Large Plasma Device (LAPD)

Transcription

1 Large Plasma Device (LAPD) Over 450 Access ports Computer Controlled Data Acquisition Microwave Interferometers Laser Induced Fluorescence DC Magnetic Field: kg, variable on axis Highly Ionized plasmas n 5 X10 12 cm 3 Plasma column up to 2000R ci across diameter Large variety of probes Reproducable 1Hz operation Now a user facility

2 . Discharge current 12 ka Discharge power 0.54 MW

3 Past- Relevant Experiments 1) Whistler wave interaction with a density striation 2) Lower Hybrid wave interaction with a density striation 3) Alfvén wave MASER 4) Alfvénic Field Line Resonances 5) Interactions with a Dense Exploding Plasma 6) Alfvén Wave Generation by Resonant Absorption 7) Turbulence in Narrow Current Channels 8) Wave Propagation in Mirror Configurations 3

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9 Lower Hybrid Waves incident on a density striation

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12 Alfvén Wave Maser

13 Examples of Geomagnetic Alfvén Wave Masers B 0 Parameter Gradient Ionosphere A l f v é n Alfvén Resonator Eart h Convection Driver Ionosphere Ionosphere Earth R e s o n a t o r B 0 Kinetic Ion Driver Ionospheric Resonator Magnetospheric Resonator

14 Plasma Source Region Essential Ingredients of a Maser Resonant Cavity Partially Transmitting Boundary Mesh Anode Reflecting Boundary Propagation Region Cathode Amplifying Medium E&M Mode Electron Drift Velocity Shear Alfvén Wave Uniform Plasma Column

15 MASER SOURCE REGION Cathode half Anode Region between anode and cathode acts as a resonant cavity full Half wave length and full wave length resonant modes illustrated Alfvén wave is amplified here Amplified Alfvén wave observed here LAPDU Plasma Device

16 Plasma without Maser Present Discharge current 3 ka Shear Alfvén wave frequency spectrum is broadband, with a peak at about.6 f ci. This peak corresponds to the maser frequency Magnetic field Time (ms) Log Spec. Amp. (db) Peak at about.6 f ci Normalized frequency (f/f ci )

17 Discharge Current Magnetic field Maser Temporal development 3 ms 6.5 ka 1. Maser action, indicated by flaring magnetic field, first occurs as discharge current is increasing. Plasma column is small and current density high 2. Maser reaches near steady state conditions as current levels off, and plasma column reaches full width 3. Final magnetic field signal is very coherent

18 Alfvén wave maser well above threshold - steady state Discharge Current 6.5 ka Fluctuating magnetic field 3 ms Essential Ingredients of a Maser Plasma Source Region RESONANT CAVITY Mesh Anode Partially reflecting boundary Maser is steady state Cathode AMPLIFYING MEDIUM Electron Drift EM wave Propagation Medium Shear Alfvén Wave Plasma Column Maser signal is very coherent Ingredient of the LAPD maser

19 FLRs - Experimental Setup Input signal is one half cycle of a sine wave with frequency f ci /2 LAPD plasma device B 0 Input signal Antenna Antenna couples azimuthal magnetic flux B-fields are observed here B-fields are measured in planes across B 0 using triaxial, differentially-wound, induction loop (B-dot) coils

20 Observed Field Line Resonances Received magnetic field signal FFT Time (µsec) n = Wavelet Source power spectrum Magnetic field power spectrum Normalized frequency.5 n=2 Frequency f=f ci Time ( µsec)

21 Shear Alfvén wave propagation in a parallel beta gradient Axial magnetic field, B 0, increases from 500 G at cathode to 1500 G at end of plasma column. Kinetic regime Inertial regime Alfven wave launched.94 m Wave observed 6.6 m Log 10 beta-bar Inertial regime Kinetic regime Distance from cathode (m) Plasma density decreases from 3.0 x cm -3 to 1.5 x cm -3, and electron temperature decreases from 5 ev to.5 ev from the cathode to the end of the plasma column. This combination of parameters leads to a three order of magnitude change in betabar d β dz = 100 m 1 β = M m β = v 2 e 2 V A

22 Iso-surface of B-perp z = 6.6 m Inertial Regime ω k >> v e z = 5.0 m z = 4.1 m Kinetic Regime ω k << v e z = 1.6 m Z =.94 m Data planes Increasing B 0 Decreasing T e Launching antenna Decreasing n e

