Confinement of pure electron plasmas in the CNT stellarator

Transcription

1 Confinement of pure electron plasmas in the CNT stellarator Thomas Sunn Pedersen CNT Columbia University In the City of New York

2 Overview Background/introductory remarks CNT s magnetic topology (a stellarator) Why study non-neutral plasmas in a stellarator? Basics of CNT operation: How we create pure electron plasmas, how we diagnose them, and typical plasma parameters Confinement studies in CNT Neoclassical predictions/expectations Transport studies: Transport is driven by rods and neutrals Rod driven transport is understood Neutral driven transport indicates unconfined orbits Numerical investigation of single particle orbits in CNT Recent experimental results: Improved confinement Conclusion

3 Stellarator magnetic surface (field lines next slide)

4 CNT s magnetic topology: A stellarator

5 CNT s magnetic topology

6 CNT s magnetic topology The nested magnetic surfaces of the Columbia Non-neutral Torus

7 CNT is the simplest stellarator ever built 1 Gourdon et al., Plas. Phys. Contrl. Nucl. Fus. Research p. 849 (1969) 2 Pedersen et al., Fusion Sci. Tech. 46 p 200 (2004)

8 CNT: First plasma Nov 2004

9 Leaking in a bit of air allows 3-D surface visualizations Electron beam at 200 ev, background gas is air at ~2*10-5 Torr

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11 Why study non-neutral plasmas in a stellarator? There are unique properties relative to pure electron plasmas in other magnetic field configurations Equilibrium is a minimum energy state (contrary to Penning and pure toroidal field traps) as a result of the stellarator topology Stability properties are different We can confine and study plasmas at any degree of neutralization all the way from pure electron to quasineutral We may be able to create, confine, and study electron-positron plasmas The transport properties of these plasmas are of interest to the fusion community Neoclassical transport in the regime of extreme electric fields a Pedersen and Boozer, PRL 88, (2002) b Boozer, Phys. Plasmas 11, p (2004) c Lefrancois et al, Phys. Plasmas 12, p. (2005) d Pedersen et al., J. Phys. B 36, p (2003)

12 Creation of pure electron plasmas Electron source: Thermionic emission from heated tungsten filament How does one fill the volume of the stellarator with low temperature electron plasma? Parallel transport fills field line on axis in ~ 1 µs Perpendicular transport fills the rest of the surfaces Reach steady state between emission and radial losses Confinement time is electron inventory divided by emission (injection) current

13 Typical plasma parameters Temperature: 2-7 ev in central region Density: ~10 12 m -3 Debye length ~ 1.5 cm Satisfies plasma criterion: CNT minor radius is ~15 cm Ion density <1% of electron density at base pressure Essentially pure electron plasma 2006: Confinement times up to 20 msec Stable equilibria - otherwise confinement would be <<1 msec Confinement still much lower than theoretical predictions This talk: Understanding of transport much improved - and so is the confinement time! J. P. Kremer et al., Phys Rev Letters (2006)

14 Stellarator neoclassical transport can be studied in CNT Fusion stellarator plasmas develop a (modest) net negative charge and a resulting negative potential if ions are poorly confined magnetically ( ion root ). This ambipolar potential gives eφ/t ~ 1, whereas in CNT s pure electron plasmas, it s eφ/t ~10-50 This leads to modest improvements in confinement (for both species) in fusion plasmas and should lead to significant confinement improvement for CNT This is essentially because the ExB drift dominates over grad B and curvature drifts (similar to LNT confinement) v ExB v B φ /B (T e B /eb 2 ) eφ T e ~ 10-50, CNT ~ 1, QNP (fusion stel.) T. Sunn Pedersen and A. H. Boozer, Phys Rev Letters (2002)

15 Particle confinement in a classical stellarator can be poor Without an electric field: Magnetically trapped particles, a sizable fraction, drift out very quickly: τ Drift a/v B ~ 10 5 sec Passing particles are confined - but it takes only one collision to turn a passing particle into a trapped particle τ p τ C ~ 10 2 sec C N T

16 Orbits are closed due to ExB drift With strong E-field: Poloidal ExB closes orbits of magnetically trapped particles C N T

17 Confinement should be enhanced due to ExB drift Assuming that the diffusive (density gradient driven random walk) transport dominates, one expects 1 that τ p ~τ c (a/λ D ) 4 τ c ~ 10 msec (10-8 Torr) A typical value of (a/ λ D ) is 10 So τ p could be 100 seconds! With strong E-field: Poloidal ExB closes orbits of magnetically trapped particles - small deviations However, random walk diffusion is not the dominant collisional transport process when the electric field is strong. The electric field directly causes convective transport 2 which scales as τ p ~τ c (a/λ D ) 2 ie about 1 second confinement should be seen C N T 1 Pedersen and Boozer, PRL (2002) 2. Berkery and Boozer, Phys. Plasmas (2007)

18 C N T Experimental measurements of confinement and transport

19 Insulated rods drive transport Two rods gives twice as much transport as one rod The rods are insulating, so they are not steady state sinks for electrons They are large electrostatic perturbations - drives ExB transport J. P. Kremer et al., Phys Rev Letters (2006)

20 Insulated rods limit confinement Insulated rods charge up negative relative to plasma to self-shield Resulting ExB drift pattern convects particles along the rod all the way to the open field lines We made a quantitative model of the rod induced ExB transport, including Debye shielding of the negatively charged rod. Very close to the rod, the transport is low (no density) and far away, it s also low (Debye screened E-field) J. W. Berkery et al., Phys. Plasmas (2007)

21 Comparison to experimental data 1/B dependence Voltage dependence Good quantitative and excellent qualitative agreement between experiment and rod model J. W. Berkery et al., Phys. Plasmas (2007)

22 Neutrals also degrade confinement

23 Rod driven and neutral driven transport is separated

24 Neutral collision driven transport The neutral driven transport is much greater than anticipated We lose an electron after ~ 1 electron-neutral collisions This transport has a component that is independent of B and a component that scales as B the second scaling has been observed in other non-neutral experiments 1,2 and may be linked to trapped particle losses. The B-field independent losses are also consistent with bad orbits 1. Stoneking et al 2. Kabantsev et al.

