International RFP Workshop
Madison, WI USA

February 28 - March 1, 2000


General Information
List of Participants
Schedule
Abstracts
Abstracts

 

Yasuyuki YAGI

Energy Technology Division

Electrotechnical Laboratory

1. GLOBAL CONFINEMENT PROPERTIES OF TPE-RX

Central electron temperature, density and ion temperature were measured for the first time in TPE-RX by using a Thomson scattering and a NPA. Global confinement properties estimated from these quantities will be presented. Temporal evolution of the confinement properties, plasma current dependence from 200 kA to 400 kA and pinch parameter dependence including a transient change like PPCD will be shown. The bottom line is that betap and tauE in the normal discharges are comparable with those of the normal discharges in MST and RFX.

2. UNDERSTANDING OF MODE-LOCKING PHENOMENA IN TPE-RX

Additional experiments and further data analyses on the mode-locking phenomena since the last WS at ETL will be presented to explain our recent understanding of the mode locking in TPE-RX. In particular, mode rotation in the non-locked discharges, threshold for the mode amplitude to wall-lock it and the difference of the global confinement properties between locked and non locked discharges will be shown. I will show that the halo current may not be the cause and that the Fitzpatrick's theory seems to be the case as has been expected.


Sergio ORTOLANI

Consorzio RFX Padova ITALY

RECENT RESULTS OF RFX

The main experimental results obtained in the last year will be shortly discussed. Among them, particular attention will be paid to the following results:

-The experimental studies and modeling of the interaction between externally produced magnetic fields and internal dynamo modes that, besides its intrinsic interest, allowed us to drive the locked mode rotation and to obtain high current pulses without carbon blooming phenomena and with a good control of hydrogen recycling at the wall;

-The results obtained by the oscillating poloidal current drive technique, that allows to induce an improved confinement repeatedly through a stationary RFP pulse. This gives further support to the possibility of achieving a non-transient confinement enhancement by applying continuous poloidal current drive;

-The discovery of the quasi-single helicity states, both transient and lasting for the whole pulse, characterized by the presence of a hot helical structure in the plasma core.


Antonio BUFFA

Consorzio RFX Padova ITALY

STATUS AND PERSPECTIVES OF RFX

The status of the RFX experiment after the fire on December 17, 1999 will be described.

The plans for reconstruction of the damaged plants and the guidelines for possible modifications of the toroidal field power supplies and for actions to widen the range of experimental scenarios will be presented.


Paolo ZANCA

Consorzio RFX Padova ITALY

NON-AXISYMMETRIC PERTURBATIONS OF THE VACUUM MAGNETIC SURFACES IN A RFP

The presence of a wide spectrum internally resonant resistive m=0 and m=1 MHD modes is a very common feature in reversed field pinch experiments. In most cases the modes are phase locked between themselves and also wall locked, so that they give rise to very severe localized plasma wall-interaction (PWI) phenomena. It is therefore particularly relevant to study the non-axisymmetric magnetic surface distortion related to the locked modes. We show that this can be done, starting from the magnetic probe arrays measurements, using an approximate analytical approach [1], under the assumption that the poloidal and toroidal harmonics of the magnetic quantities can be treated as perturbations.

We have thus found that the amplitude of the m=1 and m=0 non axisymmetric perturbation of the magnetic surfaces in RFX can be of the order of few centimeters and has a typical spatial profile related to the broad n spectrum of the modes. Such results account for the highly non axisymmetric PWI. We have also studied the empirical scaling law of the perturbation, which is found to decrease with plasma current.

[1] Zanca P., Martini S., "Reconstruction of the magnetic surfaces in a RFP with non-axisymmetric perturbations", Plasma Phys. Contr. Fus. Vol.41 n.10 (1999)


Marco VALISA

Consorzio RFX Padova ITALY

 HIGH DENSITY REGIMES IN RFX

In standard RFX discharges best performances are achieved at high density.

High density regimes are also a favorable condition for the creation of a radiative mantle to mitigate the peak heat load to the plasma facing components. Via neon seeding a complete poloidal (and toroidal) radiation belt can be generated and a high radiation fraction(Prad/Pin) with little Zeff increase can be obtained, thus proving the viability of the radiation mantle concept also for Reversed Field Pinches.

