Review of Irvine FRC

1. Review of Irvine FRC Presentation Order Erik Trask Tommy Roche Wayne Harris Topics Covered Machine overview and density diagnostics Magnetic mapping and Equilibrium comparison Neutral Particle diagnostic and spectroscopy

3. Welcome to the Lab The machine that we work on is the first machine built and used by TriAlpha

4. Experimental Layout

5. Relevant physical parameters Peak density ~5 x 1013 cm-3 Max reversed magnetic field in plasma 250 Gauss Temperatures Ions ~ 2 eV Electrons ~ 2 eV Ion Rotational Energy ~ 25 eV Wayne Harris will give more details

6. The nuts and bolts Vacuum system Pumps and monitors Energy Storage and Delivery Capacitor Banks and Field Coils Data Acquisition Hardware and Software Probes e.g. B-dot loops, langmuir probes, interferometer, etc.

7. Pumping system is adequate Two pumps VHS-6 and -10, backed with Welch 1397s Large pump was cleaned within the year Base pressure is ~ 10-6 Torr Improved from ~10-5 Torr after cleaning New Varian Multigauge controller 4 Convectorr gauges – Atmosphere to 1 mTorr 1 Bayard-Alpert ionization gauge Setpoint controller for interlocks

8. Energy storage parameters Upgraded Limiter bank from 900 volt max to 2700 volt max 2 x 6 array of 7700 uF capacitors Diode crowbar Shortened the rise time by lowering stray inductances Improved Limiter field shape by changing transmission lines Improved sightline access by spacing limiter straps Old length was ~ 45 cm. Strap spacing was ~1 – 3 cm New length is 60 cm with 5 cm spacing between straps

9. Limiter Coil Improvements Old configuration had both positive and negative transmission line connections at the south end of the set of limiter coils New transmission lines are in a twisted pair topology Path lengths are more balanced Spacing between limiter coil straps is larger to allow for unobstructed sightlines from our access ports

10. Old Limiter Configuration As you can see, strap spacing is uneven Sightlines from access ports were obstructed

11. New limiter looks nicer… Precise 5 cm spacing between straps Sightlines from all twelve access ports are clear!

12. …and works better too! Less asymmetry is observed in our magnetic signals Due to balanced path lengths for current flow Lower impedance is evident in new configuration Due to less stray inductance in the transmission lines

13. We can acquire many signals quickly 31 scopes are available for signal capture 27 BitScopes: 8 bit, 40 MHz peak sampling rate Courtesy of Alan and others at TriAlpha 3 Tektronix scopes: 8 bit, up to 1 GS/s 1 ZTEC 2 channel scope: 12 bit, 200 MS/s Acquisition software is Lab2000 Again, courtesy of Alan (Thanks!) Analysis tools are written with IDL Tommy Roche will give many examples

14. B-dots are a basic tool Four arrays 2 axial arrays Sensitive to only Bz 2 radial arrays 3 orthogonal loops, measuring Br, B?, Bz Calibrated in test coils Software correction orthogonalizes the data 2 dimensional recon- struction: Tommy Roche

15. Rogowski: Plasma current monitor Coil is placed to measure currents induced in the plasma Axial limits are the mirror coils (± 25 cm) Radial limits are the flux coil (10cm) and limiter coil (37 cm) Current waveforms depend strongly on mirror configuration Three configurations of mirror coil Mirror: Difficult to use and creates axially short plasmas Open: Axially long plasmas Cusp: Axially long plasmas and flat current profile – More stable?

16. 4th generation Langmuir probes Triple probes earlier were focus of effort Quiescent plasma are easily diagnosed FRCs are NOT quiescent! Inferred ne and Te signals were negative Cause of discrepancies - large floating potential fluctuations? Swings of ~100 V in 10 microseconds have been measured. Emphasis is now on floating double probes This topology allows the probes to follow the floating potential more closely, reducing common mode pick up.

17. Double probes results more consistent Ion saturation currents are coupled across an isolation transformer Can we use a double probe to find the temperature? Common mode signals give a measurement of the floating potential Capacitive coupling from primary to secondary of transformer Is DC coupling better? Can we also measure the high side of the bias voltage to find the temperature?

18. Two langmuir probe topologies

19. No single peak in radial Isat profile Peak densities of ~1013 cm-3 Assumes Te ~ 2 eV Profiles are lumpy More work needs to be done still Swept double probe?

20. Interferometer results are puzzling 140 GHz system Cutoff ne~2.514 cm-3 Langmuir probe data indicates densities below cutoff For Te~2 eV Interferometer data seems to show cutoff, even for just plasma and no reversal

21. Field reversal causes more cut off Interferometer signal looks cut off Line density can only be trace back to 3.5 x 1012 cm-3

22. Why is interferometer cut off? Three main possibilities Local densities could be higher than 2.5 x 1014 cm-3 Unlikely, given the Isat data Poor sightlines (old limiter configuration) do not allow full signal measurement Time changes in the line density could put the demodulated signal out of band If the bandwidth is 10 MHz, the fastest change is 20? radians per microsecond This corresponds to a density change of 6 x 1012 cm-3 per microsecond for a constant path length

23. Goal is measurement of LH wave propagation through the FRC First antenna is built Terminated-Folded Dipole Loop antenna with small loop area Resistive termination Creates large E? and small Bz RF probe built and in testing Two dipoles, each 1 cm long Orthogonal to each other Will look at E? and Ez Lower hybrid experiment in progress

24. Must measure ne and Te better Future work will include: Better Langmuir probe electronics Measurement of the electron temperature Resolution of interferometer problems Lower hybrid wave measurements

25. Main systems functional Foundation of experiment is complete Background density ~ 10-3 x ne Data acquisition capabilities are sufficient Magnetic field structures well known Density diagnostics are coming on line