Bringing the Sun down to EarthEve StensonAdvisor:Paul BellanCalifornia Instituteof TechnologyExploring the physics of plasma loopsUniversity of Idaho13 February 2009solar.physics.montana.edu/max_millennium/Outline1.) Crash course in plasma physics: Introduction to the fourth state of matter• Different types of plasma with which you may or may not be familiar• Important parameters and fundamental concepts2.) Plasma loops in the laboratory• Motivation for this avenue of investigation• How the experiment works3.) Testing the “Gobble Theory”• Quantitatively testing a proposed bulk flow model4.) Applying a simple system to the wider world• Can these results explain features in the solar corona?• More exciting avenues of investigationA few terrestrial plasmas:“More than 99% of known matter in the universe is plasma.”E.g., stars, solar wind, interstellar medium, astrophysical jets . . .High fields/currents ionize atmospheric gases.Charged particles from the sun collide with Earth’s ionosphere, exciting electrons.UV emission from noble gas plasma excites phosphors in plasma TVs.Glow discharge in a sputtererViews of the inside of JET (Joint European Torus), the world’s largest tokamak to date, with and without plasma.At the National Ignition Facility, lasers will ionize and compress a microcapsule of fuel, causing it to fuse.Plasmas cover many orders of magnitude*A plasma’s temperature may not be well defined!• “Average random speed”• 1 eV = 11,605 K• In some cases, there aren’t enough collisions to make the velocity distribution Maxwellian• Ions and electrons often have different temperatures• There can be directional heating, leading to anisotropic temperature*A few important conceptsMoving, charged particles ÅÆ Electromagnetic fieldsGoing from single particles to a “magnetized fluid:”BIBAlthough it is more accurate to think of plasmas in terms of a distribution of particle locations and velocities, this method can be unintuitive and computationally intense.By doing “weighted averages” of very detailed information, one arrives at magnetohydronamics (MHD), a description of the plasma in terms of density, pressure, and average velocity.Ideal MHD is the limit in which the magnetic flux is “frozen” into the plasma. The plasma can then be thought of in terms of magnetic structures, such as “flux tubes.”A few more conceptsquasineutralityWith the exception non-neutral plasmas (e.g., pure electron plasmas), there will typically be as many ions as electrons in any given chunk.ionizationPlasmas can be anywhere from very slightly ionized to fully ionized, and may be ionized by photons, applied voltages, induced fields, sources of electrons/ions . . .“dust”Microscopic particles mixed in with a plasma have lots of charge and lots of mass.magnetically or kinetically dominated?The dimensionless parameter β is a ratio regular kinetic pressure (proportional to density and v2) to magnetic “pressure” (proportional to B2).collision length (or rate)The distance a particle travels between collisions can be compared to other relevant distances, such as the scale of the system and the size of the cyclotron orbit.wavesThere are ions, electrons, and fields that can all oscillate, making plasmas very rich in waves.Why do experiments?Experiment can simulate computation: Resolves all scales, includes all correlations, includes all MHD and kinetic effects, ‘CPU time’ < 1 second~Stewart Prager (Wisconsin, PPPL)There are many complementary approaches to elucidating why plasmas (both natural and artificial) do the things they do:• simulations (many different codes, all have benefits and limitations)• analytic theory (approximations must be made)• observation of natural phenomena (telescopes, spacecraft)• large-scale experiments (confinement, alternative confinement)• small-scale experiments (e.g., Bellan Plasma Group)“”Past work from the Bellan grouploop with kinetic jetstandard loopS. K. P. Tripathi, P. M. Bellan, and G. S. Yun, Observation of Kinetic Plasma Jets in a Coronal-Loop Simulation Experiment, Phys. Rev. Letters 98, Art. No. 135002 (2007)Kinking as a mechanism for “closing” spheromaksThe discovery of a kinetic instabilityAn MHD theory of high-speed bulk flowsIf the azimuthal magnetic field isn’t strong enough “wrong way” ions (moving in the bulk flow) can become unstable and fly out of the flux tube.S. You, G. S. Yun, and P. M. Bellan, Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation, Phys. Rev. Letters 95, 045002 (2005).It had been shown that the self-organization process had to be 3D, and the process was unknown.P. M. Bellan, Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result, Phys. Plasmas 10 Pt 2, 1999 (2003)This “quasi-equilibrium” theory was proposed to explain why both laboratory and solar loops exhibit surprisingly uniform axial cross-sections.Experimental set-up: the chamberTop view, showing camera angle with respect to the plasma gun:29˚3D modeling by Gunsu Yun.This is a pulsed power experiment; our plasmas have a lifetime of 3-10 microseconds.Besides the copper electrodes (the “flux conserver”) of the plasma gun, all chamber walls are far enough away to have a negligible effect on the plasma.Experiment set-up: the plasma “gun”1.) Electromagnetically generated vacuum field, up to 0.3 T2.) Fast gas valves inject H2, N2, or Ar at the footpoints3.) 3-6 kV, applied to the electrodes, ionizes the gas and drives a 40-80 kA currentThe same gun can be used to create a single, bicolor loop of plasma (as shown here), or to create two adjacent loops of plasma, each made of a different gas.Experiment set-up: the plasma “gun”The “shot”:a typical hydrogen plasma loop.3 μs1.8 μs3.0 μsImaging and more:Short (16-frame) movies are taken of each plasma shot:• Camera: Imacon 200 (DRS Technologies)• Typical repetition rate: 200-600 ns• Exposure length: 20-100 nsOther diagnostics:• A magnetic probe array, located on an adjustable arm inside the chamber, can be used to measure the magnetic field in any region of the plasma. The voltage induced in small coils of wire is integrated to get B.• A photodiode array (also mounted on a moveable arm) can be used to measure x-ray emission• The voltage and current through the entire plasma are also recordedDual-gas plasmas:By supplying more than one neutral gases to the footpoints,