generalfusion Characterization of General Fusion's Plasma Devices 2015 Nimrod Summer Workshop

Transcription

1 Characterization of General Fusion's Plasma Devices 2015 Nimrod Summer Workshop Aaron Froese, Charlson Kim, Meritt Reynolds, Sandra Barsky, Victoria Suponitsky, Stephen Howard, Russ Ivanov, Peter O'Shea, Pat Carle, Michel Laberge 1/39

2 Outline Overview of General Fusion's Research Magnetized Target Fusion (MTF) Concept System Acoustic drivers Full-scale plasma injectors Field experiments (MRT family) Magnetic compression experiment (SMRT) Future plans Nimrod Applications Comparison of flux conserver geometries Poloidal flux boundary conditions Magnetic compression BCs (Charlson Kim) Summary 2/39

3 General Fusion Concept System Magnetized Target Fusion (MTF): Magnetized plasma is compressed inertially with a metal wall. 3/39

4 Design Advantages Plasma Injector 4x radial compression before injection If adiabatic compression: 1x1014 cm-3 6x1015 cm-3 40 ev >600 ev 0.2 T 3.2 T Lead-Lithium (PbLi) liner Liquid wall cannot be damaged and quickly reforms after each pulse. 2 MeV+ neutron flux to structure is 5 orders of magnitude lower than ITER 4π coverage, n,2n Pb reaction provides tritium breeding ratio of 1.5 Convenient heat extraction Low tritium solubility Acoustic drivers Steam pistons for that steampunk vibe Low cost, easily scalable 4/39

5 Acoustic Driver pressure contours from LS-Dyna simulation Uses power plant working fluid Low cost for high energy: <$0.2/Joule compared to >$2/Joule for pulsed power 100 kg piston moves at 50 m/s Piezoelectric brakes achieve better than 2us timing 5/39

6 Lead Vortex System - Minisphere 1m sphere with 14 full size drivers 15 ton molten Pb storage 100 kg/s MHD pumps Demonstrates vortex formation and collapse 6/39

7 OpenFOAM Simulation of Minisphere Material phase (left hemisphere), pressure (right) 7/39

8 Full-Scale Plasma Injectors Designed for use in concept reactor 2-stages: CT formation & Marshall gun acceleration 2.4 MJ (1.7) Marx banks 8/39

9 Future Modifications to Full-Scale Injectors Rapid convergence region followed by zero force section so that force on CT matches time profile of driving current INNOVATIVE NEW PLASMA INJECTOR New design for formation section to provide a poloidal buffer field Novel accelerator geometry allows 70% lower accelerator current at the tip New lithium gettering will dramatically improve wall effects on plasma. New diagnostics focusing on critical effects for plasma lifetime 9/39

10 Magnetized Ring Test (MRT) Device Direct formation: no acceleration stage. Comparable to CTX and SSPX designs. Lower maximum plasma density than large injectors (2e15 cm -3) Faster design iterations Size (R=10 cm, a=5.7 cm, λ=30) Gun flux (9 mwb) Designed for use in plasma compression (PCS) experiments 10/39

11 Plasma Compression Small (PCS) Tests Existing chalice geometry was designed to be easily compressible without rupturing. Above: Temperature and psi contours from VAC showing an n=1 mode that amplifies at high compression ratios Below: fast camera looking down on imploding flyer plate 11/39

12 PCS Devices (MRT & MrsT) MrsT has more containers for 2MJ capacitor banks. Argon-filled injectors can be transported and reassembled in less than 48 hours. 12/39

13 MRT Diagnostics Side View X-ray camera field of view 13/39

14 MRT Diagnostics Headplate Mirno v coils Mirnov coils 14/39

15 PCS9 Timeline Magnetic probes at R=26mm shown. MrT Shots 31614, X radial: us Slapdown: us MrT s31614 MrT s31663 RBFtotal_RBI316_Proc 1.2 2X radial: us 1.0 Fire signal sent to Veto: 140 us X radial: us 0.8 Tesla Fire signal passed by Veto: 190 us Wall moves: us Main Rogowski rise: 0 us 200 Sustain Fired: 50 us 0.2 5X radial: us µs 15/39

