RF configuration scans

1
Effect of Magnetic Configuration on the H-1NF
Heliac Plasma B.D. Blackwell , D.G. Pretty, J.H.
Harris, J. Howard, T.A.Santhosh-Kumar, F.J.Glass,
S.M. Collis and C.A. Michael Plasma Research
Laboratory, Research School of Physical Sciences
and Engineering Australian National University,
ACT 0200, AUSTRALIA
Configuration Studies Introduction The
flexibility of the heliac configuration and the
precision programmable power supplies provide an
ideal environment for studies of magnetic
configuration. The main parameter varied in this
work is the helical core current ratio, kH which
primarily varies the rotational transform iota.
Magnetic well and shear also vary as shown below.
Plasma Configuration Studies Results
Configuration Effect on Magnetic Fluctuations
high temperature conditions H, He, D B
0.5Tne 1e18 Telt50eV?i,e ltlt a, ?mfp gtgt
?conn
The H-1 heliac is a current-free stellarator with
a helical magnetic axis which twists around the
machine axis (a circular ring conductor, radius
1m) three times in one toroidal rotation. It is
a flexible heliac i composed almost
entirely of circular coils with the exception of
the helical control winding, which also wraps
around the ring conductor, in phase with the
magnetic axis of the plasma, but with a smaller
swing radius (95 mm c.f. 230 mm). Control of
this current produces a range of rotational
transform ? from 1 to 1.5 at full specification
(B0 1T, ?r? gt 0.15-0.2 m), and 0.7 to 2.2 for
B0 0.5 T, ?r? gt 0.1 m. By varying currents in
the two vertical field coil sets, flexibility is
enhanced to allow almost independent control of
two of the three parameters ?, magnetic well
(2 to 6) and shear in rotational transform.,
which can be positive (stellarator-type) negative
(tokamak-like), or near zero (lt0.1). The coils
are powered by two precision regulated computer
controlled DC supplies of 1-14kA. Ripple is kept
well below 1A to allowing precise control of
configuration, to prevent shimmer in the
magnetic surfaces and to avoid induction of
plasma currents. At 0.5 tesla, RF (20 150kW,
? ?cH) produces plasma in HHe and HD mixtures
at densities up to ltnegt 2?1018m-3, with
temperatures initially limited to lt 50eV by low-Z
impurities. ECH (? 2?ce) produces considerably
higher temperatures and centrally-peaked density
profiles. Configuration scans show a detailed
dependence of plasma density on rotational
transform, which is more pronounced for RF
production. Magnetic fluctuations are also
stronger in these plasmas, and their spectra
depend on magnetic configuration ii. There
are broad regions of low or zero density when
central ? is near ?0 5/4 and 4/3, and other
narrower, less clearly identifiable features.
Alternatively, the presence of a lower order
rational at the edge (e.g. ?a 7/5), or shear
may be important factors 2. In addition to
indicating particle confinement times, this
phenomenon may be sensitive to plasma generation
efficiency. There may be some interaction
between configuration and impurity generation, as
plasma boundaries and strike points are
varied. Various plasma conditions and formation
techniques are compared. iHarris, J.H.,
Cantrell, J.L. Hender, T.C., Carreras, B.A. and
Morris, R.N. Nucl. Fusion 25, 623 (1985). ii
D.G Pretty, J.H.Harris et al., this conference.
RF configuration scans The density and
time-evolution of RF produced plasma varies
markedly with configuration as seen here, where
kH is varied between 0 and 1.
Variation of the last closed surface with a
limiter. One possibility is that rational
transform values at the plasma edge affect plasma
confinement. The effective edge of the plasma
was controlled by a movable rod limiter. This
graph compares data with a rod limiter
penetrating 4cm into the plasma. (no limiter in
light blue)

• spectrum in excess of 100kHz
• mode numbers not yet accurately resolved, but
  appear low m 1- 8, n gt 0
• ?b/B 2e-5
• both broad-band and coherent/harmonic nature
• abrupt changes in spectrum for no apparent
  reason
• Similar phenomena inHeliotron-J, Kyoto

Density (x1018m-3 )
Time (seconds)
The graph below shows the dependence on
rotational transform twist per turn, measured
at t50ms. There are several broad regions of
low density, e.g. near 4/3, and many finer
variations. A common feature seems to be that
sudden changes occur when a resonance
(transformN/M N,M integers) occurs at a point
of zero shear (derivative of transform).

• We did not observe the expected movement of the
  sharp dips to higher k_H ( iota decreases
  inwards), but
• Three new dips were found near kH 0.54, 0.9 and
  1.0, which correspond to edge iota 4/3, 7/5 and
  10/7 respectively. This represents 3 of the five
  possible low order rationals accessible in this
  range.
• The density was generally higher, possibly due to
  impurity generation or hydrogen desorption from
  the limiter.
• Of the remaining two, 5/4 and 9/7, the former may
  lie in the experimentally inaccessible gap
  between 0 and 0.16, and the latter may be related
  to the wide valley at kH 0.4.
• So while this does not directly confirm the
  rational at edge model, it has revealed two
  more dips corresponding to rationals near the
  edge.

