pedestal plasmas

Two-step mode coupling analyses for nonlinear excitation of the ballooning mode (BM) in pedestal plasma by external helical magnetic field perturbation [Resonant Magnetic Perturbations (RMP)] are presented. This technique allows calculating the effect of higher harmonic sidebands generated by interaction of long scale RMP pump and BM. It is shown that RMP field perturbations can modify the BM growth rate and frequency through nonlinear Reynolds stress and magnetic stress. In particular, it is shown that both stresses can efficiently excite high wavenumber BM fluctuations which, in turn, can enhance the transport in the pedestal. Another notable feature of this analysis is the existence of short scale (high-ky) nonlinear instability at Alfven time scale near the ideal BM threshold boundary.

Control of reversible magnetization switching by pulsed circular magnetic field in glass-coated amorphous microwires

Magnetization reversal in magnetic microwires was studied in the presence of external mechanical stress and helical magnetic fields using the magneto-optical Kerr effect. It was found that a combination of tuned magnetic anisotropy and a direct current or pulsed circular magnetic field activated different types of magnetization reversal scenarios. The application of the pulsed magnetic field of 10 ns time duration induced a transient controlling action to switch the magnetic states without activating a domain wall motion. This created a promising method for tuning the giant magneto-impedance effect.

Instabilities of an annulus flow between rotating cylinders in a helical magnetic field.

The stabilities of an annulus flow between the rotating inner and outer cylinders with an external helical magnetic field are studied by using the quasistatic approximation. It is shown numerically that for the spiral base flow with a zero axial pressure gradient, the helical magnetic field yields a helical traveling wave at a critical Reynolds number. This wave mode is revealed to be the most unstable mode by linear stability analysis. At higher Reynolds numbers, the first wave mode is superposed by a second antisymmetric helical wave mode, which travels with a higher phase velocity than the first mode. When the Reynolds number is increased further, the flow becomes turbulent, but the key features of the flow structure are still dominated by the first and the second wave modes. Furthermore, when a finite axial pressure gradient is applied to guarantee a zero axial flow rate, the annulus flow is found to be more unstable than the case with zero axial pressure gradient.

Parametric analysis of magnetic islands subject to halo-current perturbation in disrupting tokamak plasmas

Results of simulation and parametric analysis of magnetic island production by helical magnetic perturbation generated under non-axisymmetric halo current are presented. Predictions are made for a cylindrical ITER-size plasma in conditions of disruption. Calculations are carried out with the TEAR code based on the visco-resistive MHD approximation. The radial distribution of the magnetic flux perturbation is calculated with account of the external helical field produced by halo current. The equations for the magnetic flux perturbation describe the dynamics of the tearing mode depending on plasma rotation. In sequence, this rotation is affected by electromagnetic forces depending on the tearing mode magnetic field and external magnetic perturbation. The coupled diffusion-type equations for the helical flux function and for the plasma rotation velocity are numerically treated in a similar way. The magnetic island behavior is analyzed for different plasma parameters expected at the Current Quench stage of disruption. The calculated width of the produced magnetic islands extends to a significant part of plasma minor radius.

Interpreting the time variable RM observed in the core region of the TeV blazar Mrk 421

In this work we interpret and discuss the time variable rotation measure (RM) found, for the first time over a 1-yr period, in the core region of a blazar. These results are based on a one-year, multi-frequency (15, 24, and 43 GHz) Very Long Baseline Array (VLBA) monitoring of the TeV blazar Markarian 421 (Mrk 421). We investigate the Faraday screen properties and its location with respect to the jet emitting region. Given that the 43 GHz radio core flux density and the RM time evolution suggest a similar trend, we explore the possible connection between the RM and the accretion rate. Among the various scenarios that we explore, the jet sheath is the most promising candidate for being the main source of Faraday rotation. During the one-year observing period the RM trend shows two sign reversals, which may be qualitatively interpreted within the context of the magnetic tower models. We invoke the presence of two nested helical magnetic fields in the relativistic jet with opposite helicities, whose relative contribution produce the observed RM values. The inner helical field has the poloidal component ($B_{\rm p}$) oriented in the observer's direction and produces a positive RM, while the outer helical field, with $B_{\rm p}$ in the opposite direction, produces a negative RM. We assume that the external helical field dominates the contribution to the observed RM, while the internal helical field dominates when a jet perturbation arises during the second observing epoch. Being the intrinsic polarization angle parallel to the jet axis, a pitch angle of the helical magnetic field $\phi\gtrsim 70^\circ$ is required. Additional scenarios are also considered to explain the observed RM sign reversals.

