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Blackwell, GEM XV Conference 2/2008
The Effects of Plasma Shape on Stability and Confinement in the H-1NF Heliac.
B. D. Blackwell , D.G. Pretty, J. Howard , D.R. Oliver , S.T.A.Kumar , G. Potter, F. Detering , D. Byrne, C.A. Nuhrenberg*, M. McGann, R.L. Dewar , M.J. Hole , M. Hegland and **J.H. Harris Australian National University, *Max Planck IPP Greifswald, and **Oak Ridge National Laboratory
Outline
H-1 HeliacConfigurations, parameters, diagnostics
MHD Data Data mining, SVD
InterpretationAlfvénic Scaling
H-1’s unique twist control as an additional fit parameter
Scaling discrepancies
Radial Structure
Confinement Effects
Summary
Blackwell, GEM XV Conference 2/2008
H-1NF: National Plasma Fusion Research Facility
A Major National Research Facility established in 1997 by the Commonwealth of Australia and the Australian National University
Mission:• Detailed understanding of the basic physics of magnetically confined hot plasma in
the HELIAC configuration• Development of advanced plasma measurement systems• Fundamental studies including turbulence and transport in plasma• Contribute to global research effort, maintain Australian presence in the field of plasma
fusion power
The facility is available to Australian researchers through the AINSE1 and internationally through collaboration with Plasma Research Laboratory, ANU.1) Australian Institute of Nuclear Science and Engineering
International collaboration played an important role in the success of H-1 in obtaining facility funding
Contract extended until 2010: includes some operational funding and limited collaborative funding
Blackwell, GEM XV Conference 2/2008
H-1 CAD
Blackwell, GEM XV Conference 2/2008
H-1NF Photo
Blackwell, GEM XV Conference 2/2008
H-1 Heliac: Parameters
3 period heliac: 1992Major radius 1mMinor radius 0.1-0.2mVacuum chamber 33m2 excellent access
Aspect ratio 5+ toroidal
Magnetic Field 1 Tesla (0.2 DC)Heating Power 0.2MW 28 GHz ECH
0.3MW 6-25MHz ICH
Parameters: achieved to date::expected n 3e18 :: 1e19
T <200eV(Te)::500eV(Te)
0.1 :: 0.5%
Blackwell, GEM XV Conference 2/2008
H-1 configuration (shape) is very flexible
• “flexible heliac” : helical winding, with helicity matching the plasma, 2:1 range of twist/turn
• H-1NF can control 2 out of 3 oftransform ()magnetic well andshear (spatial rate of
change)
• Reversed Shear Advanced Tokamak mode of operation
Edge Centre
low shear
medium shear
= 4/3
= 5/4
Blackwell, GEM XV Conference 2/2008
Large Device Physics on H-1
Confinement Transitions, Turbulence (Shats, 1996--)
– High Confinement mode (’96)– Zonal Flows 2001– Spectral condensation of
turbulence 2005
Magnetic Island Studies – H-1 has flexible, controlled and verified geometry– Create islands in desired locations (shear, transform)– Langmuir probes can map in detail
Alfvén Eigenmodes– May be excited in reactors by fusion alphas, and destroy
their confinement– (more)
D3D tokamak H-1Pinj (MW)
1
2
3
Edge ne (1019 m–3)10
Edge Te (keV)
0.1
0.2
0.3
1600 2000 2400 2800t (ms)
Edge Te (eV)
t (ms)
6
3
Prf (kW)
ne (r/a = 0.3) (1017 m–3)
80
40
0
030
20
10
20 40 60
Blackwell, GEM XV Conference 2/2008
Plasma Production by ICRF
ICRF Heating:
•B=0.5Tesla, = CH
(f~7Mhz)
•Large variation in ne with iota
Backward WaveOscillator Scanning Interferometer (Howard, Oliver)
Axis
time
Blackwell, GEM XV Conference 2/2008
Configuration scan: rich in detail
ICRF plasma configuration scan
Mode spectrum changes as resonances enter plasma
No simple explanation for “gap”
left side corresponds to zero shear at resonance
A clear connection with rational twists per turn – but what is it?
5/4
4/37/5
5/44/3
7/5
gap
D. Pretty, J. Harris
Plasma density
increasing twist
increasing twist
Magnetic fluctuationsapproach “high temperature” conditions: H, He, D; B ~ 0.5T;
ne ~ 1e18; Te<50eV
i,e << a,
mfp >> conn
• spectrum in excess of 5-100kHz
• Low mode numbers: m ~ 0 - 7, n ~ 0 - 9 b/B ~ 2e-4
• both broad-band and coherent/harmonic nature
• abrupt changes in spectrum randomly or correlated with plasma events
Python open source datamining tools
David Pretty: http://code.google.com/p/pyfusion
Blackwell, GEM XV Conference 2/2008
Identification with Alfvén Eigenmodes: ne• Coherent mode near iota = 1.4, 26-60kHz,
Alfvénic scaling with ne
• m number resolved by bean array of Mirnov coils to be 2 or 3.
• VAlfvén = B/(o)
B/ne
• Scaling in ne in time (right) andover various discharges (below)
phase
1/ne
ne
f 1/ne
Critical issue in fusion reactors:
VAlfvén ~ fusion alpha velocity
fusion driven instability!
