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.
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