Measurement of Tokamak Plasma with the External Helical Field Using a High-Speed Camera in TOKASTAR-2

Measurement of Tokamak Plasma with the External Helical Field Using a High-Speed Camera in TOKASTAR-2

Construction of a plasma confinement device TOKASTAR-2 with tokamak-helical hybridization concepts

In the research of confinement of torus plasma using magnetic fields, a concept of superposition of the external helical magnetic field to tokamak plasma has been widely proposed. One of the objectives of this concept is to improve the stability of plasmas, for example, to suppress plasma current disruption. A tokamak-helical hybrid plasma confinement device TOKASTAR-2 has recently been constructed to investigate the effect of helical field to tokamak plasma. First plasma was just produced in June, 2009. TOKASTAR-2 machine has Ohmic heating central coils, eight toroidal field coils and two outboard helical field coil segments. The major radius Rp and the averaged minor radius <ap> of the toroidal plasma with nearly circular cross-section is expected to be approximately 0.1 m and 0.04 m, respectively. The toroidal magnetic field Bt is typically 0.1 T. The plasma start-up experiment using the electron cyclotron heating (ECH) by the radio frequency wave with the frequency of 2.45 GHz and the injected power of 2.0 kW is in progress in this machine as the pre-ionized phase of tokamak operation. In this paper, the overview of TOKASTAR-2 device and present states of the study are presented. According to the preliminary experiment, we confirmed the effect of outer vertical or helical magnetic field application to suppress bursting density fluctuation in the pre-ionized ECH plasmas.

Effect of the external helical fields on the plasma boundary shape in JET

Externally applied helical magnetic fields are now often used on tokamaks for various purposes. This paper presents results of studies of the effect of the external fields, produced by the error field correction coils (EFCCs) on JET, on the plasma boundary shape. Significant 3D distortions, predicted in the previous studies, have been confirmed using upgraded magnetic diagnostics and high-resolution Thomson scattering diagnostics. A simple method of estimating the edge distortion using magnetic diagnostics calibrated on the kinetic measurements is proposed and demonstrated.

Control of Neoclassical Tearing Mode by Electron Cyclotron Current Drive and Non-Resonant Helical Field Application in ITER

On tokamak plasmas like ITER, it is necessary to stabilize neoclassical tearing mode (NTM) because the NTM reduces plasma temperature and fusion power output. For the analysis of stabilizing NTM in fusion plasmas, the electron cyclotron current drive (ECCD) and the non-resonant external helical field (NRHF) application are simulated using the 1.5-dimensional equilibrium/transport simulation code (TOTAL code). The 3/2 NTM is stabilized by only external helical field, but the 2/1 mode is not stabilized by only external helical field in the present model. The stabilization time becomes shorter by the combination of ECCD and NRHF than that by ECCD alone.

Initial Plasma Confinement Experiments in Tokamak-Helical Hybrid Device TOKASTAR-2