Blackwell, GEM XV Conference 2/2008
Mode Decomposition by SVD and Clustering• Initial decomposition by SVD
~10-20 eigenvalues• Remove low coherence and low
amplitude• Then group eigenvalues by
spectral similarity into fluctuation structures
• Reconstruct structuresto obtain phase difference at spectral maximum
• Cluster structures according to phase differences (m numbers)
reduces to 7-9 clusters for an iota scan
Grouping by SVD+clustering potentially more powerful than by mode number
– Recognises mixturesof mode numbers caused by toroidal effects etc
– Does not depend critically on knowledge of thecorrect magnetic theta coordinate
• 4 Gigasamples of data – 128 times
– 128 frequencies
– 2C20 coil combinations
– 100 shots
Data mining increasing twist
Mode numbers consistent with twist
• Mode analysis of the clusters found is complex, but is consistent with the rotational transform (twist/turn)
dominant azimuthal m=3
dominant toroidal mode n=4 (aliased to n=1)
Iota=4/3
Blackwell, GEM XV Conference 2/2008
Identification with Alfvén eigenmodes: k||, twist
Why is f so low? VAlfven~ 5x106 m/s res = k|| VAlfvén = k||
B/(o)
• k|| varies as the angle between magnetic field lines and the wave vector
• for a periodic geometry the wave vector is determined by mode numbers n,m
• Component of k parallel to B is - n/mk|| = (m/R0)( - n/m)
res = (m/R0)( - n/m) B/(o)
• Low shear means relatively simple dispersion relations – advantage of H-1Alfvén dispersion (0-5MHz)
Near rationals, resonant freq. is low
Alfvén dispersion (0-50kHz)
Small near resonance
Blackwell, GEM XV Conference 2/2008
Identification with Alfvén eigenmodes k|| 0 as twist n/m
res = k|| VA = (m/R0)( - n/m) B/(o)
• k|| varies as the angle between magnetic field lines and the wave vector
k|| - n/m
• iota resonant means k||, 0
Expect Fres to scale with iota
David Pretty
Resonant
ota
= 4/3
Twist
Blackwell, GEM XV Conference 2/2008
Two modes coexisting at different radii
Cross-power between ne and Mirnov coils shows two modes coexisting (0,180o)
Probably at different radii as frequency is different, mode numbers the same
Blackwell, ISHW/Toki Conference 10/2007
CAS3D: 3D and finite beta effectsCarolin Nuhrenberg
Beta induced gap ~5-10kHz
Coupling to “sound mode”
Gap forms to allow helical Alfven eigenmode, and beta induced gap appears at low f
Helical Alfvén eigenmode
Good scaling with ne,
Correction is ~ 0.7 – typical of 3D effect
David Pretty
Helical gap mode likely at coalescence of 7/5 and 10/7
as n1-n2 3 = m1-m2 2
n=10, m=7
n=7, m=5
Blackwell, GEM XV Conference 2/2008
Radial Mode StructureJesse Read 865poster today
Rad
ius
configuration
phase
Cross-power between light emission and Mirnov coils shows phase flips in radius and configuration
Blackwell, GEM XV Conference 2/2008
Scaling discrepancies
res = k|| VAlfvén = k|| B/(o)
• Numerical factor of ~ 1/3 required for quantitative agreement near resonance
– Impurities (increased effective mass) may account for 15-20%– 3D MHD effects ~ 10-30% (CAS3D), still a factor of 1/1.5-2x required
• Magnetic field scaling is unclear• Unknown drive physics
– VAlvfen ~5e6 m/s – in principle, H+ ions are accelerated by ICRH, but
poor confinement of H+ at VA (~40keV) makes this unlikely.
– Electron energies are a better match, but the coupling is weaker.– H+ bounce frequency of mirror trapped H+ is correct order?
Blackwell, GEM XV Conference 2/2008
Alfven Spectroscopy – potential as diagnostic
Potential to be iota diagnostic (if shear low)
Original iota calibration at reduced field
Corrected iota agrees better with dispersion
Santhosh Kumar
Transform decreases by 0.5% at high field
iota I2
1.400 iota 1.408
discrepancy halved
Effect of Magnetic Islands
Giant island “flattish” density profile
Central island – tends to peakPossibly connected to core electron root
Blackwell, GEM XV Conference 2/2008
• Several Configurational effects demonstrated:– Confinement, magnetic fluctuations spectra
• Alfvénic modes observed• Data mining – unsupervised reduction into significant clusters• New Diagnostics (J. Howard Thurs PM)
H-1 National Facility– Configurational flexibility– Large device physics accessible
Confinement Transitions, Alfvén Eigenmodes, Magnetic Islands
– Test Bed for Advanced Diagnostic Development, e.g. to develop reactor edge plasma diagnostics
Alfvénic modes observed– H-1’s unique “twist” control is a valuable diagnostic parameter, especially
at low shear– Increased dimensionality of data space is handled by datamining– Strong evidence for Alfvénic scaling between ne, k||, f– Unclear B scaling (resonant plasma production), frequency is low by a factor
of ~ 1.5 - 3. – Interferometer and PMT array valuable mode structure information– Currently David Pretty is applying technique to data at
Heliotron J (Kyoto), TJ-II (Madrid) and W7-AS (Max Planck IPP)
– “Alfven Spectroscopy” an emerging field, information about rotational transform
– Confinement inside magnetic islands is very interesting!
SummarySee also Posters todayJesse Read 865Frank Detering 856
Jesse Read 865