RFX Reversed Field Pinch Experiment Research Articles

Active MHD control experiments in RFX-mod

The RFX reversed field pinch experiment has been modified (RFX-mod) to address specific issues of active control of MHD instabilities. A thin shell (τBv∼50 ms) has replaced the old thick one (τBv∼500 ms) and 192 (4 poloidal × 48 toroidal) independently powered saddle coils surround the thin shell forming a cage completely covering the torus. This paper reports the results obtained during the first year of operation. The system has been used with various control scenarios including experiments on local radial field cancellation over the entire torus surface to mimic an ideal wall (‘virtual shell’) and on single and multiple mode feedback control. Successful virtual shell operation has been achieved leading to: a 3-fold increase in pulse length and well controlled 300 ms pulses(∼6 shell times) up to ∼1 MA plasma current; one order of magnitude reduction of the dominant radial field perturbations at the plasma edge and correspondingly 100% increase in global energy confinement time. Robust feedback stabilization of resistive wall modes has been demonstrated in conditions where rotation does not play a role and multiple unstable modes are present.

Impurity transport during pulsed poloidal current drive experiment in the reversed field pinch experiment RFX

The characteristics of the impurity transport in the RFX reversed field pinch experiment during enhanced confinement regimes, such as those obtained performing pulsed poloidal current drive and oscillating poloidal current drive, have been investigated. It is found that during the enhanced confinement period, the ratio between the Ly α line of C VI and the resonance line of C V increases systematically well beyond the expected enhancement due to the temperature improvement.Experimental data have been reproduced by means of a one-dimensional, time dependent, collisional–radiative impurity transport code, which simulates several C and O lines, continuum emission and radiated power. In standard RFX plasmas the typical diffusion coefficient is ∼20 m2 s−1 at the plasma centre decreasing to ∼1 m2 s−1 at the edge. Best simulations of the spectroscopic data during the enhancement confinement phases are obtained by expanding the low diffusion region at the edge from r/a = 0.9 to r/a = 0.8±0.05 and shrinking the neutral density profile at the edge.

Effects of pulsed poloidal current drive on the edge region of areversed field pinch plasma

The plasma edge region of the RFX reversed field pinch experiment has beenstudied during pulsed poloidal current drive (PPCD) operation at low plasmacurrent. The edge region is largely affected during PPCD, and evidence ofimportant modifications of electric and magnetic field and ofE×B drift velocity are reported. A substantialdecrease of electrostatic transport has been observed during the PPCD phaseand this effect is ascribed to a combination of displacement and plasma columnshrinking.

Study of the scaling of magnetic fluctuations in the RFX reversed fieldpinch1

The scaling of magnetic fluctuations with the Lundquist number (S) isstudied in the RFX reversed field pinch (RFP) device for different operationalregimes. For the first time in a RFP experiment a complete set of diagnosticsto measure all the quantities needed to estimate S is available. Due to thehigh plasma currents (up to 1.1 MA) obtained in RFX, a broad range of Svalues is available for scaling. The results are compared with numericalMHD simulations (SPECYL code) providing a reasonable agreement(αexp = -0.18±0.02, αSPECYL = -0.22). Themodification to the expected scaling law due to viscous effects is alsodiscussed.We also perform a comparison of the amplitude of magnetic fluctuationsin discharges with different characteristics: standard, pulsedpoloidal current drive (PPCD), quasi single helicity and rotating lockedmode induced by an external rotating toroidal field modulation. Areduced level of fluctuations is obtained during PPCD operations, althoughthe dependence on S is the same. Concerning confinement time, at fixedI/N≈3×1014 A m, a favourable scaling with S is found. Itsinterpretation in terms of the simple Rechester-Rosenbluth model is brieflydiscussed.

Electrostatic transport reduction induced by flow shear modification in a reversed field pinch plasma

The E × B velocity and its shear have been modified in the edge of the RFX reversed field pinch experiment by a biasing experiment performed with electrodes inserted into the plasma. Causality between an increase in the E × B velocity shear and a decrease of the particle flux driven by electrostatic turbulence has been observed. The decrease of the particle flux has been found to be mainly due to a change of the relative phase between density and plasma potential fluctuations. The results confirm the role of sheared flow in transport suppression in reversed field pinches and show remarkable similarities with those found in other magnetic configurations.

