Transcript The Australian Plasma Fusion Research Facility: Recent

The Australian Plasma Fusion Research Facility:
Recent Results and Upgrade Plans
B. D. Blackwell 1), D.G. Pretty 1), J. Howard 1), S.T.A. Kumar 2), R. Nazikian 3), J.W. Read 1) C.A. Nuhrenberg 4), J. Bertram 1),
D. Oliver 5), D. Byrne 1), J.H. Harris 6), M. McGann 1), R.L. Dewar 1), F. Detering 7), M. Hegland 8), S. Haskey 1), M. J. Hole 1).
1) Plasma Research Laboratory, The Australian National University, ACT 0200, Australia. 2) Present Address: Department of Physics, University of Wisconsin-Madison, USA.
3) Princeton Plasma Physics Laboratory, NJ, USA. 4) Max-Planck-Institut für Plasmaphysik, Greifswald 5) Present Address: Research Group, Boronia Capital, Sydney Australia
6) Oak Ridge National Laboratory, Tn, USA.7) Diversity Arrays Technology Pty Ltd, Yarralumla, ACT 2600, Australia. 8) Mathematical Sciences Institute, ANU, Australia
Abstract: The “flexible Heliac” coil set of helical axis stellarator H-1 (major radius R=1m, and average
minor radius <r> ~ 0.15-0.2 m) permits access to a wide range of magnetic configurations. This has
enabled investigation of the effect of plasma configuration on Alfvénic range instabilities, magnetic
island studies, and the development of a number of innovative imaging and 2D diagnostics.
Alfvén modes normally associated with energetic populations in larger scale fusion experiments are
observed, in the absence of any obvious population of energetic particles. Using H-1’s unique
combination of flexibility and variety of advanced diagnostics RF-generated plasma in H-1 is shown to
have a very complex dependence on configuration of both the electron density and the nature of
fluctuations in the MHD Alfvén range. The magnetic fluctuations range from highly coherent, often
multi-frequency, to approaching broad-band (df/f ~ 0.02-0.5), in the range 1-200kHz.
Application of datamining techniques to a wide range of configurations classifies these fluctuations and
extracts poloidal and toroidal mode numbers, revealing that a significant class of fluctuations exhibit
scaling which is i) Alfvénic with electron density (within a constant factor) and ii) shear Alfvénic in
rotational transform. An array of optical and interferometric diagnostics is combined with the magnetic
probe arrays to provide initial information on the internal structure of the MHD modes, and associated
3D effects. The configurational dependence is closely related to the presence of low order rational
surfaces; density falls to very low values near, but not precisely at these rational values.
Results from a uniquely accurate magnetic field mapping system, combined with a comprehensive model
of the vacuum magnetic field in H-1 show that magnetic islands should not dominate the confinement
of the configuration, and indicate that the strong dependence of plasma density on configuration may
be attributable to variations in plasma generation favouring the presence of islands. Magnetic islands
have been deliberately induced to study their effect on lower temperature plasma to allow the use of
Langmuir probes. It was found that islands can cause both flattening and peaking in the plasma
density profile.
Finally, plans for a significant upgrade are described, including improved heating, vacuum and diagnostic
systems. A “satellite” linear device will be constructed employing helicon heating in hydrogen with a
target density of 1019m-3. The main aim of this device is to develop diagnostic techniques on fusionrelevant advanced materials under conditions of high plasma and power density.
Configuration Studies, Magnetic Islands
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 .
Facility Upgrade: Aims
New Imaging Techniques  Mode Structure
Synchronous Intensified Imaging
Results from a high speed Princeton Instruments PIMAX 3
intensified camera viewing a vertical
cross-section of the plasma gated in
synchronism with the mode are shown
in the figures below for
κh = 0.33 and 0.63. Camera
icon shows viewpoint.
Major/minor radius
Vacuum chamber
Aspect ratio
Magnetic Field
Heating Power
n
T

 Consolidate the facility infrastructure required to implement the
ITER forum strategy plan
 Try to involve the full spectrum of the ITER forum activities
More specifically:
Improve plasma production/reliability/cleanliness
RF production/heating, ECH heating, baking, gettering, discharge cleaning
Improve diagnostics
Dedicated density interferometers and selected spectral monitors permanently in
operation
Increase opportunities for collaboration
Increasie suitability as a testbed for ITER diagnostics
1m/0.1-0.2m
33m2 good access
5+
toroidal
1 Tesla (0.2 DC)
0.2MW 28 GHz ECH
0.3MW 6-25MHz ICH
3e18
<200eV
0.2%
Access to Divertor – like geometry,
island divertor geometry
 ~US$7M over 4 years for
infrastructure upgrades
Upgraded RF Systems
RF (7MHz) will be the “workhorse”
Datamining – Automatic Identification of Modes
Probability density in the multi-dimensional space of nearest
neighbour phase difference can be used to quickly identify modes in
new data. In the figure below, a very large difference in the ordering
of (log) probability of the modes involved in the activity before and
after ~37ms clearly shows the clusters to which the two dominant
modes belong.
• Subtracting DC (above) from synchronized images reveals mode
structure (below).
• Even parity, four zero crossings – implies m=2 or m=4 mode
(believed to be 5/4 mode).
New system doubles power: 2x40-180kW systems.