Tokamak and helical systems have been widely recognized as efficient toroidal magnetic plasma confinement devices. TOKASTAR-2 [1] is a new plasma confinement device which has both confinement properties. There are outer helical field coils with the toroidal period n=2 or 1 outside of eight toroidal field coils. One of main purpose of this experiment is to evaluate effects of external helical field application on tokamak plasma confinement. Present status of TOKASTAR-2 is in the phase that electron cyclotron resonance of radio frequency wave creates plasmas. In this conference, analysis of magnetic field surface, plasma production near electron cyclotron heating (ECH) resonance layer, estimation of electron density and temperature, and radial density profile in this device are presented. Electron density and temperature were measured using single or double Langmuir probe. Central density and temperature are estimated in order of 10m and 10eV respectively. In the measurement of plasma density profile in space, using double probe, plasma density profile is analyzed assuming that measured probe current is proportional to plasma density at respective measuring position of probe. Figure 1 shows temporal evolution of probe current which were measured at each position of radial direction. In this case, simple toroidal magnetic field was applied and plasma current was not induced. Figures (a),(b),(c) and (d) of Fig. 1 denote temporal evolution of probe current at major radius R=17, 13, 9, 7cm, respectively. As shown in this figure, during 2~3 ms, peak of probe signal is observed at the inner radial position at the early stage. The peak position goes to the outside gradually. During 3~5ms, signal level is almost constant at R=17cm. Moreover during 5~6ms, peak of signal is observed at inside direction. Therefore, initiated plasma at the inside position moves outward, and return to the inside, which corresponds to the movement of ECH layer. Analysis of magnetic field surface and ECH resonance layer position in the poloidal cross section is being carried out by using computer calculation codes. New experimental results of outer helical field application will be also presented in the poster.

Determination of the resonant harmonics of the error field from dynamic magnetic measurements in a tokamak

The possibility is discussed of determining the amplitude and phase of a static resonant error field in a tokamak by means of dynamic magnetic measurements. The method proposed assumes measuring the plasma response to a varying external helical magnetic field with a small (a few gauss) amplitude. The case is considered in which the plasma is probed by square pulses with a duration much longer than the time of the transition process. The plasma response is assumed to be linear, with a proportionality coefficient being dependent on the plasma state. The analysis is carried out in a standard cylindrical approximation. The model is based on Maxwell’s equations and Ohm’s law and is thus capable of accounting for the interaction of large-scale modes with the conducting wall of the vacuum chamber. The method can be applied to existing tokamaks.

Nonlinear simulations of magnetic Taylor-Couette flow with currentfree helical magnetic fields

The magnetorotational instability (MRI) in cylindrical Taylor-Couette flow with external helical magnetic field is simulated for infinite and finite aspect ratios. We solve the MHD equations in their small Prandtl number limit and confirm with timedependent nonlinear simulations that the additional toroidal component of the magnetic field reduces the critical Reynolds number from O(10 6 ) (axial field only) to O(10 3 ) for liquid metals with their small magnetic Prandtl number. Computing the saturated state we obtain velocity amplitudes which help designing proper experimental setups. Experiments with liquid gallium require axial field ∼ 50 Gauss and axial current ∼ 4 kA for the toroidal field. It is sufficient that the vertical velocity uz of the flow can be measured with a precision of 0.1mm/s. We also show that the endplates enclosing the cylinders do not destroy the traveling wave instability which can be observed as presented in earlier studies. For TC containers without and with endplates the angular momentum transport of the MRI instability is shown as to be outwards.

Error field locked modes thresholds in rotating plasmas, anomalous braking and spin-up

The mechanisms of nonlinear interaction of external helical fields with a rotating plasma are investigated analyzing the results of recent systematic experiments on the Joint European Torus (JET) [A. Gibson et al., Phys. Plasmas 5, 1839 (1998)] that widen the previous data base collected on Compass-D [T. C. Hender et al., Nucl. Fusion 32, 2091 (1992)], Doublet III-D [R. J. La Haye et al., Nucl. Fusion 32, 2119 (1992)] and JET. The empirical scaling laws governing the onset of “error field” locked modes are re-assessed and interpreted in terms of existing driven reconnection theories and with new models. In particular the important mechanisms of plasma rotation braking, and spin up, associated with error fields are analyzed in detail and interpreted.

Generic structure of externally driven tearing modes instabilities

A major limit to steady state and advanced high βp operation of tokamaks of reactor class is due to the onset of tearing modes that develop magnetic and may cause loss of energy confinement or a major disruption. Here the structure of a classical problem about the effects of external control helical fields is analysed and it is shown to offer a general paradigm of response of low order classical and neoclassical tearing modes to a wide class of external perturbations. New results of principle on the structural stability of the response model are obtained, leading to a clear interpretation of the role of “seed islands” in the onset of neo-classical tearing modes and the role of finite ion larmor radius corrections to Ohm’s law.

