Simulation of ionospheric plasma heating experiments in the versatile toroidal facility

Remote sensing techniques employed to diagnose ionospheric modification experiments are intrinsically ambiguous, uncorrelated with "ground truth." To overcome this limitation, laboratory experiments are performed in the model ionosphere of the Versatile Toroidal Facility (VTF). The VTF contains a thermionically produced, weakly magnetized ( wce < wpe) background plasma of either hydrogen or argon. The HF "pump" wave of ionospheric experiments is modeled by 2.45 GHz microwaves, launched perpendicular to the magnetic field and the density gradient of the VTF in the ordinary mode. The peak plasma density is several times greater than the critical density (nc ~/= 7.4xI0 16 m-3 ), and the microwaves reflect, forming a standing wave Airy pattern. Wave spectra produced near reflection are measured using a miniature double probe and microwave receiver along with a fast oscilloscope. This combination is capable of simultaneously measuring spectra in two 250 MHz bands, one near DC and the other near the 2.45 GHz pump, to μs resolution. In addition, absolute electric field strengths and wavenumber spectra can be estimated. To explore the extent to which the VTF experiments simulate ionospheric heating, similarity rules are derived from the governing equations and applied to the two plasmas. A set of ten dimensionless parameters results, six of which match satisfactorily between the two plasmas. Three others can be neglected, leaving only one unmatched parameter: the ratio T/Ti, which in the VTF is about 12 and in the ionosphere is near unity. Consideration of boundary conditions limits the scope of the simulation to the first Airy maximum. The main observational results of VTF heating experiments are: (1) Langmuir wave sidebands both up- and down-shifted from the pump frequency that decrease monotonically to the noise floor in tens of MHz, (2) lower hybrid waves in a broad band from 35 - 150 MHz, with maximum power occurring at 50 - 90 MHz, (3) both Langmuir and lower hybrid waves appear in bursts of duration and period in the 2- 100 ms range, depending upon radius, (4) Langmuir and lower hybrid bursts are anti-correlated at the edge of the plasma but become uncorrelated in the core, and (5) the electric field, both of the pump and the plasma sidebands, varies by a factor of 100 in a burst period, from 1.3 to 130 kV /m for the pump (expected: 10.8 kV/m). The main features of ionospheric heating were reproduced in these experiments: down- and up-shifted high frequency sidebands, extreme time-variability of electric field amplitude, large pump wave absorption, and significant electron heating. The observed spectral bursts suggest the concentration of electric field into small time-varying regions. The periods and parameter dependencies of the bursts resemble results of three-dimensional simulations of Langmuir turbulence. However, the upshifted Langmuir waves predicted by strong Langmuir turbulence (SLT) and nonlinear scattering theory are not observed in the VTF. A consistent account of the VTF observations is obtained by combining the caviton collapse cycle of SLT and the parametric production of lower hybrid waves by energetic Langmuir waves. As the high frequency electric field concentrates in cavitons, the threshold for the Langmuir decay instability is exceeded, generating lower hybrid waves in anti-correlated bursts. Because of the similarity of the VTF experiments to ionospheric heating, the observation of lower hybrid wave production during heating may also be borne out by future field experiments with diagnostics capable of viewing field-aligned modes.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Nuclear Engineering

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2001

Author dc:creator

• Dalrymple, Nathan Edward

Advisor dc:contributor.advisor

• Min-Chang Lee.

• Nuclear Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/8866

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/8866