Measurement Of Poloidal Magnetic Field Research Articles

Evaluation of plasma current quench time using a discrete Rogowski coil in the KSTAR tokamak

The plasma current was earlier measured by a Rogowski coil on the wall inside the vacuum vessel of the KSTAR tokamak. After installing in-vessel components, this measurement included toroidal eddy currents induced on the in-vessel components between plasma boundary and the wall in the transient phases. The plasma current is now evaluated by using the sum of local poloidal magnetic field measurements, as a discrete Rogowki coil measurement, at the plasma facing components along a poloidal circumference that does not encompass the eddy currents. These two measurements agree to within a few percent in the flat-top phase of the discharge. The current quench time previously overestimated by the Rogowski coil measurement can be now evaluated from the linear fit on the wave-form of the plasma current excluding the eddy current during the disruption, so it is more accurately estimated as 2.7 ± 1.0 ms for the plasma current of 0.3–1.1 MA. In addition, an estimate of the eddy currents is obtained by subtracting the discrete Rogowski coil measurement from the Rogowski coil measurement in the transient phases, and its magnitude is validated by comparing with the toroidal current directly measured at the gap resistors connecting two adjacent passive stabilizers.

Electromagnetic wave polarization state evolution in weakly anisotropic and nonuniform media with dissipation

The change of the polarization state of electromagnetic beam propagating in weakly anisotropic and smoothly inhomogeneous media with dissipation is analysed. On the basis of a quasi-isotropic approximation, which provides the consequent asymptotic solution of Maxwell's equation, the differential equation for the evolution of four component Stokes vector is derived. Obtained equation generalizes previous results for the nonadsorbing media and is written in terms of the dielectric tensor of birefringent media with dissipation. The formalism is illustrated by an example of magnetised plasma with dissipation due to the electron collisions. Full Text: PDF ReferencesK.G.Budden, Radio Waves in the Ionosphere (Cambridge U. Press 1961).V.I.Ginzburg, Propagation of Electromagnetic Waves in Plasma (Gordon & Breach 1970).Yu.A.Kravtsov, ""Quasiisotropic" Approximation to Geometrical Optics", Sov. Phys. Dokl. 13, 1125 (1969).A.A. Fuki, Yu.A. Kravtsov, and O.N. Naida, Geometrical Optics of Weakly Anisotropic Media (Gordon & Breach, Lond., N.Y. 1997).Yu.A. Kravtsov and Yu.I. Orlov, Geometrical optics of inhomogeneous media (Springer Verlag, Berlin, Heidelberg 1990). CrossRef Yu.A. Kravtsov et al., "Waves in weakly anisotropic 3D inhomogeneous media: quasi-isotropic approximation of geometrical optics", Physics-Uspekhi 39, 129(1996). CrossRef F.De Marco, S.E.Segre, "The polarization of an e.m. wave propagating in a plasma with magnetic shear. The measurement of poloidal magnetic field in a Tokamak", Plasma Phys. 14, 245 (1972). DirectLink S.E.Segre, "A review of plasma polarimetry - theory and methods", Plasma Phys. Control. Fusion 41, R57 (1999). CrossRef M. Born and E. Wolf, Principles of Optics (Pergamon, Oxford 1980). CrossRef B.Bieg et al., "Quasi-Isotropic Approximation of Geometrical Optics Method as Adequate Electrodynamical Basis for Tokamak Plasma Polarimetry", Physics Procedia 62, 102 (2015). CrossRef B.Bieg et al., "Two approaches to plasma polarimetry: Angular variables technique and Stokes vector formalism", Nucl. Instr. Meth. Phys. Res. Sect. A 720, 157 (2013). CrossRef S.E.Segre, "New formalism for the analysis of polarization evolution for radiation in a weakly nonuniform, fully anisotropic medium: a magnetized plasma", J. Opt. Soc. Am. A 18, 2601 (2001). CrossRef

Poloidal Magnetic Field Measurements in a Reversed Field Pinch Plasma by a Far?Infrared Polarimeter

A multichannel far‐infrared (FIR) polarimeter has been installed in RFX, a Reversed Field Pinch (RFP) plasma experiment, to measure the poloidal magnetic field profile. The polarimeter uses a CH3OH FIR laser operating at λ=118.8 μm. Faraday rotation measurements on five of six parallel diagnostic chords are used in preliminary investigations of poloidal field profiles. The experimental results are generally found in good agreement with the μ&p model predictions. The choice of the profile of μ = μ0j·B/B2 is discussed. For the reconstruction of the magnetic field profiles a numerical filamentary current equilibrium code is described, where polarimetric data are included as constraints. An alternative method based on the best‐fit of a three‐parameter μ profile to the polarimetric data is also reported. Both methods provide reliable reconstructions of the plasma magnetic field and the results indicate the existence in RFX of hollow μ profiles.

Line ratio method for measurement of magnetic field vector using Li-multiplet (2 2S−2 2P) emission

A method has been developed, for use in magnetic confinement devices, to measure the magnetic field vector B⃗ in plasmas or gases. It utilizes the intensities of the π and σ components of the resonance multiplet emitted by lithium atoms subjected to a strong Zeeman effect. A difference in dependence of these intensities on the inclination angle θ between B⃗ and the line of sight allows one to determine the direction of B⃗, provided the intensity ratio of the π and σ components ξ(θ) is measured. The magnitude of B⃗ is routinely inferred from the width of the multiplet. The principles of the measurement are elaborated in detail for the case of a fast Li beam (20–100 keV) used to diagnose a fusion plasma. The deviation of the population of the m states from the statistical one due to a dominant direction for the relative velocity during the excitation of the atoms by plasma ions has been analyzed and corrections to ξ(θ) are calculated. The geometry employed for the measurement is investigated in order to minimize the uncertainties due to systematic and random errors. A procedure for in situ calibration is outlined. As proof of the principle the results from poloidal magnetic field measurements in ohmic and H-mode pulses on the Joint European Torus are analyzed. As expected, much higher components of the poloidal magnetic field BZ and BR have been found at the plasma edge in H-mode pulses indicating the sensitivity of the measurements to the bootstrap current. An accuracy of 10–20% for the poloidal magnetic field component BZ and BR and 1% for B has been reached. Reasonable agreement has been observed between the expected and obtained accuracy. The uncertainty in ξ(θ) is found to be close to the statistical limit at ξ(θ)>6%. The prospects for current density measurement at the plasma edge, which remains a key issue for achieving advanced performance of modern tokamaks, are examined in terms of making use of the developed technique. It is concluded that prospects are good provided the best available Li-beam guns, with equivalent neutral current ∼5 mA, are used.

Poloidal magnetic field measurements and analysis with the DIII-D LIBEAM system (invited)

For over 30 yrs, neutral lithium beams have been employed as a localized, noninvasive diagnostic on a variety of plasma experiments worldwide, providing a number of key physics measurements. On DIII-D the LIBEAM diagnostic has been designed to provide precise measurements of the local poloidal magnetic field in the edge region, a parameter of basic importance to understanding the stability of high performance tokamaks. We utilize the Zeeman splitting and known polarization characteristics of the collisionally excited 670.8 nm Li resonance line to interpret local magnetic field components viewed using a closely packed (ΔR∼5 mm) array of 32 view chords. A dual photoelastic modulator/linear polarizer combination serves to amplitude modulate the light in exact correspondence to its input polarization state. Subsequent narrowband spectral filtering using etalons and standard interference filters is used to isolate one of the three Zeeman components, and the polarization state of that component is recovered using a PC-based, multichannel digital lock-in detection system. Edge magnetic pitch angle profiles for a variety of shots have been reconstructed using a small number of chords and detailed analysis of the lock-in and dc signal levels. Present system performance appears to be limited by etalon performance as well as various broadening mechanisms in the beam that tend to decrease the polarization fraction in the observed component. A careful analysis of this effect and some strategies for improving the measured polarization will be presented.

A rotating coil probe for the magnetic field measurement on a long pulsed tokamak

In order to determine the magnitude and the position of the plasma current in a long pulsed tokamak such as the International Thermonuclear Experimental Reactor (ITER), it is urged to establish a reliable method which is free from the zero-level drift of the integrator as well as the radiation damage for the steady-state magnetic field measurement. For this purpose, we have developed a hybrid system, a combination of a conventional magnetic probe for the measurement of fast varying magnetic field and a rotating coil magnetic probe for that of slowly varying field. The rotating coil is energized by an air turbine to avoid electromagnetic interference and the induce signal with a constant rotation frequency is picked up through a transformer to eliminate mechanical contacts. An automatic gain control circuit is also designed for the compensation of rotation speed fluctuation. The system is proved to achieve a flat frequency response with a proper choice of cross over frequency for high- and low-frequency systems. The rotating coil probe is tested for over 170 h without any trouble. The probe was applied to the poloidal magnetic field measurement on the TRIAM-1M long pulsed tokamak, and proved to work satisfactorily.

Poloidal magnetic field measurements on TEXT-Upgrade

A polarimetry capability has been added to the UCLA multichord far-infrared heterodyne interferometer system on TEXT-Upgrade. The polarimeter utilizes a phase technique in order to measure the Faraday rotation angle of the incident laser beam upon transmission through the plasma. From these data one derives information on the poloidal magnetic field, current density and safety factor profiles. The response of these profiles to electron cyclotron resonance heating and fast current ramps is investigated.

Active feedback stabilization of axisymmetric modes in highly elongated tokamak plasmas

Active feedback stabilization of the vertical instability is studied for highly elongated tokamak plasmas (2 ⩽ κ ⩽ 3) and evaluated in particular for the TCV configuration. It is shown that the feedback can strongly affect the form of the eigenfunction for these highly elongated equilibria, and this can have detrimental effects on the ability of the feedback system to detect properly and stabilize the plasma. A calculation of the vertical displacement that uses poloidal flux measurements, poloidal magnetic field measurements, and corrections for the vessel eddy currents and active feedback currents was found to be effective even in the cases with the worst deformations of the eigenfunction. These deformations are also examined to determine how they affect equilibria with various degrees of shaping, and it is seen that the magnitude of the deformation of the eigenfunction is a strong function of the plasma elongation

Poloidal magnetic field measurement system in JT-60

A diagnostic system using a helium beam for local poloidal magnetic field measurements (Zeeman polarimeter system) has been constructed. The Zeeman spectrum of the helium 31P-21S line was observed in JT-60. The intensity and energy of the neutral helium beam were up to 0.6 A and 200 keV, respectively. Resolution of the Zeeman polarimeter was 0.01 nm which was realized by using a Littrow-type spectrometer. Photon counting efficiency was 0.065. Moreover, an in situ wavelength calibration could be done, using an Ar laser, within an error of 0.001 nm.

Equilibrium analysis of current profiles in tokamaks

An efficient method is given for self-consistent reconstruction of the tokamak current profiles and their associated magnetic topology using the magnetohydrodynamic (MHD) equilibrium constraint from external magnetic measurements, kinetic profile measurements, internal poloidal magnetic field measurements, and topological information from soft X-ray (SXR) measurements. Illustrative examples for beam heated H-mode divertor discharges in the DIII-D tokamak are presented, using the experimentally measured kinetic profile information and external magnetic data from the existing diagnostics. Comparative reconstructions of the current profile using various combinations of diagnostics are given. Also presented is an alternative magnetic analysis method in which the MHD equilibrium is reconstructed using external magnetic data and a constraint on the edge pressure gradient. The results of a sensitivity study are given which show that the axial safety factor q(0) can be more accurately determined when additional information from internal poloidal magnetic measurements is used in conjunction with the external magnetic, kinetic and SXR topological data.

MEASUREMENT OF POLOIDAL MAGNETIC FIELD OF TOKA-MAK BY LASER EXCITED FLUORESCENCE OF HIGH ENERGY Li BEAM

In this paper the principle, experimental setup and technique of the tokamak poloidal magnetic field measurement by laser excited fluorescence of high energy Li beam are presented. The main diffeculties and its limitations are also mentioned.

The measurement of poloidal magnetic field in a Tokamak by the change of polarization of an electromagnetic wave

Presents a theoretical development of a proposal for measuring the current distribution in a Tokamak from the change of polarization of an electromagnetic wave traversing the plasma. A simple numerical method is given for calculating the resulting polarization and is illustrated by examples. The effect of the radial variation of toroidal field is discussed. The analytic solution in the form of a series is used to obtain some properties of the output polarization.

Mode conversion and harmonic generation at the upper hybrid layer in toroidal plasmas

The behavior of an extraordinary wave normally incident along a decreasing magnetic field gradient in a warm collisionless toroidal plasma is considered. Full electromagnetic Vlasov theory is used and the density gradient lengths are assumed to be much larger than all wavelengths. In the case for which the upper hybrid layer is accessible, it is shown that the incident wave almost completely converts into a backward Bernstein mode at the layer. The Bernstein mode then propagates until it is absorbed near the cyclotron layer. The second harmonic generated by this pump field is calculated, and its usefulness for poloidal magnetic field measurements in tokamaks is discussed. The theory is found to be in qualitative agreement with previous experimental results.

The polarization of an e.m. wave propagating in a plasma with magnetic shear. The measurement of poloidal magnetic field in a Tokamak

Studies the evolution of the polarization state of an electromagnetic wave propagating in a non-homogeneous plasma with a sheared magnetic field. A differential equation is given describing such evolution in the case of slowly varying plasma parameters. Assuming the model of a cold collisionless plasma one obtains an approximate solution giving the output polarization as a function of the input one and of the plasma parameters along the path. These results are applied to the case of a Tokamak; it is shown that in the some particular cases the output polarization is a relatively simple functional of the poloidal magnetic field. Therefore measurements of polarization can be used to determine the distribution of the poloidal field.

The polarization of an e.m. wave propagating in a plasma with magnetic shear the measurement of poloidal magnetic field in Tokamak

The polarization of an e.m. wave propagating in a plasma with magnetic shear the measurement of poloidal magnetic field in Tokamak