Final Technical Report
The nonlinear physics of electron magnetohydrodynamics (EMHD) in plasmas. Time-varying wave magnetic field exceeding the background magnetic field produces highly nonlinear whistler mode since the wave dispersion depends on the total magnetic field. There exists no theory for such whistler modes. The present experimental work is the first one to explore this regime of nonlinear whistlers. A field-reversed configuration has been found which has the same vortex topology as an MHD spheromak, termed a whistler spheromak. Whistler mirrors have compressed and twisted field lines propagating in the whistler mode. Their helicity properties have been studied. Whistler spheromaks and mirrors have different propagation and damping characteristics. Wave collisions have been studied. Head-on collisions of two whistler spheromaks form a stationary field-reversed configuration (FRC) without helicity. When whistler spheromaks are excited the toroidal current flows mainly in the toroidal null line. It is only carried by electrons since ion currents and displacement currents are negligible. A change in the poloidal (axial) magnetic field induces a toroidal electric field which drives the current. Magnetic energy is dissipated and converted into electron kinetic energy. This process is called magnetic reconnection in 2D geometries, which are simplifications for theoretical convenience but rarely occur in nature. A crucial aspect of reconnection is its rate, determined by the electron collisionality. Regular Coulomb collisions can rarely account for the observed reconnection rates. In the present experiments we have also observed fast reconnection and explained it by electron transit time damping in the finite-size null layer. Electrons move faster than a whistler spheromak, hence transit through the toroidal null line where they are freely accelerated. The transit time is essentially the collision time but no particle collisions are required. Strong electron heating and visible light emissions are only observed in whistler spheromaks and FRCs but not in mirrors or asymmetric configurations lacking magnetic null lines. The collisionless electron energization in a toroidal null line usually produces non-Maxwellian distributions. Off the null axis electrons gain more perpendicular than parallel energy. Distributions with T{sub {perpendicular}} > T{sub {parallel}} lead to whistler instabilities which have been observed. A whistler spheromak is a source of high-frequency whistler emissions. These are usually small amplitude whistlers propagating in a complicated background magnetic field. The waves are emitted from a moving source. High frequency whistlers propagate faster than the spheromak, thus partly move ahead of it and partly in the reverse direction. In test wave experiments wave growth opposite to the direction of the hot electron flow has been observed, confirming that Doppler-shifted cyclotron resonance instabilities account for the emission process. Propagating whistler mirrors produce no significant instabilities except when they interact with other fields which exhibit null lines. For example, a whistler mirror has been launched against a stationary FRC, resulting in strong FRC heating and whistler instabilities. In the whistler mirror configuration the antenna near-zone field produces a toroidal null line outside the coil which can also become a source for whistler emissions. Finally, nonlinear EMHD research has been extended to initially unmagnetized plasmas where a new nonlinear skin depth has been discovered. When a small-amplitude oscillating magnetic field is applied to a plasma the field penetration is governed by the skin depth, collisional or collisionless depending on frequency, collision frequency and plasma frequency. However, when the magnetic field increases the electrons become magnetized and the field penetration occurs in the whistler mode if the cyclotron frequency exceeds the oscillating frequency. This phenomenon has been observed. A loop antenna creates a dipole field which is frozen into the plasma for a half cycle and becomes the background field for the wave launched by the next half cycle. The field topology consists of field-reversed dipoles of decreasing strength with distance. The propagation region depends on field amplitude but not on the skin depth. Our research has been published in 13 scientific papers.
- Research Organization:
- Univ. of California, Los Angeles, CA (United States)
- Sponsoring Organization:
- Office of Science , SC-24.2, NSF/DOE Partnership
- DOE Contract Number:
- FG02-06ER54905
- OSTI ID:
- 963717
- Report Number(s):
- DOE/ER5405-4; TRN: US1203723
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
AMPLITUDES
CONFIGURATION
CYCLOTRON FREQUENCY
CYCLOTRON RESONANCE
DIPOLES
ELECTRIC FIELDS
ELECTRONS
FIELD-REVERSED THETA PINCH DEVICES
HEATING
HELICITY
KINETIC ENERGY
LANGMUIR FREQUENCY
MAGNETIC FIELDS
MAGNETIC RECONNECTION
MAGNETOHYDRODYNAMICS
MIRRORS
PLASMA
TOPOLOGY
WHISTLER INSTABILITY
WHISTLERS
Experimental research
electron magnetohydrodynamics
whistler spheromaks
nonlinear skin effect