New movable shielded antenna to complement “bare” antenna
(water and gas cooled).
Advantages:
Very wide range of magnetic fields in Argon
New system allows magnetic field scan while keeping the resonant
layer position constant.
e.g. to test Alfven scaling MHD
Additional ECH source (10/30kW14/28GHz) for higher Te
• Taking images at different time delays (between PLL pulse and
camera gate) shows mode rotation (video on PC)
• Mode structure and rotation direction can be determined from
reconstructions.
Dashed Line is max likelihood mode before
transition, solid line after
Improved Impurity Control
Impurities limit plasma temperature (C, O, Fe, Cu)
High temperature (>~100eV) desirable to excite spectral lines
relevant to edge plasma and divertors in larger devices.
Strategy - Combine:
• Glow discharge cleaning for bulk of tank
• Pulsed RF discharge cleaning for plasma facing components.
antenna (cooled) and microwave source (2.4GHz)
• Low temperature (90C) baking
• Gettering
Configuration scans
The density of RF produced plasma varies markedly with
configuration as seen here, where kH is varied between 0 and 1.
  43
  65
Small Linear Satellite Device –
Plasma Wall Interaction Diagnostics
Effect of Magnetic Islands on Confinement
Experiments in the vicinity of iota ~3/2 were performed in Argon
plasma over a range of parameters, which, for lower neutral densities,
showed there is a small increase in confinement within the island
(below), and a steepening of the potential profile in the vicinity of the
core. Investigations into similarity with core electron root enhanced
confinement are ongoing.
Purpose:
Above: Vertical view of plasma light
synchronised with the Mirnov signals
and averaged over many cycles.
Right: Intensity profile at the crosssection of the dashed line
Below Right: Same profile of ne from
interferometer – shows that the
intensity is a good proxy for ne. The
m=4 simulation is a toy model in
Boozer space for illustration. (CAS3D
simulation is beneath m=4 model)
  75
Testing various plasma wall interaction diagnostic concepts
e.g.
Doppler spectroscopy, laser interferometry
coherence imaging, imaging erosion monitor
ne ~ 1019 m-3
Features:
P ~ 1MW/m2
Much higher power density than H-1
H-1 cleanliness not compromised by material erosion diagnostic
tests
Simple geometry, good for shorter-term students, simpler projects
Shares heating and magnet supplies from H-1
Mirror
coils
Intensity profile along dashed line
Helicon
source
Contours of plasma density radial profile as configuration is varied:
Sudden changes in density are associated with resonance at zero shear
Optical
Diagnostics
Helicon H+ source
concept
Localised Enhanced
Particle Confinement
a)
b)
HAE
Fig. 5: Classification of the configuration scan (kh). Three of the clusters
found are shown, Cluster 6 with mode numbers n/m=5/4, cluster 5
(n/m=4/3) and cluster 46 with n=0. thin lines are contours of rational
rotational transform
5
5
/4
Iota at 3/2 (left)
– peaking at O-point
Iota just below 3/2 (above figure)
– sudden transition to bifurcated state
Plasma is more symmetric than in quiescent
case.
Plausible that islands are generated at axis.
/4
4
/3
7
/6
6
/5
4
/3
6
/5
5
/4
configuration parameter k h
Fig. 6: Rescaling by ne to show Alfvénic dependence on configuration parameter
kh. Lines show expected Alfvén frequency at the stationary point in rotational
transform profile when the corresponding resonance is in the plasma, and at a
fixed radius (<a>~15cm) if not.
Many unanswered questions……
Symmetry? How to define Er with two axes?
Core electron root?.
Alfven Spectroscopy
Comparison of transform determination using Alfvén eigenmode
resonance and direct e-beam mapping(a). The discrepancy between
the transform obtained from the symmetry point in the “V” structure
of the observed frequency (b) and the computed transform value is
halved (c) if the computed transform is corrected for a small
distortion in the magnetic field coils due to the magnetic forces
inferred from the results (a) of electron beam mapping at high field.
normalised toroidal flux s
s
 induced gap
CAS3D Eigenmode calculations
CAS3D cylindrical (a) and 3D (b) frequency spectra. They grey dots
are sound mode continua for high mode numbers, which need to be
included for convergence, especially in H-1 which has a nonnegligible mirror (bumpy) component in the 1/B2 spectrum.
Note a gap mode (HAE) near top right, and low frequency modes are
up-shifted by a  -induced gap ~5kHz At the frequencies the modes
lose their m=3 or 4 character and become more electrostatic, is
suggestive of the Beta-induced Alfvén Eigenmode (BAE) or the
related Geodesic Acoustic Mode (GAM). A more extensive spectrum
is below.
Upgrade to Mirnov Array Systems
To complement the two existing poloidal arrays a toroidal/helical
magnetic probe array has been installed, consisting of 16 sets of 3
mutually perpendicular coils housed inside a vacuum-tight thin
stainless steel bellows Proximity to the plasma produces large signal
amplitudes, and traversal of regions of various magnetic curvature
may enable ballooning and interchange modes to be distinguished.
The Toroidal Mirnov array terminating in a special coil set enclosed in a
metallised glass tube for high frequency response. The existing poloidal array is
in the foreground (bean shaped tube), and CAD detail of a coils set is inset.
Helical
plasma
by John Wach
50 kN /coil support structure
14000Amp conductors and cooling