Edge temperature and density measurements with a thermal helium beam in the RFX reversed field pinch

The electron temperature and density at the edge plasma of the reversed field pinch experiment RFX are measured from the intensity ratio of selected spectral lines emitted by an atomic thermal helium beam injected into the plasma discharge. This technique, already performed in several Tokamaks, has been, for the first time, applied to a reversed field pinch to extend the edge density and temperature measurements to high-current discharges where intrusive probes cannot withstand the high-power fluxes. The temperature at the edge increases with the plasma current and decreases with the plasma average electron density. These edge values have been used to analyse the behaviour of the impurity influxes. The influence of the edge temperature and density on the efficiency of carbon penetration has been studied by means of a Monte Carlo simulation of the plasma behaviour at the edge.

Experimental and simulated neon spectra in the 10-nm wavelength region from tokamak and reversed field pinch plasmas

Experimental neon spectra (in the 10-nm region), from the tokamak Tore Supra and the reversed field pinch experiment RFX, have been simulated. The spectra include lines from three neon ionization states, namely Ne(7+), Ne(6+), and Ne(5+) ions. Collisional radiative models have been built for these three Ne ions, considering electron collisional excitation and radiative decay as populating processes of the excited states. These models give photon emission coefficients for the emitted lines at electron density and temperature values corresponding to the experimental situations. Impurity modelling is performed using a one-dimensional impurity transport code, calculating the steady-state radial distribution of the Ne ions. The Ne line brightnesses are evaluated in a post-process subroutine and simulated spectra are obtained. The parts of the spectra corresponding to a single ionization state do not depend on the experimental conditions and show good agreement with the simulated single ionization state spectra. On the other hand, the superposition of the three spectra depends on the experimental conditions, as a consequence of the fact that the ion charge distribution depends not only on the radial profiles of the electron density and temperature, but also of the impurity transport coefficients. Simulations of the Ne spectra (including transport) give confidence in the atomic physics calculations; moreover, they allow the determination of the transport coefficients in the plasma region emitting the considered ionization states.

Energy flux driven by electrostatic turbulence in the RFX edge plasma

The electron energy flux driven by electrostatic turbulence has been measured for the first time in the edge region of the RFX reversed field pinch experiment using triple Langmuir probes. The energy flux has been found to be almost entirely convective. The total energy loss due to electrostatic turbulence has been estimated to account for 30% of the overall transport losses.

Erosion, redeposition and boronization lifetime in RFX

In this paper the boronization lifetime in the reversed field pinch experiment RFX is discussed. It is found that the redeposition of carbon-rich layers, which grow during plasma exposure, cover the boron containing layers preventing the getter effect through formation of stable boron oxides. Enhanced localized erosion on graphite tiles, which constitute the first wall of the experiment, due to magnetic disturbances or field errors is identified as the source of carbon which codeposits together with hydrogen over the boronized layers. Samples covered with different boron containing layers have been exposed to the plasma, analyzed by surface techniques and compared. The redeposition process is confirmed to be the main cause determining the boronization lifetime.

Improved performance of the Thomson scattering system in RFX

A Thomson scattering diagnostic is used in the RFX reversed field pinch experiment to measure single pulse, multipoint electron temperature (Te) profiles. Recent significant improvements have provided reliable and routine operation of the diagnostic. Custom developed CAMAC charge analog-to-digital converters have been introduced, which sample the plasma light 170 ns after the laser light and have greater sensitivity. The spectrometer configuration and the calibration procedure have been optimized. These upgrades have finally led to better fitted scattering spectra and more regular estimates of Te profiles, allowing the measurement of temperatures as low as 20 eV. The introduction of a second spectrometer has recently added a further ten positions to the original ten point profile, resolving the often steep Te edge gradient. On axis values are in good agreement with other Te measurements from SXR diagnostics (either SiLi and double filter method). Te profiles are typically rather flat, but some dependence on the plasma parameters, like density and pinch parameter, has been drawn. Significant Te profile modifications have been observed during transient events like pulsed poloidal current drive and pellet injection. The diagnostic will be completed soon, allowing production of 27 point profiles, with the introduction of the third spectrometer. Double pulse operation of the laser is in progress.

