Excitation of electrostatic, whistler, and electromagnetic waves at double layers and double-layer-like structures

Transcription

1 Excitation of electrostatic, whistler, and electromagnetic waves at double layers and double-layer-like structures Nils Brenning, Ingvar Axnäs, Michael Raadu, Mark Koepke*, and Einar Tennfors Space- and Plasma Physics, Royal Institute of Technology, Stockholm, Sweden *West Virginia University, Morgantown, USA U e?? e?? Double layer z (emitting) Cathode sheath

2 Overview 1. Electrostatic 2. Whistlers 3. (vacuum) Electromagnetic

3 Apparatus Electrostatic E-probe array

4 1. Electrostatic Spatial extent: a few cm where the elecron beam enters the high potential plasma Average potential profile of Double-Layer Average density profile E (kv/m) z (mm) z (mm) Maximum amplitude of electric field oscillation normalized to reference probe signal Potential profiles of a pic simulation, at 10 times during one half period

5 1. Electrostatic Frequency: about the local fpe. Monocromatic, with periods of multiple frequencies. [arb. units]

6 1. Electrostatic The zero crossing method gives instantaneous frequencies of the dominating mode at frequency f1. Jump structures sometimes inticate mode competition at the transitions.

7 1. Electrostatic When the amplitude of the dominating f1 it high, then the near frequency space is kept clear of competing modes. Amplitude of f1 (arb. units) + 20 MHz Arnold tongue? Important for whistler excitation!! f = f1 f2 (MHz)

8 1. Electrostatic, summary Always there Eigenmode(s) Gap around strongest mode Steady for microseconds Single modes and transitions

9 2. Whistlers SELECTED CASE During a mode transition between 380 MHz and 400 MHz, two strong eigenmodes co-exist for half a microsecond. During that time, a whistler wave packet is excited, satisfying fwhistler = f1 f2 = 20 MHz

10 2. Whistlers TYPICAL CASES Whistlers are excited when there are mixed modes present, as during eigenmode transitions. Little whistler signals are seen even when the transitio f does not match the 20 MHz resonance cone (on which the magnetic probe is placed). However, LARGE whistlers only arise when there are two matching f = 20 MHz eigenmodes present in the electrostatic oscillations.

11 2. Whistlers Temporal distribution: μs wave packets, a few % of the time - frequency 7 40 MHz fge A 12.5 MHz whistler wave packet passes over an array of dbz/dt probes, spaced 45 mm in the z direction

12 2. Whistlers Spatial distribution: - along group velocity resonance cones, and - along a central channel. group group = phase = arcsin 2 ge B

13 2. Whistlers Frequency: - strongest around half the local gyro frequency fge fge This experiment. Ljungberg, 1995.

14 2. Whistlers This experiment Mirror B field Cathode sheath Magnetic probe Ljungberg, 1995 Constant B field Double Layer E field probe

15 2. Whistlers, summary A few % of the full time Monocromatic burst, μs. Wings and a central channel Excited by the electrostatic eigenmodes Radiated power scales fast with input power

16 3. (vacuum) Electromagnetic Temporal: high amplitude bursts of about 1 μs duration, a few % of the time. Low amplitude radiation there most of the time. Frequency: same as the electrostatic oscillations (follows in phase). Emission: anticorrelated with whistlers. Strong radiation only for monocromatic electrostatic. Spatial: fills the vacuum tank outside the dense plasma where it it cutoff.

17 Space interest cartoon 2. Vacuum e-m (only into less dense plasma) 1. Ei W e waves E v e DL E w 3. Whistlers, ducted along flux tube B i 4. Whistlers, along resonance cones i DL s in space will emit characteristic radiation

18 Last slide Electrostatic Electromagnetic Whistlers