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1936 Phys. Plasmas, Vol. 8, No. 5, May 2001 Egedal et al.
FIG. 1. Poloidal cross section of VTF. The solid contour
lines represent the poloidal magnetic field strength.
The dashed contour lines correspond to constant levels
of the poloidal magnetic flux, , which coincide with
magnetic field lines.
and the toroidal field B , it is possible to modify the shape
and location of the ECRH-ring. Highly reproducible target
plasmas are produced during shots lasting up to 100 ms and
taken typically every 2 minutes.
The snapshots in time of the steady state density profiles
shown in Fig. 2 were measured by a 16-tip Langmuir probe
moved though the plasma 29 plasma shots per profile. For
each magnetic configuration the location of the ECRH-ring
is indicated by the red lines. The plasma is confined partly by
mirror effects 12 and partly electrostatically.
The profile in Fig. 2A was obtained in the absence of
toroidal magnetic field. In this case the magnetic moment is
not conserved for the particles on field lines passing through
the region of the absolute magnetic null. As a consequence,
the measured density is reduced along the separatrix. In Fig.
2B a small toroidal component (B 30 mT is added, improving
the confinement of the particles on the separatrix.
The 1/R dependence of the toroidal magnetic field
strength makes it possible to place the ECRH-ring to the
right of the X point. The density profiles obtained in two
such examples are given in Figs. 2C and 2D.
The profiles in Fig. 3 were obtained by sweeping the
bias on the multiple Langmuir probe tips. Electron temperatures
in the range of 20 eV are typical for the VTF plasmas.
The plasma potential is, for most of the possible configurations,
in the range of 60 to 100 V.
III. RECONNECTION DRIVE SYSTEM AND RESPONSE
OF THE PLASMA
For driving magnetic reconnection VTF is equipped with
a 1 mH ohmic coil system fed through a semiconductor
switch by a1mFcapacitor. 12 The capacitor is charged to 3
kV. As shown in Fig. 4, the LC resonance circuit produces
sinusoidal current pulses inducing electric field pulses of 10
V/m in the center of the vacuum vessel.
The density contours in Fig. 5A represent another example
where the plasma is produced entirely to the right of
the X line. According to ideal MHD it should not be possible
for this plasma to cross the separatrix into the quadrants to
the left of the X line. However, as seen in Figs. 5B and
5C, during the electric field pulses the plasma drifts unhindered
across the separatrix. The direction of the drifts in the
plasma agrees with the EB drift. The measurements described
in the next section show that the drift velocity is
given by E /B cusp .
During the reconnection experiments, the time evolution
of the magnetic flux over the plasma cross section is measured
by a multiple magnetic probe. This movable probe has
30 pick-up coils each with a total area of 0.5 cm 2 for each
of the three direction in space, providing a spatial resolution
of 1 cm in the radial direction. Due to the reproducibility of
the VTF plasma the resolution in the vertical direction can be
made arbitrarily fine by moving the probe in between plasma
shots. The signals are integrated ( int 0.5 ms and amplified
gain100 before digitization.
These magnetic measurements reproduce the magnetic
fields obtained in the absence of plasma. In turn, these measurements
agree with the fields calculated on the basis of the
currents applied in the VTF magnetic coils system. Hence,
we see no evidence of currents in the plasma. Taking into
account the sensitivity of the magnetic probes, it can be concluded
that the total toroidal current induced in the plasma is
less than 5 A. If classical Spitzer resistivity were valid for the
experimental configuration, a current of the order of 10 kA
should have been observed.
Significant plasma currents are only observed in profiles
where the maximal poloidal magnetic field in the cross section
is less than 2% of the toroidal field see Sec. VI.
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