The Quest for Questions: Reflective Summary of Astrophysical Discussions

1. The quest for questions: a reflective summary of the discussion Monday:edited during Tues. discussion R. Paul Drake University of Michigan Rice Workshop May 2007

2. The approach we developed • Find compelling astrophysical questions • Identify underlying issues that can be addressed in laboratory experiments • Discuss the potential contribution of experiments in this context

3. 1. • What is the role of antimatter in the most energetic events of the universe? • Gamma ray bursts • Pulsar dynamics • AGN jets • Underlying issue • What happens when electron-positron pair plasmas interact? • How do shocks in colliding pair plasmas behave? • Potential experiments • Jason Myatt described a design for Omega • Comment by RPD • This topic seems promising to me for formation of a collaborative team (with Rochester people) aimed long term at NIF

4. 2. • Why are jets spectacularly collimated over enormous distances? • Young Stellar Objects (perhaps not relevant) • Galactic jets • Underlying issues • Stability behavior of large currents carried by MHD plasmas over distances of many skin depths • Unique behavior of Poynting dominated jets • Potential experiments • Evolution of jet experiments with plasma guns and Z pinches (?)

5. 3. • How does tenuous plasma stop super-relativistic particles? • Pulsar winds • Gamma Ray Bursts • Underlying issues • Development and saturation of Weibel and particle acceleration in plasma • Radiation from Weibel-produced structures • Potential experiments • Weibel has been seen in relativistic lasers but not diagnosed in detail • Could do an experiment with beam in gas • nb ~ 1018 cm-3, ngas ~ 1019 cm-3, beam length ~ 300 µm to 3 mm, background skin depth ~ few µm, beam charge ~ 1 to 10 nC, use probe beam, Faraday rotation, emission • Potential facilities -- EP, Vulcan, ZR

6. 4. • How do shock waves produce cosmic rays? • Supernova remnant shocks • Modeling of SNR 1006 has failed • Termination shocks • Underlying issues • What is the electromagnetic structure of collisionless shocks? • How are particles heated from the thermal distribution so they can be “injected” into long-term acceleration? • How do such shocks amplify magnetic fields? • Potential experiments • Need facility that can do high beta and high Alfven Mach number with small collisions and strong magnetization of the ions • This is feasible

7. 5. This question needs work or to drop • How does turbulence dissipate energy in astrophysical plasmas? • An issue from the solar wind to intergalactic space • Accretion disks • Underlying issues • What is the nature of MHD turbulence in space and astrophysical plasmas? • How is energy in magnetic fields delivered to particles? • Away from stars, where does the magnetic energy come from? • The nature of the heating/acceleration produced by magnetic reconnection? • Potential experiments • Need facility that can do high beta and high Alfven Mach number with small collisions and strong magnetization of the ions • This is feasible • Next-generation (less collisional, I think) reconnection experiment

8. 6. Poynting dominated flows somehow • Maxim?

9. There seems to be a trend here • Need facility that can do high beta and high Alfven Mach number with small collisions and strong magnetization of the ions • A “thinking big” wish list • Vacuum chamber of 50 to 100 cubic meters • Flexibly configurable internal coil set • Substantial auxiliary heating • Two 5 kJ ns laser beams (to launch shocks and photoionize) • A (10-100 ps) petawatt laser (to provide relativistic particles) • This could be a community user facility • It would revolutionize the study of high-beta plasma dynamics • Much of its research would have astrophysical relevance With the addition of internal coils, LAPD might evolve in this direction • UCLA Proposal parameters • D/rLi ~ 100 • b ~ 5 • MA ~ 2 (vA = 63 km/s) • v Shock ~ 120 km/s • THe > 100 eV postshock • B ~ 100 G in shock region

10. Things that did not fit and notes from discussion • It is interesting for many astrophysical systems to get • B2/ec2 > 1 and R pe/c >> 1 • Large “” and 10-100 skin depths • Dusty plasmas • Unifying idea: nonlocality in collisionless systems • Possible question: what’s the difference between collisional and collisionless systems? • Results depend strongly on boundary conditions • Lab experiments could explore • Example: voltage source vs current source vs resistive power dissipation

11. Notes from discussion • White paper needs to specify definite system applications • An example from HED lab astro should be in white paper … • Magnetically dominated plasmas -- not clear to me what can be done. • Edison: assign discussion leaders.