Simulating the effect of external helical magnetic fields on MHD stability of tokamak plasma

The effect of external helical magnetic fields on the stability of a rotating plasma in an axially symmetric tokamak machine is simulated numerically. Threshold amplitude effects are investigated using the NFTC code for the complete nonlinear dynamical 3-D system of MHD equations.

Improvement of the density limit with an external helical field on JFT-2M tokamak

The density limit is increased by the application of an external helical field in the JFT-2M tokamak. The effect of the magnetic stochasticity due to the external field is investigated to study the mechanism of the improved density limit related to the edge plasma behaviour. The improvement is correlated with the retardation of the increase in the plasma inductance. At the improved density limit, local radiation loss is modified by the helical field, in which that from the vicinity of separatrix X-point is remarkably reduced, while that from outboard edge is slightly increased. The formation of a positive radial electric field at the plasma edge is also observed in the presence of the helical field.

Kinetic effects in stochastic topologically nontrivial field

The kinetic description of a collisionless system of particles in an external stochastic field possessing helicity is examined. The kinetic equation which contains an additional term proportional to the helicity is obtained. The solution describing evolution of the distribution function for arbitrary initial conditions is found. Particular examples of such an evolution and some new kinetic effects connected with helicity are discussed. It is shown that a particle beam with inhomogeneous velocity profile in an external stochastic helical field generates a new particle flow in transverse direction.

Effect of an external helical field on a rotating magnetic island

Nonlinear MHD calculations of the m/n=2/1 tearing mode in a low beta cylindrical tokamak are carried out, taking account of the plasma rotation with the effect of the magnetic field induced by an external helical current. A rotating magnetic island is locked to the external helical field through the damping torque Jϕh × Brh, where Brh is the radial component of the external helical field and Jϕh is the plasma current induced by the helical field and the plasma rotation. When the rotating flow velocity is high enough, the damping torque produces a strong velocity shear near the resonant surface until locking of the magnetic island occurs. After mode locking, the width of the magnetic island becomes larger than that in the case without a helical field because of phase matching between the external helical field and the locked magnetic island. The stabilizing effect of the velocity shear can be sustained by keeping the velocity profile in a transient state by applying an external helical field with alternating directions. This method also ensures that the magnetic island does not become mode locked by other mechanisms such as interaction with a resistive shell or an error field, which is often followed by major disruptions

Global structure of MHD modes in tokamaks

Weak external helical fields can act as local resonant disturbances in a tokamak plasma. Such fields can therefore be used as ‘active magnetic probes’ in the study of MHD instabilities in tokamak plasmas. For this purpose, ℓ = 2/n = 1 and ℓ = 3/n = 1 helical windings have been installed on the HT-6B and HT-6M tokamaks. Diagnostic signals (using magnetic probes, soft X-ray detectors, spectroscopy, electron cyclotron emission, etc.) from MHD instabilities have been obtained and studied during discharges with programmed resonant helical fields (RHFs). It was found that an optimized value of the ℓ = 2 RHF strongly suppressed not only the m = 2/n = 1 magnetic perturbation but also the 3/1, 4/1 and 1/1 modes. It was unexpected that the ℓ = 3 RHF could suppress all m = 2,3,4 modes, as well as (and even more strongly) the m = 1 mode. The ℓ = 2 and ℓ = 3 RHFs could also substantially modify the sawtooth oscillation. During these processes, all MHD modes with different m numbers were tightly coupled, which indicates a global coherent mode structure over the whole plasma rather than individual mode behaviour.

Thermodynamics of free electrons in helical magnetic fields

The low-temperature thermomagnetic properties of a free-electron gas coupled to an external helical magnetic field of variable strength and pitch are calculated. The diamagnetic behavior of the system is shown to exhibit some novel remarkable effects which differ substantially from the constant field Landau theory. In the strong field regime (B∼104 G) the magnetization, susceptibility, and helicity modulus are evaluated, yielding some new physical results.