A fast moving diagnostic system on the RFX experiment

A fast rotating graphite arm equipped with two Langmuir probes has been developed to investigate the temperature, density, and plasma potential profiles at the edge in the reversed field pinch experiment RFX. The system, driven by an electromagnetic torque, allows a 5 cm radial insertion in about 20 ms with a magnetic field of 0.3 T. The motion of the arm takes place on a plane having an angle of 45° with respect to the direction of the poloidal magnetic field. The probe collecting area is constant during the movement. The initial and final positions of the graphite arm are monitored by electrical contacts. To reconstruct the motion of the equipment, an equivalent lumped-constant network has been developed. The equivalent resistance and inductance are measured before each discharge. By this system radially resolved measurements of edge plasma parameters in a single RFX plasma discharge have been obtained.

A new linear amplifier for the RFX electron energy analyzer

A simple low cost novel linear amplifier has been developed for the electron energy analyzer employed on the reversed field pinch experiment RFX. The amplifier is designed to provide a 1 kV output voltage from a 10 V input. The current rating is ±50 mA. It utilizes a configuration of four blocks in series, each of them with an output totem-pole configuration. The rise and fall times are of a few milliseconds. A fast protection against output short circuit has been implemented. The amplifier has been used in various experimental campaigns on RFX and multiple time resolved measurements of the edge electron energy distribution in a single plasma discharge have been obtained.

Electrostatic Turbulence and Transport in the Velocity Shear Layer of a Reversed Field Pinch Plasma

The electrostatic turbulence in the edge plasma of the RFX reversed field pinch experiment has been studied using Langmuir probes and a homodyne reflectometer. The radial particle flux driven by electrostatic fluctuations has been measured in the region where a double $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ velocity shear layer occurs. It is found that almost 100% of the particle flux at the edge is driven by electrostatic fluctuations. The naturally occurring $\mathbf{E}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ velocity shear has been shown to be close to that required by the Biglari-Diamond-Terry criterion for turbulence radial decorrelation.

Thomson scattering measurements in the RFX reversed field pinch

The first systematic measurements of the electron temperature (Te) spatial profile have been obtained in the reversed field pinch experiment RFX with a single pulse Thomson scattering (TS) diagnostic. Scattered light from a ruby laser pulse (E⩽15 J, Δt=30 ns) is collected through three objectives from 10 positions along a diameter in the plasma equatorial plane, with a spatial resolution of 2.5 cm. Plasma discharges with current in the range 700–900 kA have been investigated finding evidence of a quite flat Te profile. Data dispersion significantly greater than experimental uncertainties provides an indication of remarkable plasma fluctuations. Results are in good agreement with Te measurements from other single chord spectroscopic diagnostics (SiLi detector and SXR double filter), showing a reliable operation down to an electron density ne=3×1019 m−3. Integration of this apparatus with a ND:YLF laser system for multipulse Thomson scattering measurements, sharing the same input optics, is under way.

Upgrade of the RFX CO2 interferometer using in-vessel optics for extended edge resolution

The extension to 16 chords of the eight-chord two-color MIR interferometer of the RFX reversed field pinch experiment (R=2 m, a=0.46 m) is presented. The new measurements are done through a 100 mm equatorial port with beams which are reflected back by corner cube metal mirrors mounted inside the vacuum vessel. Moreover, for four of the new chords, the beams are also steered by copper mirrors mounted inside the access pipe, whose edges are at 20–30 mm from the plasma surface. In this way measurements along lines of sight that are at a mean distance of 67 and 21 mm from the vessel first wall are possible. This results in a significant improvement of the density profile reconstruction towards the plasma edge. All the optics inside the vacuum vessel are fixed and the beam layout and focusing have been optimised to take in the misalignments due to the vacuum vessel thermal deformations. The system proved to work well despite mechanical vibrations as large as ≈1.5 mm at frequencies of ≈200 Hz. Stability of alignment and survival of the optics in the proximity of the plasma is also satisfactory.

Plasma flow in the outer region of the RFX reversed field pinch experiment

Langmuir probes and collector samples have been exposed to the edge plasma in the RFX reversed field pinch experiment to investigate the plasma flow. Parallel and perpendicular drifts have been observed for primary ions and carbon impurities. Two models valid for a magnetized and an unmagnetized plasma have been applied and compared in order to derive the parallel and perpendicular drift velocities. The perpendicular flow is accounted for by the radial electric field and shows a velocity shear, consistent with the change in sign of the radial electric field in the edge region. The parallel flow increases going into the plasma

Electron temperature measurements with high time resolution in RFX

Core electron temperature measurements have been performed in the large, high-current RFX reversed field pinch experiment (a = 0.46 m, R = 2 m, target plasma current = 2 MA) with the double filter technique. With this technique the soft x-ray emission is measured by two Si detectors, covered by different Be-filters, and viewing the same region of the plasma. By choosing the filters in such a way that only the emission from the continuum part of the spectrum is measured, the ratio of the two signals is a function of the highest electron temperature along the line of sight of the detectors. With this diagnostic, the electron temperature has been measured in RFX with a bandwidth of approximately 2 kHz. This allows the direct measurement, for the first time in a RFP on a single shot basis, of coherent oscillations, associated with relaxation events and showing under some conditions a sawtooth-like character. The measurements of electron temperature obtained with this method are in good agreement with those performed with other diagnostics.

Stochastic magnetic and ambipolar electric fields in the plasma edge region of RFX

The edge region of the reversed field pinch experiment RFX has been investigated with Langmuir and calorimetric probes. The energy flux measurements reveal a spatial structure that is consistent with the presence of a superthermal tail in the energy distribution function of the electrons, as expected according to the kinetic dynamo theory (KDT). In the framework of this model, the value of the magnetic field line diffusion coefficient in the edge region has been derived. The radial electric field obtained from the plasma potential gradient is opposite in sign to the ambipolar electric field expected in a stochastic magnetic field. The discrepancy is discussed in terms of particle recycling at the wall

Development and tests of a simple multifoil spectrometer for highly time-resolved line intensity measurements in the RFX experiment

A simple and cost-effective spectrometer dedicated to line-intensity monitoring in the soft X-ray range has been realized, installed and tested in the RFX reversed field pinch experiment for the study of magnetically confined fusion plasmas. The technique of photon energy selection by means of multilayer material filters has been applied to the instrument described here, which, together with the associated electronics, has been designed in an original compact version for RFX. This present version allows on-line monitoring with good time resolution (10 kHz bandwidth) of line emission from carbon and oxygen in their H- and He-line states (which fall within the soft X-ray energy range), but, with modifications of the set of material filters, different wavelength ranges can be chosen and then other lines can be monitored. A detailed description of the instrument is given here and the data analysis technique is presented. Particular attention is dedicated to a critical analysis of the technique and to the discussion of the experimental errors which may be associated with this measurement; new results on the estimate of systematic errors and experimental improvements to eliminate the problem of continuum radiation are presented. As a demonstration of the proper operation of the instrument, data taken under a wide range of conditions in 0.5-0.6 MA RFX plasmas are shown and discussed. Good agreement of the measurements with other spectroscopic diagnostics already operating on the machine is found; by exploiting the high time resolution of the instrument, interesting results concerning the RFP configuration`s dynamics are obtained. Based on this prototype, a multichord diagnostic has been designed for imaging the impurity emission radial profiles.

The summary database system for the RFX reversed field pinch experiment

The RFX Summary Database System contains the most important experimental data, and provides tools to collect and insert the data and to interactively study their dependence. It was developed to allow a fast search for shots with given characteristics for follow-up, detailed shot analysis, and study of the scaling laws of the RFX plasmas. The database uses the Rdb/VMS relational database and is accessed using Structured Query Language syntax. A program automatically inserts the most important data for each shot while the users can access the database with their own programs using high-level routines. An Interactive Data Language interface to the database allows the users to interactively insert and extract data from the database, combine, or modify the data as they want, plot and fit the extracted or computed data, and perform general multiple regression analysis.