Confinement properties of Neon and also Xenon seeded plasmas do not depend specifically on the achieved level of the edge radiation, but are instead well correlated with the heating power level.

A well known lower operational boundary exists in the I/N parameter space. Such limit appears to be very similar and just above the Greenwald limit for Tokamaks (I/N=1 10-14 A/m).

At high current especially (above 0.9MA), when the lower threshold of 2 10-14 A/m is reached the discharge may terminate prematurely, in times of the order of 1-2 ms, in a way similar to the disruptive processes of Tokamaks.

The question then arises whether the apparent density limit is of extrinsic or intrinsic nature.


Roberto PIOVAN

Consorzio RFX Padova ITALY

ACTIVE CONTROL IN RFX

Active systems have been used in the last two years for control of tearing modes and field errors in RFX.

Active control of the dynamics of MHD modes is a main engineering issues in RFP and tokamak. In RFX a m=0, n=1 rotating magnetic field has been generated by the toroidal field winding. This rotating field is able to induce a continuous synchronous rotation or a stepping rotation of the wall locked modes. Open loop control has been extensively exploited; an open further improvement is the feedback control of rotation by varying the speed rotation of the forcing field. Field errors at the poloidal and equatorial cuts of the RFX conductive shell are relevant in the wall-locking of the Localized Helical Deformation. For this reason, active control of field errors at the two poloidal gaps of RFX was implemented, at one gap in spring 1998, at the other one 1 year later. 11 local correction coils were mounted across each gap, each coil being fed by means of an independent fast DC/DC power supply.

The quality of the field error correction achieved at the gaps suggested to extend this approach also to the portholes on the outer equatorial gap of RFX. 22 local correction coils have been designed to be mounted on the equatorial portholes, fed by 22 independent fast DC/DC power supplies. This introduces a new possibility to induce mode rotation in the presence of a thick shell by applying a driving rotating field through the equatorial local control coils.

The talk presents first an overview of the technology used in the new control systems of RFX. It then reports the experimental results achieved in the control of tearing modes and field errors.


Vanni ANTONI

Consorzio RFX Padova ITALY

EDGE BIASING IN RFX

Plasma flow velocity and its shear have been increased at the edge during biasing experiments with electrodes in RFX. The electrodes were inserted up to 7 cm and the plasma current was below 400 kA. A reduction of the electrostatic particle and energy fluxes has been measured accompanied by an increase of the plasma density. The transport reduction has been shown to be lead by a change in phase between plasma potential and density fluctuations. During biasing experiments the impurity content has been monitored showing and increase in the boron content which has been taken into account in the particle balance. The density increase, numerically simulated by 0 and 1 dimensional models, indicates an increase of the particle confinement time.

In order to interpret the experimental values of the toroidal flow velocity and its modification due to the electrode operation model, a momentum balance equation has been applied. Anomalous perpendicular viscosity due to anomalous particle diffusion has been assumed and the equations have been written in cylindrical coordinates. Preliminary results of a numerical code based on this model show a good agreement with the experimental data without biasing and accounts for the toroidal velocity variations induced by the electrode current.


Susanna CAPPELLO

Consorzio RFX Padova ITALY

SINGLE AND MULTIPLE HELICITY STATES REVISITED IN 3D MHD DYNAMICAL SIMULATIONS AND RELATED MAGNETIC DIFFUSION PROPERTIES.

Recent observations (1996-1999) in different experiments, and especially in RFX, have motivated renewed efforts in studying how the 3D MHD turbulent dynamics switches into a 2D laminar regime characterized by Single (SH) or Quasi-Single (QSH) Helicity spectrum.

In this talk we will present the status of our research in this area. The dependence of the dynamical bifurcation on the Lundquist (tr/tA), Prandtl (tr/tV) and Q parameters are considered: results obtained till now show that the Hartmann number H=tA/(tr tV)1/2 is a good control parameter. Magnetic diffusion properties of these configurations are also discussed. In particular, it will be shown that chaos healing occurs when the dominant mode of QSH states reaches amplitudes such that its separatrix is expelled.


