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B. D. Blackwell 1), D.G. Pretty 1), J. Howard 1), S.T.A. Kumar 2), R. Nazikian3), J.W. Read 1)C.A. Nuhrenberg4), J. Bertram 1), D. Oliver 5), D. Byrne 1), J.H. Harris 6), M. McGann 1), R.L. Dewar 1), F. Detering 7), M. Hegland8), 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-InstitutfürPlasmaphysik, Greifswald5) 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 fusion-relevant advanced materials under conditions of high plasma and power density.
Facility Upgrade: Aims
New Imaging Techniques Mode Structure
Improve plasma production/reliability/cleanliness
RF production/heating, ECH heating, baking, gettering, discharge cleaning
Dedicated density interferometers and selected spectral monitors permanently in operation
Increase opportunities for collaboration
Increasie suitability as a testbed for ITER diagnostics
Access to Divertor – like geometry,island divertor geometry
Synchronous Intensified Imaging
Major/minor radius 1m/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
~US$7M over 4 years for infrastructure upgrades
Upgraded RF Systems
RF (7MHz) will be the “workhorse”
New system doubles power: 2x40-180kW systems.
New movable shielded antenna to complement “bare” antenna
(water and gas cooled).
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
Datamining – Automatic Identification of Modes
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 .
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:
Dashed Line is max likelihood mode before transition, solid line after
Small Linear Satellite Device – Plasma Wall Interaction Diagnostics
The density of RF produced plasma varies markedly with configuration as seen here, where kHis varied between 0 and 1.
Effect of Magnetic Islands on Confinement
Testing various plasma wall interaction diagnostic concepts
e.g. Doppler spectroscopy, laser interferometry coherence imaging, imaging erosion monitor
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
Above: Vertical view of plasma light synchronised with the Mirnov signals and averaged over many cycles.
Right: Intensity profile at the cross-section 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)
ne ~ 1019 m-3
P ~ 1MW/m2
Intensity profile along dashed line
Contours of plasma density radial profile as configuration is varied:Sudden changes in density are associated with resonance at zero shear
Helicon H+ source concept
Localised Enhanced Particle Confinement
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
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.
Many unanswered questions……Symmetry? How to define Er with two axes?
Core electron root?.
Upgrade to Mirnov Array Systems
CAS3D Eigenmode calculations
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.
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.
by John Wach
50 kN /coil support structure
14000Amp conductors and cooling