Date post: | 27-Mar-2015 |
Category: |
Documents |
Upload: | sean-scott |
View: | 215 times |
Download: | 2 times |
Ion Heating
Presented byGennady Fiksel, UW-Madison
for CMSO review panelMay 1-2, 2006, Madison
CMSO
Scope of the problem
• In many laboratory and space plasmas the ions are hotter than expected (e.g. in Reversed Field Pinch ions are hotter than expected from collisional e/i heating).
• Frequently the ions heating exhibits explosive behavior with a strong temperature increase in a short time.
• Examples: Hot ions in solar corona. High Ti and fast ion heating in reconnection experiments and RFP.
• The phenomenon is very robust and well documented.
• At the same time it is poorly understood.
CMSO
Ions are hot in solar corona
• Spectroscopic measurements indicate the ions in solar corona (r > 1.5R) are very hot.
• Indications that the ions are especially hot in polar corona holes, where the electrons are relatively cold.
• Heavier ions are hotter than protons.• Anisotropic velocity distribution function
Ion velocity distribution functionin solar coronaE. Marsch et al, NPG, 10, 2003
100 km/s ≈ 100 eV for H
CMSO
Heavier ions are hotter
From: Cranmer et al., ApJ, 511, 481 (1998)
TO/TO 10
Hydrogen Oxygen
TO > TH
Strong perpendicular heating of oxygen
CMSO
Plan of action
• Different mechanisms proposed for solar ion heating
• Focus first on laboratory work.• Understand, within the framework of the Center, the connection of ion heating and magnetic self-organization.
• Expand our understanding to extra-terrestrial plasma.
CMSO
Some features of laboratory ion heating
• Anomalously high ion temperature observed in Reversed Field Pinches, spherical tokamaks, merging spheromaks.
• Energy source - magnetic field energy released during reconnections.
• Large fraction of the energy is deposited either in the form of ion thermal energy or ion flows.
• Different heating for light majority ions and heavy minority ions.
CMSO
Diagnostics
• Active and passive ion Doppler spectroscopy for minority ions
• Rutherford scattering diagnostic for majority ions
• Diagnostic neutral beams• Probes (current, magnetic, optical)• Good spatial and temporal resolution. • The Center diagnostics: sophisticated, often unique, up to the challenging measurements.
CMSO
Magnetic energy is released
Reconnection EventsExplosive growth ofmagnetic fluctuations
MST Experiment
MST - Ion heating during reconnections
Impurities (C5+)
Majority ions
Ions are heatedTC >> TD during reconnection. Agrees with observations in solar corona.TC ≈ TD away from crash.
CMSO
Co- low magnetic fluctuationsCounter- high magnetic fluctuations
Merging spheromaksCo-helicity orCounter helicity
Co- no ion heatingCounter- strong ion heatingElectron temperature the same
magnetic fluctuations
ion and electron heating
MRX - strong ion heating during counter-helicity spheromak merging
Courtesy of H. Ji
CMSO
Bi- and unidirectional flows in the lab and the Sun
SSX M. Brown
V (km/sec)
0 20 40 60-20-40-60
Unidirectional
Bi-directional
Courtesy of M. Brown
Sun (SOHO)Innes, Nature, 1997Innes, Solar Physics, 1997
?
CMSO
Ion heating and tearing fluctuations
• Magnetic reconnections in MST are caused by rapid growth and non-linear interaction of tearing modes.
• How to relate it to ion heating?• Challenge:
– Rapid growth of T requires high power 10 MW
– The heating depends not only on the mode amplitude but also on their spatial localization and non-linear interaction.
CMSO
Example of multiple reconnections -edge modes removed - no ion heating
Edge modes
Core modes
Magnetic energy
Ion temperature
Edge modes removed
CMSO
Theoretical approach
• Theory studies of ion heating have been concentrated on tearing modes induced ion flows and their viscous damping.
CMSO
Flow flowchart
Sheared flow Compressible flow
Cross-field flow
Uniform electric
field
Compressible flow
Sheared flow
Parallel flow
Kinetic simulations
e/i/Z componentplasma
EstimatesBraginski
Dielectric tensor withcollisional
corrections.Weak
collisionality
Kinetic computations
exact collisionaloperator.Arbitrary
collisionality.
CMSO
Sheared flow Compressible flow
Cross-field flow
Flow chart
Dielectric tensor withcollisional
corrections.Weak
collisionality
Kinetic computations
exact collisionaloperator.Arbitrary
collisionality.
• Heating stronger for impurities
• Heating rate is consistent with experiment IF :– flow thermal speed – scale ion gyroradius
• Strong ion flows have been observed in SSX. Have not been observed in MST.
• However, if the spatial and temporal resolution is not sufficient such flows can be misinterpreted as high Ti.
CMSO
Flow chart
• Strong E|| observed in experiment
• Electrons drag impurities which are heated through collisions with bulk ions.
• Strong electron and impurity flows (vflowvthermal) required for heating.
Uniform electric
field
Compressible flow
Sheared flow
Parallel flow
Kinetic simulations
e/i/Z componentplasma
CMSO
• Interplay of ion heating and core/edge modes• Collisional dissipation of parallel flows
– parallel and perpendicular gradients – large parallel viscosity – neoclassical effects
Future plans - theory
• Kinetic treatment. • Nonlinear resistive MHD modeling. • Nonlinear two fluid computations with NIMROD.
CMSO
Future plans - experiment
• New diagnostics and improved resolution measurements– upgrade of CHERS diagnostic neutral beam– new Mach probe - ion heating from flow dissipation
– new optical probe - local Doppler spectroscopy
• Dependence of ion heating on Z/M– compare to theory – identification of the heating mechanism– compare to solar- and space plasma.
• Ion heating and tearing modes. • Joint measurements and shared diagnostics on MST, MRX, and SSX.
The End