Roberto PACCANELLA

Consorzio RFX Padova ITALY

RECENT RESULTS ABOUT HELICAL STATES IN RFX

In this talk we report recent experimental and theoretical results about the coherent helical structures which are observed in the RFX plasma core. They form as a result of transitions to states where the n-spectrum of the m=1 modes is dominated by a single (1,nmax) geometrical helicity. These helical states are dubbed Quasi Single Helicity (QSH) states and their existence is consistent with the predictions of theory and numerical simulations.

We describe their magnetic and thermal properties, which have been diagnosed with a variety of spatially resolved diagnostics We show in particular that the core helical structure is hotter than the surrounding plasma.

The effect of aspect ratio on the onset of helical states will also be discussed.

The results of a detailed calculation of helically symmetric ohmic RFP equilibria will also be discussed, together with some considerations on the effect of the magnetic boundary conditions for the onset of QSH states.


Lionello MARRELLI

Consorzio RFX Padova ITALY

CORE ELECTROSTATIC FLUCTUATIONS IN RFX

We report the measurements of core electron temperature fluctuations in RFX.

They have been detected by applying the Singular Spectrum Analysis (SSA), a statistical technique used for years to analyze short and noisy time series in various research fields, to measurements performed in the RFX experiment with a multifilter soft x-ray spectrometer .

SSA is mainly a single time series version of the Principal Component Analysis. We use it in conjunction with non-parametric statistical tests and we can filter out the white noise from the experimental series and, therefore, isolate the trend from the fluctuating part of the filtered time series. The measurements have been done in various plasma conditions, and in particular during Pulsed Poloidal Current Drive experiments. During these experiments we observe in fact a strong and controlled reduction of the magnetic fluctuations.

Core temperature fluctuations are typically not larger than few percents, as those in the electron density. We find that they are well correlated both qualitatively and quantitatively with magnetic fluctuations. In particular we observe that core electrostatic fluctuations decrease during PPCD. Moreover the level of electron temperature fluctuations can be accounted for by considering them as originated by magnetic fluctuations. We propose an approximate estimate of the electrostatic heat flux in the RFP core and we conclude that electrostatic fluctuations have a small impact on the global transport (at least for the plasma analyzed up to now).


 

Zensho YOSHIDA

University of Tokyo, Japan

DOUBLE BELTRAMI FIELD AND GENERALIZED RELAXED STATE - H-MODE BOUNDARY LAYER AND ADVANCED HIGH-BETA CONCEPT

The Beltrami condition, demanding alignment of vorticity and flow, represents a relaxed equilibrium state in a vortex dynamics system. In the two-fluid MHD theory, the simultaneous Beltrami conditions both on the electron and ion equations of motion yield a double Beltrami equation that leads to magnetic field and flow velocity structures far richer than the ones contained in the Taylor relaxed states. Applying the theory for the H-mode boundary layer, we obtain a self-consistent two-fluid equilibrium where the magnetic field, shear flow and radial electric field make a natural coupling. The theoretical estimates of the flow velocity and the electric field are in good agreement with experimental observations in tokamaks. Extending the magnitude of the flow (and the plasma potential), we can generate a high-beta equilibrium. The Proto-RT device can produce toroidal non-neutral plasmas. The experiment is aimed at studies on various shear flow effects that include the challenge to an innovative high-beta confinement.


SADAO MASAMUNE

Kyoto Institute of Technology, Japan

ROTATING RESONANT HELICAL FIELD EXPERIMENT IN THE STE-2 RFP

Toroidally rotating internally resonant helical field (RHF) has been applied to the STE-2 RFP (a/R=0.1m/0.4m, Ip >50kA, discharge duration <1ms), with a view to driving mode and/or plasma rotation. At present, the effective duration of the RHF is restricted to be less than 0.3-0.4ms by the capabilities of helical field power supplies. A toroidal array of Br coils has revealed that almost stationary m/n=1/8 magnetic disturbance is dominant without the RHF. When the RHF is applied with relative amplitude of higher than 0.4%, the magnetic fluctuation is observed to rotate with a velocity slightly lower than the phase velocity of the RHF. Improvement of the power supplies is under way.