25 Numerical analysis of electron orbits in CNT Given these experimental findings, we should investigate the particle orbits in CNT in much more detail CNT student Benoit Durand de Gevigney has developed a code that calculates the particle orbits in CNT

26 1: No electric field (single particle - no space charge) Magnetically trapped particles expected to drift out - CNT is not optimized Single particle orbit movie:

27 1: No electric field (single particle - no space charge) Magnetically trapped particles expected to drift out - CNT is not optimized Statistics: Start ev particles out and follow them

28 2: Strong electrostatic potential conforming to flux surfaces Expectation: Excellent orbits, forced to rotate poloidally by ExB Movie of single particle orbit

29 2: Strong electrostatic potential conforming to flux surfaces Expectation: Excellent orbits, forced to rotate poloidally by ExB

30 3: Electrostatic potential varying on magnetic surfaces Until fall 2007, and for all the experiments discussed so far, the internal coil vacuum jackets and the vacuum chamber were the electrostatic boundary condition for our plasmas (grounded). We model this complicated boundary condition (crudely) in our 3D equilibrium code. This gives us the full 3D electrostatic potential - showing rather large variation on a magnetic surface. That is input into orbit follower.

31 3: Electrostatic potential varying on magnetic surfaces

32 3: Electrostatic potential varying on magnetic surfaces Is there a substantial fraction of such unconfined orbits? Yes, this is a big problem on the outer surfaces (small problem on the innner surfaces)

33 Intuitive picture of collisionless loss orbits with E ExB (perpendicular motion) carries the electron along the electrostatic potential contour The parallel motion of passing electrons (combined with rotational transform) carries the electrons along the magnetic surface, moving them poloidally By switching between potential contours and magnetic surfaces, particles can make enormous radial excursions See Benoit Durand de Gevigney s poster for more transport modeling results - including comparisons with experimental results

34 Flux surface conforming electrostatic boundary With a flux surface conforming electrostatic boundary condition, we expect to see significantly improved confinement Installed summer 2007 Proper alignment with magnetic surfaces was a slow process 13 individual copper sections Can be used as a capacitive probe too (work in progress) Meshes can be biased individually to perturb the plasma

35 Results with new mesh installed 190 ms Order of magnitude improvement! Rod driven transport is significantly lower now (factor of 6) We also increased our maximum operating B-field to 0.15 T from 0.1 T What about the neutral driven transport? (We expected that to be improved) See Paul Brenners poster for more on these experimental confinement results

36 Results with new mesh installed 190 ms The picture is not yet clear regarding neutral collisions. A preliminary conclusion: The neutral driven transport is strongly reduced but primarily due to reduction in the neutral pressure (neutral collision frequency). It looks like electrons are still lost after order unity collisions. See Brenner s poster for more results.

37 Transport Jumps When operating with conditions that increase radial transport significantly (low B, high bias voltage, high neutral pressure, several rods) we observe abrupt transport jumps These can be as large as a factor of 2 They decrease confinement time by about that much See Michael Hahn s and Paul Brenner s posters for more on these observations

38 Electron-positron plasmas in CNT? Unique and relatively simple plasma physics due to perfect mass symmetry: How? No ion acoustic waves Other wave types collapse into just a couple of waves The hydrogen atom of plasma physics Have not been created yet on Earth Inject positrons into an initially pure electron plasma, neutralizing it First, create a pure electron plasma target : Cold (small Debye length) Well confined No internal material objects Plasma stays around after injection and retraction Develop instantaneous source of ~10 12 of cold positrons (C. Surko, J. Danielson et al., UCSD, work in progress) Develop injection method with >10% efficiency (we have ideas)

39 Summary Non-neutral plasmas on magnetic surfaces: Unique physics Largely unexplored territory Radial transport: Driven by rods, and neutrals Rod transport well understood Evidence of poor orbits Confinement has been improved to 190 msec (previous record: 20 msec) Mostly due to reduction in rod driven transport, strangely We still have plenty of mysteries Electron-positron plasmas: We are making progress

40 Acknowledgements CNT group CNT undergraduates John W. Berkery Naveed Ahmad Allen Boozer Sarah Angelini Quinn Marksteiner Charles Biddle-Snead Michael Hahn Dennis Boyle Benoit Durand de Gevigney Joanna Corby Paul Brenner Paul Ennever Xabier Sarasola Martin Avi Grumet Remi Lefrancois Stacey Hirsh Jason Kremer Elliot Kaplan PPPL (CNT design support) Mark Kendall Neil Pomphrey Josh Narciso Wayne Reiersen Mike Shulman Fred Dahlgren Katherine Velas Kyoto Inst. Tech. (and more) Haru Himura More CNT results: Upcoming talk and poster session