Configuration Control Computer Control of
H-1NF H-1 is controlled by a PLC system
(Programmable Logic Controller). Each of 4
PLCs is a small, real-time computer executing
20-40 complete monitoring cycles every second.
These run a very simple operating system, unlike
some windowing systems, and are therefore very
reliable. In conjunction with the control
computer, and the CITECT human interface
software, these automate all power systems,
cooling systems and the sequence that comprises a
plasma pulse. The CITECT software has the power
and graphical user interface advantages of a
Windowing system, but is not critical to safe
operation. Sample CITECT H-1NF control panels are
below
Heliotron-J Neutral Beam HeatingECH
Heliotron-J ECH only

Poloidal mode number measurements, H1
phase
Expected for m 2
Sudden changes in density associated with
resonance at zero shear
magnitude
Dynamic configuration scans The programmable H-1
power supplies allow changes in currents in tens
of milliseconds. This was exploited to explore
the dependence of plasma density on the immediate
previous history of plasma formation. History
dependence may be expected because both the ECH
and RF production methods depend fundamentally on
formation of an initial plasma, but for different
reasons. A minimum plasma density is required
for fast wave propagation in the ICRF frequency
range. For second harmonic electron cyclotron
heating, no such minimum exists, but the electron
pressure determines the absorption rate.

• bean-shaped 20 coil Mirnov array
• Phase vs poloidal angle is not simple
• Boozer coords
• External to plasma
• Propagation effects
• Large amplitude variation
• Phase problem reduced at higher m, amplitude
  problem worsens.
• Significant interpretation problem in advanced
  confinement configurations

Coil number
ECH plasma Figure 3 shows an ECH produced
plasma (200kW 28GHz gyrotron, 2?CE at 0.5T).
With a 10ms pulse, and rf preionization of
1?1017m-3, a diamagnetic temperature of
100-200eV was observed provided gas feed was
carefully controlled (p lt 2?10-6 Torr). A highly
localised ionization rate was observed in the
emission from argon doping, and at higher gas
fills, a peak density in excess of 3?1018 was
obtained, with a lower temperature. Impurity
levels, estimated by comparative spectral line
intensities, were lower than in the rf
discharges. The ECH system used for the
configuration experiments was limited to 10ms
pulses. In most cases this was adequate to
produce plasma close to steady state, provided
there was a very small amount of RF
pre-ionization, typically 5-10 of the target
density..
3 period heliac 1992 Major radius 1m Minor
radius 0.1-0.2m Vacuum chamber 33m2 good
access Aspect ratio 5 toroidal Magnetic Field ?1
Tesla (0.2 DC) Heating Power 0.2MW 28 GHz
ECH 0.3MW 6-25MHz ICH Upgrade one to 400-500kW
Parameters achieved planned n 3e18
1e19 T lt100eV(Ti)0.5-1keV(Te) ? 0.1 0.5
FIGURE 3. EC heating in H-1, B0.5T, HD 11, gt
100kW ECH.
Coil number
See poster D.G. Pretty
2-4

• mode numbers related to rationals

2-4
4-5, 7-8
Mostly 0-3
3-8
Main Control Screen (Overview) Subsystems are
summarised in each box, red and green indicating
faults and normal operation. Each sub system has
a detailed screen such as that below.
FIGURE 2. The magnetic configuration is scanned
in 100ms by ramping down the helical core
current. The preionization occurs in the kH
0.95 configuration and the ECH plasma is
generated in the 0.75 configuration.
Comparison of Dynamic and Static Configuration
Scans The figure below shows two RF dynamic scans
of configuration in the range 0.65 lt kH lt 0.95
overlaid on static a configuration scan. Apart
from a displacement to higher kH, which
corresponds to a time delay of 20ms, the general
behaviour is similar. The scan A went from
high density to low density, showing that density
actually falls in the vicinity of kH 0.75,
rather than failing to rise. Possible origins of
such a time delay include particle confinement
time (10ms), and plasma toroidal current L/R
time.
Density (x1018m-3 )
Data Mining handles large quantities of data

• 4 Gigasamples of data
• 128 times
• 128 frequencies
• 2C20 coil combinations
• 100 shots
• Data mining allows sub sampling, exploring and
  rule extraction
• Initial work with Weka java and Gabor
  transforms for time freq analysis

kH (helical ratio)
FIGURE 4. Configuration scans in ECH plasma.
The solid line shows a single scan, ? is the
result of a dynamic configuration scan.
H-1NF Programmable Logic Controllers(PLC)
Initial results show that the ECH-produced
plasmas had similar overall dependence of plasma
density on plasma configuration to that of
RF-produced plasma, but with reduced fine detail.
The higher density of the dynamical scan point
is partly due to the use of slightly reduced
power in the systematic scans, to improve
reliability. There is an apparently greater
gross dependence on iota, which may be due to the
shorter path length for ECH absorption in more
elongated plasma.
Density (x1018m-3 )
H-1NF Pulse Sequencer The PLCs implement a 32
step, software configured sequencer which is
displayed below through a real-time Excel
interface. This is they key component in
providing a sequence of H-1 pulses with varying
currents under remote control, to enable the
configuration studies to be done more precisely,
reliably, and on a finer resolution than could be
achieved through manual operation.
The Programmable logic controller controlling
pulse sequencing, vacuum and cooling is on the
right, and below is the PLC for the DC motor
generator and the 3 RF/microwave heating
systems. .
kH (helical ratio)

• Conclusions
• H-1 shows a strong configuration effect on RF
  plasma production
• clear relationships with iota.
• Three scales are observed
• very slow variation, consistent with the
  variation in gross shape (mainly elongation) with
  iota,
• broader, more persistent gaps or valleys.
• very sharp features (?i/i .02), which are more
  ephemeral
• Broad features fit best with points where ?i/?r
  0
• Sharp features correspond to rational iota near
  the edge clear identification is made for 3 out
  of 5 of the lowest order rationals.
• Effects on RF produced plasma appear to be more
  complex than for ECH, suggesting that some of the
  contributing phenomena are source dependent.