23 Generation of a Dense plasma (by laser irradiaition) in a background magnetoplasma

24 Excluded Magnetic Field B0 = 1.5 kg δz = 2 cm τ = 0.38 µs Neon E lpp R bubble = 3µ 0 2 π B 0 1/3 4 cm v = cm / s v = cm / s Target (x=0) τ bubble 2R b v = 0.7 µs

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26 Intense Microwave interactions at plasma resonance locations 27

27 Fast electron generation by microwaves Schematic Cartoon of Problem Resonant Layer ω 0 = ω pe (x) x Alfvenic signal Alfvenic Signal N 0 z Confinement Magnetic Field High-Power RF ω Ω i Plasma Edge Horn or Antenna Radiates ω 0 B. Van Compernolle Phd->Brussels W. Gekelman P. Pribyl G. Morales Related to Ionospheric Modification Experiments 28

28 Fast Electrons (x = -20 cm, y = 2 cm) v de = 3 x 10 8 cm/s dn e /n 0 = 10-4 Wave oscillations Fast electron peak

29 Self-consistent Low-Frequency Features Downstream from Exciter Alfvenic B-field B y /B Parallel Electron Distribution Function f(u z ) x (c/ω pe ) Parallel Current Density J z (q e n c q e ) to B x (c/ω pe ) 0.1 fast tail 0.01 current u z (V A )

30 An Alfvén Wavepacket is Excited by the Nonlinear O-mode Pulse Space-time (z,t) history of transverse magnetic field B y (x,t) at a fixed transverse position x shows a field-aligned structure that propagates with Alfvén speed, V A, away from exciter Time Slope =V A Z, B 0

31 Fast Electrons

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33 Alfven Wavepacket Excited in Experiment by O-Mode Pulse B 0 Measured snapshot of wave magnetic field at several axial positions away from O-mode beam injection O-mode injection here

34 Turbulence in Narrow Current Channels

35 Geometry and data collection capacitors Fixed magnetic probe Current sheet antenna δx =2mm Transistor switch Setup Current = 70A Voltage = 75 V He, B=500G 1.5 kg Fixed flow probe Photograph T shutter = 1 µs View down axis of machine Movable flow probe

36 LIF system Dye Laser 12 MW out 320nm-690 nm YAG pump laser 50 MW 10ns 500 nm Pulse stretcher 10ns-> 100ns

37 Density Density Perturbation as seen with Laser Induced Fluorescence

38 T=690µsec Parallel Ion Flow in a Perpendicular Plane Time of peak flow Mach number y (cm) R ci = 3.29 mm δ = 3.8 mm M = 1 ln 2 I Sat Upstrem I Sat Downstream x (cm)

39 Density plane as a function of time Each location is an average over 20 plasma discharges Watch for the repeated filling in of the density depression 10cm

40 Alfvén wave propagation in multiple magnetic mirrors

41 Mirror Array Configuration in Well Diagnosed LAPD Plasma South North rectangular loop Various mirror array configurations are powered by 10 independent magnet power supplies; B 0 ~ 0.5 kg 2.0 kg. Helium plasma column density FWHM ~0.60 m, 17 m length (1 shot per second cathode discharge) Microwave interferometers for column plasma density calibration (port 23) (n peak 1 x /cm 3 ) Triple probes for local Te, n i, V f measurements (port 13, 15, 19, 35) SAW antennas: small disk (p51); rectangular loop (p47) copper rod (p46 to p49); B-dot probes for local B SAW measurements (port 14, 16, 18, 20, 36, 38)

42 Computation Setup With beach beach small Br approximation is used in the beach section. The beach section is set flatter than experiment. The effective ion collision frequency is introduced in the beach to resolved ion cyclotron resonance excitation.

43 Closer Look: Simulation Results and Experiment Data B 0 Antenna Port 14 Port 36

44 Some Planned Experiments for the Muri project: 1) Creation of Mirror-Trapped Electron Populations 2) Interaction of Trapped Electrons with Rotating Magnetic Fields, Alfvén Waves, Whistler Waves, Lower Hybrid Waves 3) Measurement of Antenna Radiation Patterns 4) Wave particle Interactions in the Presence of Fast Ions 46

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46 two independent coils 4 turns each 50<f<300 khz B = G Rf current A

47 Antennas, Probes

48 Ion beam (under-development) 25 kv, 3 Amps, He

49 future magnetic probes

50 E field detectors smaller than D ~30µm Development of Microprobes collaboration with UCLA Engineering (Jack Judy), LANL Velocity Analyzer 2 grids, collector, can be made 100µ X 100µ 10µm tips, 20µm spacing, 60 µm-gap 2 Publications RSI, JMEMS