16 Plasma Confinement Progress Tesla Compre ssion Time 2.0 Feb 2013 Dec 2013 Poloidal Field February µs thermal life Self-heating to 290 ev May June µs Oct /39

17 Nimrod Stability Analysis Both PCS shots and SMRT shots tend to die from an n=1 instability (when q0 increases to nearly 1). This n=1 growth can sometimes be reproduced in 3D VAC simulations, but VAC is a shock-capturing code, so it has high numerical diffusivity. Charlson and I are trying to reproduce the same n=1 instability with Nimrod. I am scanning the linear stability for static geometries over beta and f2/f1 (current profile peakedness). Charlson has developed BCs for magnetic compression and is looking for dominant instabilities (esp. n=1) and robust initial conditions. We rarely see a strong n=1. The n=3 usually grows faster. 17/39

18 Linear Stability Details Beta and current profiles are specified by the standard Grad-Shafranov parameters. F=f0+f1(1-ψ)+f2ψ(ψ-1) μ0p(ψ)=p0+p1(1-ψ)+p2ψ(ψ-1) Shaft current is I0[MA]=5f0[T m]. f1 determined by poloidal flux, usually ψ=15 mwb. p1 determined by beta on axis. Negligible background pressure: p 0=0.01 Pressure profile always peaked: p 2/p1=0.25 Other parameters: elecd=10, iso_visc=5, par_visc=20, k_plle=1e4, k_perpi=10, ohms='mhd', nd_diff=1 18/39

19 β=10%, f2/f1=-0.1: Resistive Interchange Unstable mode is m/n=2/3. Growth rate (and stability) affected by resistance. Unstable mode n=3 Spatial parity indicates pressure-driven mode. m/n=2/3 odd radial parity m/n=2/3 even toroidal parity 19/39

20 β=0.1%, f2/f1=0.1: Internal Kink Mode Unstable mode is m/n=1/2. Fast growth rate that is not affected by resistance. Unstable modes n=2 Spatial parity indicates current-driven mode. m/n=1/2 even radial parity m/n=1/2 odd toroidal parity 20/39

21 Linear Stability Map Example Most unstable modes (n=1-7) shown for each point. Map for spheromak (low shaft current) in MRT geometry. 21/39

22 Linear Stability Map Example Most unstable modes (n=1-7) shown for each point. Map for spheromak (low shaft current) in MRT geometry. interchange resistive modes internal kink 22/39

23 Stability Improvements q Control One MRT variant has a separate circuit to control the shaft current and, therefore, the safety factor profile. Will be used for next PCS shot in September. Shaft current can be held above 1 MA for 1 ms. Current increases lifetime by factor of q0<1 q0>1 23/39

24 Stability Map for Chalice Geometry Constant q trajectory (I 0=250->600kA) in green 24/39

25 Stability Map for Compressed Chalice Larger coverage than previous map (inset box). No obvious n=1 instability on current trajectory. 25/39

26 Stability Improvements FC Geometry Current Geometry Cylindrical Compression Compressed plasma shape is reverse-d, quite unstable; technique was easier to implement New Geometry Spherical Compression Compression maintains D-shape of plasma. 26/39

27 Spherical Tokamak-Style Device Simulations predict good plasma stability Larger flux conserver (R=11.5cm, a=8cm, 36mWb gun flux) LS-Dyna simulation shows gas in pink and Al flyer in blue. 27/39

28 Stability Map for ST Geometry Constant q trajectory (I 0=250->1900kA) in green 28/39

29 Stability Map for Compressed ST Larger coverage than previous map (inset box). Very good stability along constant q trajectory. 29/39

30 SMRT Device (Magnetic Compression) Grad-Shafranov equilibria Intended to allow for better repeatability of PCS experiments. Six 1-turn coils with slot placement 120-degrees apart (6% n=3 error). Levitation of 15mWb CT requires 15kA current, max current 120kA. 30/39

31 SMRT Magnetic Compression Series of Nimeq equilibria showing effect of increasing current in compression coils. Stability does not change significantly with compression. Problem: Compression field has 25 us ramp time and will not penetrate far into the aluminum walls. 31/39

32 Poloidal Flux Boundary Conditions Similar to Corsica, I generate a set of toroidal currents that are displaced slightly outside the mesh. The current in each coil is optimized to produce the desired poloidal flux (here zero) at the mesh edge. