Poloidal Magnetic Field Profile Research Articles

Reconstruction of poloidal magnetic field profiles in field-reversed configurations with machine learning in laser-driven ion-beam trace probe

The diagnostic of poloidal magnetic field ( ) in field-reversed configuration (FRC), promising for achieving efficient plasma confinement due to its high β, is a huge challenge because is small and reverses around the core region. The laser-driven ion-beam trace probe (LITP) has been proven to diagnose the profile in FRCs recently, whereas the existing iterative reconstruction approach cannot handle the measurement errors well. In this work, the machine learning approach, a fast-growing and powerful technology in automation and control, is applied to reconstruction in FRCs based on LITP principles and it has a better performance than the previous approach. The machine learning approach achieves a more accurate reconstruction of profile when 20% detector errors are considered, 15% fluctuation is introduced and the size of the detector is remarkably reduced. Therefore, machine learning could be a powerful support for LITP diagnosis of the magnetic field in magnetic confinement fusion devices.

The Heavy-Ion Beam Diagnostic of the ISTTOK Tokamak-Highlights and Recent Developments.

The unique arrangement of the heavy-ion beam diagnostic in ISTTOK enables one to measure the evolution of temperature, density and pressure-like profiles in normal and AC discharges. The fast chopping beam technique provided the possibility to reduce the noise on the measurements of the plasma pressure-like profile and for the precise control of the plasma column position in real time. The consequent improvements in S/N levels allowed the observation of the effects of runaway beam magnetic energy conversion into plasma local heating. In addition, it made it possible to follow the evolution of the quiescent plasma maintained during AC transitions when the plasma current is null. The use of a new operation mode in the cylindrical energy analyzer provided an improved resolution up to five times in determining the fluctuations of the plasma potential as compared to the normal operation mode. Such analyzer is extremely compact (250 mm × 250 mm × 120 mm) and provides a unique geometry in order to cover the whole plasma diameter. The detector configuration choice gives the possibility for the simultaneous measurements of plasma poloidal magnetic field, plasma pressure-like and plasma potential profiles together with their fluctuations.

Investigation of Dual Polarization Reflectometry for Determination of Plasma Density and Magnetic Field in a Spherical Tokamak

The paper deals with dual polarization reflectometry as a method for determination of plasma density and poloidal magnetic field profile in low aspect ratio tokamaks. A numerical iteration algorithm is developed and solutions for integral equations for O- and X-modes are presented. The numerical calculations confirm the use of the iterative algorithm for effective reconstruction of the density and magnetic field profile.

Status and characterization of the lithium beam diagnostic on DIII-D

The 30 keV lithium beam diagnostic on DIII-D is suitable to measure both the radial electron density and poloidal magnetic field profiles in the pedestal. The refurbished system features a new setup to measure the Doppler shift allowing accurate alignment of the spectral filters. The injector has been optimized to generate a stable lithium neutral beam with a current of I = 15-20 mA and a diameter of 1.9 ± 0.1 cm measured by beam imaging. The typical temporal resolution is Δt = 1-10 ms and the radial resolution of ΔR = 5 mm is given by the optical setup. A new analysis technique based on fast Fourier transform avoids systematic error contributions from the digital lock-in analysis and accounts intrinsically for background light correction. Latest upgrades and a detailed characterization of the system are presented. Proof-of-principle measurements of the poloidal magnetic field with a statistical error of typically 2% show a fair agreement with the predictions modeled with the Grad-Shafranov equilibrium solver EFIT within 4%.

First measurements of the multichannel far-infrared polarimeter on RFX-mod

A multichannel far-infrared (FIR) polarimeter has been recently installed and improved in RFX-mod to measure the Faraday rotation angle along vertical chords on a poloidal plasma section. Polarimetric data, associated with measurements of the electron density, permit the reconstruction of the poloidal magnetic field profile, Bθ. The entire diagnostic is described and its main sections outlined. Emphasis is placed on the work performed on the polarimeter to reduce the fluctuations affecting the old diagnostic signals and to increase the S/N ratio. In the recent installation of the polarimeter the optical line was more carefully designed and the mirror holders have been made in insulating material to avoid any interaction with the variable magnetic fields. Moreover all the optics have been fixed on an inertial granite platform. Examples of the first Faraday angle measurements performed on five chords are presented and discussed. The measured Faraday rotation angles are compared to a theoretically calculated value, based on the μ&p model, showing a good agreement between experimental and predicted data in the central region of the plasma. The comparison between experimental and predicted data is reported and discussed in the present work.

Internal magnetic field structure and parallel electric field profile evolution during the sawtooth cycle in MST

Temporal dynamics of the magnetic field, parallel current density, parallel electric field and safety factor profiles in the core of a high-temperature reversed-field pinch plasma are experimentally resolved. Measurements are realized using a high-speed polarimeter–interferometer diagnostic which is employed to simultaneously determine the electron density, toroidal current density and poloidal magnetic field profiles. Combined with external magnetic measurements, these data allow determination of the equilibrium profile dynamics during individual sawtooth magnetic relaxation events. At the sawtooth crash, the E∥ profile has a large positive peak on-axis, and a negative peak of slightly smaller value near the reversal surface demonstrating the need for a dynamo to sustain the plasma equilibrium.

High-speed three-wave polarimeter-interferometer diagnostic for Madison symmetric torus

A high-speed three-wave polarimeter-interferometer diagnostic has been developed on the Madison symmetric torus reversed field pinch to provide simultaneous measurement of electron density and poloidal magnetic field profile evolution. With increased digitizer bandwidth, polarimetry noise due to aliasing and cross-talk is minimized, and time response improved. System performance is demonstrated by measurements of equilibrium profile evolution during fast events such as the sawtooth crash and pellet injection.

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.

Improvement of confinement in tokamaks by weakening of poloidal magnetic field near boundary

Theory of turbulent equipartition and experiment indicate that density, pressure, and temperature profiles follow the poloidal magnetic field profile. Therefore, it is suggested to change the magnetic geometry between core and boundary by toroidal conductors and/or plasma current. As a result, density and temperature gradients will become steeper, and stored energy will be higher with low boundary plasma parameters. The suggested new mode of confinement may substantially simplify achieving of ignition.

The multichord far infrared polarimeter of the RFX experiment

A multichannel far infrared (FIR) polarimeter has been installed on the reversed field experiment (RFX) to measure the poloidal magnetic field profile, BΘ. The polarimeter uses a FIR laser (λ=118.8 μm) whose beam is transmitted to the machine via a 31 m long, nitrogen filled beam line. Faraday rotation measurements are reported for five of six parallel chords crossing the 1 m diameter RFX plasma on a poloidal cross section. The density profile is known from a CO2 interferometer. Low noise deuterated L-alaine doped triglycene sulphate (DLATGS) detectors are used. An accuracy of about 0.2° is obtained in the measured Faraday rotation angle, which is sufficient to estimate the characteristics of the BΘ profile. Removal of the noise induced by mechanical vibrations allowed acceptable measurements of Faraday rotation angles.

Internal magnetic field measurement in tokamak plasmas using a Zeeman polarimeter

In a tokamak plasma, the poloidal magnetic field profile closely depends on the current density profile. We can deduce the internal magnetic field from the analysis of circular polarization of the spectral lines emitted by the plasma. The theory of the measurement and a detailed design of the Zeeman polarimeter constructed to measure the poloidal field profile in the ADITYA tokamak are presented. The Fabry-Perot which we have employed in our design, with photodiode arrays followed by lock-in detection of the polarization signal, allows the measurement of the fractional circular polarization. In this system He-II line with wavelength 4686 A is adopted as the monitoring spectral line. The line emission used in the present measurement is not well localized in the plasma, necessiating the use of a spatial inversion procedure to obtain the local values of the field.

Study of toroidal current penetration during current ramp in JIPP T-IIU with fast response Zeeman polarimeter

Toroidal current penetration is studied in current ramp experiments on theJIPP T-IIU tokamak. The poloidal magnetic field profile in the peripheral region of a plasma (0.5 ⩽ r/a ⩽ 1.0) has been measured directly with a newly developed fast response Zeeman polarimeter. The experimental results indicate that a skin effect of the toroidal current density is clearly observed during both the current ramp-up (CRU) and the current ramp-down (CRD) experiments. The experimentally obtained toroidal current density profiles are well described by the profiles calculated on the assumption of neoclassical electrical resistivity, although the difference between the neoclassical electrical resistivity and the Spitzer electrical resistivity is not clearly distinguished because of the relatively collisional peripheral plasma region. Quasi-linear Δ' analysis of tearing modes for the measured current density profile is consistent with the time behaviour of the coherent magnetohydrodynamic (MHD) modes such as m = 4/n = 1 or m = 3/n = 1 (m is the poloidal mode number and n is the toroidal mode number) often observed during the CRU phase. An obvious enhancement of the current penetration by the MHD modes is not detected in this ramp-up experiment, where the relative amplitude of the poloidal field fluctuations is less than 0.8%.

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.

Measurement of motional Stark spectrum of a neutral beam emission in JIPP T-IIU plasma

Polarization measurements of the Balmer α line of a beam emission are performed using a newly developed four-polarizer optical system on the JIPP T-IIU tokamak. Poloidal magnetic field profile in a plasma is obtained successfully in current ramp-up experiments. Electron density dependence of the relative emissivity of σ and π components in the motional Stark spectra is also investigated experimentally. The results suggest that the relative emissivity is independent of the electron density in the range of 2 × 10 19-9 × 10 19 m −3.

Evolution of the poloidal magnetic field profile of the ISTTOK plasma followed by heavy ion beam probing

The evolution of the poloidal magnetic field profile during the initial current ramp-up in the ISTTOK tokamak plasma has been followed with a heavy ion beam diagnostic. The experimental results are compared with numerical simulations. The validity of the retrieved poloidal field profiles is discussed.

Multichannel far-infrared polarimeter system on TEXT-Upgrade

A far-infrared polarimeter diagnostic has been added to the UCLA multichord heterodyne interferometer system on TEXT-Upgrade. The polarimeter utilizes the phase technique of Rice [Rev. Sci. Instrum. 63, 5002 (1992)] 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. Due to the excellent port access and the polarimeter/interferometer scanning capability, the Faraday rotation profile can be measured across the entire plasma cross section.

The heavy ion beam diagnostic for the tokamak ISTTOK

In this paper we describe the heavy ion beam diagnostic for the tokamak ISTTOK, which has been designed to determine the temporal evolution of the plasma density, poloidal magnetic field and plasma potential profiles. This diagnostic makes use of a new type of high density caesium plasma source, a multiple cell detector and a fast data acquisition system. We describe the numerical code for trajectory and beam attenuation simulations, a method for the experimental determination of the poloidal field profile, the ion gun and the detection, control and data acquisition systems. Calibration tests and the first experimental results are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Crossed-sightline correlation of thermal ECE for remote measurement of absolute magnetic fields (abstract)

Many of the diagnostics that work on present magnetic fusion experiments will fail badly in the high radiation field of the burning fusion devices which are under conceptual design now. Two quantities of particular interest are the poloidal magnetic field profile versus minor radius, and radial measurements of β near the plasma center. Knowledge of the poloidal magnetic field is sufficient to infer the q profile and/or the current profile which is vital to understanding the MHD stability of the plasma and controlling the current profile. Knowledge of the ratio β of the plasma pressure to the magnetic field pressure radially resolved in the vicinity of the plasma center is also extremely useful for control and diagnosis of the plasma. A proposed new diagnostic uses crossed-sightline correlation of the thermal fluctuations in the electron cyclotron radiation emitted by the plasma to infer the local magnetic field very accurately. The field is measured at the crossing point of two sightlines into the plasma. The sightlines cross at small angles, typically 8° at the plasma center to about 18° at the plasma edge. By scanning the mirrors defining the two sightlines the absolute magnetic field across the plasma minor radius can be measured to an accuracy of better than 1 part in 103, with time resolution of the order of 10 ms per data point. This allows calculation of the q profile over most of the minor radius to an accuracy of better than 10% (given density and temperature measurements), and extremely accurate measurements of the plasma beta from r/a=0 to r/a=0.2.

Ion cyclotron range of frequencies stabilization of sawteeth on Tokamak Fusion Test Reactor

Results obtained from experiments utilizing high-power ion cyclotron range of frequencies (ICRF) heating to stabilize sawtooth oscillations on Tokamak Fusion Test Reactor (TFTR) [Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] are reviewed. The key observations include existence of a minimum ICRF power required to achieve stabilization, a dependence of the stabilization threshold on the relative size of the ICRF power deposition profile to the q=1 volume, and a peaking of the equilibrium pressure and current profiles during sawtooth-free phases of the discharges. In addition, preliminary measurements of the poloidal magnetic field profile indicate that q on axis decreases to a value of 0.55±0.15 after a sawtooth-stabilized period of ∼0.5 sec has transpired. The results are discussed in the context of theory, which suggests that the fast ions produced by the ICRF heating suppress sawteeth by stabilizing the m=1 magnetohydrodynamic (MHD) instabilities believed to be the trigger for the sawtooth oscillations. Though qualitative agreement is found between the observations and the theory, further refinement of the theory coupled with more accurate measurements of experimental profiles will be required in order to complete quantitative comparisons.

Determination of the poloidal magnetic field profiles in a tokamak by polarization spectroscopy of an impurity ion line.

Profiles of the poloidal magnetic field in Ohmically heated, sawtoothing discharges are determined in the Texas Experimental Tokamak (TEXT) from a precise measurement of the partial circular polarization of an impurity-ion line, TiXVII 3834 \AA{}. The sawtooth-period-averaged safety factor (inverse of the rotational transform) on the plasma axis is found to be close to 1.0, independent of its value on the plasma edge. The measurements indicate a decoupling of the current and temperature profiles and a flat current density profile in the center of the discharge.