Kiyoshi HAYASE

ETL, Japan

SUMMARY OF DIVERTOR EXPERIMENT IN TPE-2M

Characteristic behavior of scrape-off plasma in the divertor region and stability consideration will be given.


S. SHIINA

NIhon Unviversity, Japan

BOTSTRAP CURRENT IN LOW ASPECT RATIO REVERSED FIELD PINCH

We are concerning with low aspect ration RFP equilibria focusing on bootstrap current fraction which affects strongly the economical attractiveness of steady state powe plants, in which partially relaxed state model RFP configuration reasonably close to a stable minimum state at finite beta is considered.


Jenny-Ann MALMBERG

Division of Fusion Plasma Physics, Alfven Laboratory, Royal Institute of Technology, SE-100 44, Stockholm, Sweden

A COMPARISON OF LOCKED MODES IN TWO REVERSED _ELD PINCH DEVICES

The toroidally localized, phase- and wall-locked magnetic structure, known as the locked mode phe- nomena, has frequently been observed in many RFP plasmas. In this study, we have compared the locked mode phenomena in the medium-sized RFP, EXTRAP-T2 (R/a = 1.24/0.183 m) and the large-sized RFP experiment, TPE-RX (R/a = 1.72/0.45 m). TPE-RX has a close-_tting, multi-layered conducting shell system (b/a=1.08 for the thin shell) whereas EXTRAP-T2 has a resistive shell (b/a = 1.09) with a penetration time of 1.5 ms which is about ten times shorter than the pulse duration. In EXTRAP-T2, magnetic structures, which correspond to the phase locking of multiple toroidal modes with the same helicity as the internal magnetic field are frequently observed. They are also known as the 'slinky' in the original device OHTE. These stuctures lock to the wall, thus forming a clear locked mode structure. In TPE-RX, modes frequently lock to the wall, but unlocked discharges also exist. Statistics of the toroidal distribution of the wall locked modes shows that field errors play a role in the wall locking process in EXTRAP-T2. In TPE-RX, field errors generated at the poloidal shell gap are often related to the wall locking phenomena [1]. Furthermore, the shape of the m=0 and m=1 contribution to the locked mode has been studied in the two devices using a last closed flux surface reconstruction. Locked modes in EXTRAP-T2 have a more narrow toroidal extent than those in TPE-RX, as can be expected from the higher aspect ratio of EXTRAP-T2. A dependence of the magnetic fluctutations on the plasma current and density has been found in both machines. The amplitude of the normalized radial magnetic field increases slightly with increasing current and density in EXTRAP-T2 whereas a decrease with increasing current can be observed in TPE- RX [1]. However, studies of the degree of phase alignment of the modes involved in the locked mode shows that the probability of locking increases with increasing current and density. Figure 1: Reconstruction of the last closed flux surface in EXTRAP-T2 [1] Yagi, Y., et al., Phys. of Plasmas 6 (1999) 3824.


J.R. Drake1 and A. Hedqvist

1Alfvén Laboratory, KTH, Stockholm, Sweden

2Department of Atomic and Molecular Physics, KTH, Stockholm, Sweden

Association EURATOM/NFR

Spitzer and non-Spitzer loop voltage data from T2.

In a reversed-field pinch (RFP) the applied loop voltage, VL, and toroidal current determine the total power input. On the one hand, a toroidal loop voltage, VK, is required to sustain a constant total magnetic helicity in the plasma volume. On the other hand a "Spitzer" loop voltage, VS, can also be derived from the total Ohmic power input in the plasma volume based on the Spitzer parallel resistivity. These voltages can be compared to give insight into the efficiency of the MHD-fluctuation driven self-organisation process and the related MHD dynamo. A critical question in RFP research concerns whether the applied loop voltage is just that required to sustain helicity.

A loop voltage data base is being evaluated for the EXTRAP T2 reversed-field pinch experiment in order to study possible loop voltage anomalies. In the presentation, the three loop voltages, VL, VK and VS are compared for a number of cases and impurity influx conditions.

