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,

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


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


H-1 CAD


H-1NF Photo


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%


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


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


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


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


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!


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


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


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


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


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


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?


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


• 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

Documents

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,