Ion energization during magnetic reconnection in MST RFP
description
Transcript of Ion energization during magnetic reconnection in MST RFP
![Page 1: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/1.jpg)
MMFWMadison, Wisconsin
6 May 2011
D.J. Den Hartog, R. M. Magee, S.T.A. Kumar, V.V. Mirnov(University of Wisconsin–Madison)
D. Craig (Wheaton College)G. Fiksel (Laboratory for Laser Energetics)
J.B. Titus (Florida A&M University)
Ion energization during magnetic reconnection in MST RFP
![Page 2: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/2.jpg)
Ions are heated impulsively during magnetic reconnection.
• Energy is transferred from the equilibrium magnetic field to ion thermal energy.
• Heating time (100 μs) is much faster than i-e collision time (10 ms).
• Power flow from equilibrium magnetic field is larger than Ohmic input power.
Pmag ~ 10 kJ/ 100s = 100 MW
Pohmic ~ 5 MW
![Page 3: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/3.jpg)
Outline
• Magnetic reconnection in MST
• Majority ion energy distribution– Neutron flux measurements – Neutral particle energy spectra
• Impurity ion temperature– CHERS measurements of local C+6 Tperp and Tpar
Small population of fast ions generated during reconnection
Anisotropy with Tperp > Tpar during heating
![Page 4: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/4.jpg)
The Madison Symmetric Torus is a large, moderate current reversed field pinch.
R = 1.5 m Ip ~ 400 kA ne = 0.4 - 2.0 x 1019 m-3
a = 0.52 m B = 0.5 T Ti,e = 0.2 - 2 keV
![Page 5: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/5.jpg)
Magnetic reconnection in MST is impulsive and periodic.
• Reconnection events are characterized by a burst of resistive tearing mode activity.
![Page 6: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/6.jpg)
Much is known about ion heating in MST.
• Equilibrium magnetic field is the ultimate energy source.• The heating rate is very large (3-10 MeV/s).• The majority ion heating efficiency ~ m1/2. • Fully-developed magnetic turbulence is required (i.e. m=0 is a
necessary condition).• Impurities tend to be hotter than the majority ions.
However, a comprehensive theoretical model of the heating mechanism remains elusive.
![Page 7: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/7.jpg)
Measurements of majority ion energy distribution
![Page 8: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/8.jpg)
Neutron flux measurements provide information about ion energies.
• D-D fusion reaction produces neutrons,
• Neutron emission rate is a function of ion energy and density,
• A small number of fast ions can produce as many neutrons as a thermal plasma.
![Page 9: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/9.jpg)
Neutron flux measurements do not agree with predictions using Maxwellian assumption.
• Measured neutron flux is much larger than expected for thermal ions
![Page 10: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/10.jpg)
D0
D+
electrostaticenergy analyzer
He stripping cell
electron multiplier
Information about fi can be obtained from neutral flux measurements.
• The neutral flux is related to fi(v,x) by
• Attenuation (α) and neutral density profile (na) are known, so information about fi can be extracted.
![Page 11: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/11.jpg)
Derived ion energy spectrum reveals a significant tail in ion distribution function.
• fi(E) is well-modeled by
fi(E) = A exp(-E/T) + B E-γ
after reconnection
before reconnection
• Spectral index• varies with density• decreases rapidly during
reconnection events
![Page 12: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/12.jpg)
Fast ion density 2-6% in low density case, <1% in high.
![Page 13: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/13.jpg)
Ion acceleration mechanisms
![Page 14: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/14.jpg)
• Characteristics:• core amplitude ~ 50 V/m• duration ~ 100 μs• extends across minor radius to
suppress current in the core and drive current in the edge
• Ion acceleration from parallel electric field has been used elsewhere (MAST, ZETA) to explain suprathermal ion population.
• Plausible scenario for MST.
E|| induced during reconnection can accelerate ions to high energies.
(Courtesy of W. Ding.)
![Page 15: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/15.jpg)
Ions bouncing off of moving magnetic mirrors can gain energy (Fermi acceleration).
• First proposed by Fermi (1949) to explain high energy cosmic rays.
• Applied to Earth’s magnetosphere to explain high energy electrons (Drake, 2006).
• Predicts beta dependent power law energy distribution (γ = 3.7 for β=0.16).
![Page 16: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/16.jpg)
Measurements of impurity ion temperature
![Page 17: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/17.jpg)
CHERS can measure both Tperp and Tpar locally.
• CHarge Exchange Recombination Spectroscopy measures C+6 impurity ion temperature.
(Courtesy of S. Oliva.)
![Page 18: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/18.jpg)
Impurity ion temperature anisotropy is observed during reconnection heating.
• Tperp > Tpar during heating implies perpendicular heating mechanism.
• ΔTpar decreases with density, ΔTperp does not.
• Anisotropy increases with density, contrary to expectation from collisional isotropization.
![Page 19: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/19.jpg)
Tperp,DTperp,C
Tpar,C Tpar,D
energy energy
Energy flows through multiple channels.
Cranmer et. al. Astrophys. J. 518, (1999)
![Page 20: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/20.jpg)
Inverse density dependence of ΔTpar reproduced by model with varying Zeff
• Known impurities (C, B, O, N, Al) included in proportion to give:
Zeff = 4.2 in low density Zeff = 2.0 in high density
![Page 21: Ion energization during magnetic reconnection in MST RFP](https://reader033.fdocuments.us/reader033/viewer/2022051821/5681634c550346895dd3e333/html5/thumbnails/21.jpg)
Summary
• High energy tail appears in majority ion distribution function.• Generated at reconnection.• Well-described by power law.• A few percent of total density, with energy ~ 1 - 5+ keV.• Ion runaway and Fermi acceleration are possible mechanisms.
• Impurity ion anisotropy appears during heating with Tperp > Tpar.
• Implies perpendicular heating mechanism (ICRH or stochastic heating).
• Density dependence of anisotropy may be due to changing relative impurity content.