 

 

 


Alex HANSEN

University of Wisconsin - Madison, USA

 Electromagnetic Torques Exerted on Single and Multiple Modes in the Reversed-field Pinch

 

In the MST reversed-field pinch there are typically several dominant tearing modes (m=1, n=5-9) present which rotate in a phase-locked fashion. In order to provide rotation control via placing an electromagnetic torque on one of these modes, we have constructed a system that produces a magnetic perturbation that is resonant with the (m=1,n=6) mode. Even though this perturbation is only resonant with one mode, all of the dominant modes respond to it. This is consistent with theory, which predicts that there are nonlinear torques between the modes, and therefore an external torque on one mode could result in an effect on all of them. As an indicator of this nonlinear torque, we have measured the triple product of the dominant tearing modes, and have found that it correlates with the mode and plasma rotation. We have also estimated the nonlinear torque using the results of the static magnetic perturbation experiments.


Neal CROCKER

University of Wisconsin - Madison, USA

Measurement of Current Fluctuations and Charge Transport During Reconnection

The Reversed Field Pinch (RFP) plasma configuration normally exhibits strong magnetic field fluctuations due to resistive tearing modes. As a result, field line reconnection and associated phenomena such as magnetic islands and field line stochasticity play an important role in the RFP. A hallmark of magnetic reconnection is that the current density is expected to become large (singular in the limit of vanishing electrical resistivity) at the location of the reconnection. One of the regions in an RFP where reconnection is expected to occur is the reversal surface, in the plasma edge. The current "sheet" associated with this reconnection has been measured in the Madison Symmetric Torus (MST) with the aid of a pair of insertable Rogowskiis. Various data analysis techniques have been employed to elicit spatial information about the parallel current density perturbations observed in MST's edge. These perturbations show a spatial structure consistent with reconnection at the reversal surface. Somewhat surprisingly, they show a broad radial structure, comparable to the expected island width for reversal surface resonant tearing modes in MST, rather than a highly radially localized "sheet" structure. Using a radial b-dot coil built into one of the insertable Rogowskii probes, it was possible to measure the magnetic fluctuation driven charge transport due to these current density perturbations. Magnetic fluctuation driven charge transport across a flux surface is proportional to the flux surface correlation of the parallel current density perturbation and the radial (surface normal) component of the magnetic field perturbation. The measured charge transport is small. This is in agreement with theoretical expectation , although it is found that the reason for the small value of charge transport varies with radius.


Jan SCHEFFEL

RFP CONFINEMENT AND FLUCTUATION SCALINGS

Jan Scheffel and Dalton D. Schnack*

Fusion Plasma Physics (Association EURATOM-NFR)

Alfvén Laboratory, Royal Institute of Technology

www.fusion.kth.se

* Science Applications International Corporation, San Diego, CA, USA

A series of high resolution, 3-D, resistive MHD numerical simulations of the

reversed-field pinch (RFP) are performed to obtain scaling laws for poloidal beta and

energy confinement at Lundquist numbers approaching 10 6 . Optimum plasma

conditions are attained by taking the transport coefficients to be classical, and

ignoring radiation losses and resistive wall effects. We find, for Q = 1.8, the scalings

poloidal beta bq µ (I/N) - 0.40 I - 0.40

, energy confinement time tE µ (I/N)0.34 I 0.34 , and on-axis

temperature T(0) µ (I/N)0.56 I 0.56

.

Experimental results at T2, RFX and MST agree well with the above numerical

results.

Magnetic fluctuations are found to decrease only weakly with Lundquist

number; we find < br 2

> µ S - 0.28

. Stochastic core field lines will consequently be

maintained also at high currents and temperatures. Similar fluctuation scaling results

have earlier been found in zero-pressure numerical simulations and experimentally

at RFX and MST.

The computer simulations were performed at low aspect ratio (1.25) to save

memory and computing time. We show, however, that the results do not depend

sensitively on aspect ratio. In fact, given boundary conditions, only two parameters

are required to characterize each time-averaged state.

The weak confinement scalings lead us to conclude that the conventional,

ohmically heated RFP does not appear to extrapolate towards reactor conditions

within the present model. The results thus emphasize the importance of

experimentally demonstrating control of the RFP current profile in order to

minimize the dynamo fluctuations, reduce the corresponding thermal losses, and

improve energy confinement.