Large Plasma Device (LAPD) Over 450 Access ports Computer Controlled Data Acquisition Microwave Interferometers Laser Induced Fluorescence DC Magnetic Field: 0.05-4 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
. Discharge current 12 ka Discharge power 0.54 MW
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
Lower Hybrid Waves incident on a density striation
Alfvén Wave Maser
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
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
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
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 2 6 10 Time (ms) 6.2 6.4 Log Spec. Amp. (db) -30-40 Peak at about.6 f ci.2.6 1.0 Normalized frequency (f/f ci )
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
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
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
Observed Field Line Resonances Received magnetic field signal FFT 20 40 60 80 Time (µsec) n = 2 4 6 8 1.0 Wavelet Source power spectrum Magnetic field power spectrum Normalized frequency.5 n=2 Frequency f=f ci 0. 20. 40. 60. Time ( µsec)
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 1.0 0.0-1.0-2.0 2.0 Inertial regime Kinetic regime 4.0 6.0 8.0 10.0 Distance from cathode (m) Plasma density decreases from 3.0 x 10 12 cm -3 to 1.5 x 10 11 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
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
Generation of a Dense plasma (by laser irradiaition) in a background magnetoplasma
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 = 1.4 10 7 cm / s v = 1.1 10 7 cm / s Target (x=0) τ bubble 2R b v = 0.7 µs
Intense Microwave interactions at plasma resonance locations 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
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
Self-consistent Low-Frequency Features Downstream from Exciter Alfvenic B-field 0.05 0.04 B y /B 0 0.03 0.02 0.01 0-0.01 100 Parallel Electron Distribution Function f(u z ) -0.02 30 35 40 45 x (c/ω pe ) 10 0.006 Parallel Current Density J z (q e n c q e ) to B 0 1 0.004 0.002 0-0.002-0.004-0.006 30 35 40 45 x (c/ω pe ) 0.1 fast tail 0.01 current -1-0.5 0 0.5 1 u z (V A )
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
Fast Electrons
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
Turbulence in Narrow Current Channels
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
LIF system Dye Laser 12 MW out 320nm-690 nm YAG pump laser 50 MW 10ns 500 nm Pulse stretcher 10ns-> 100ns
Density Density Perturbation as seen with Laser Induced Fluorescence
T=690µsec Parallel Ion Flow in a Perpendicular Plane Time of peak flow +0.05 Mach number y (cm) -0.25 R ci = 3.29 mm δ = 3.8 mm M = 1 ln 2 I Sat Upstrem I Sat Downstream x (cm)
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
Alfvén wave propagation in multiple magnetic mirrors
Mirror Array Configuration in Well Diagnosed LAPD Plasma South 23 46 49 51 North 14 16 18 20 36 38 rectangular loop 13 15 19 35 47 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 10 12 /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)
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.
Closer Look: Simulation Results and Experiment Data B 0 Antenna Port 14 Port 36
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
two independent coils 4 turns each 50<f<300 khz B = 80-100 G Rf current 100-300 A
Antennas, Probes
Ion beam (under-development) 25 kv, 3 Amps, He
future magnetic probes
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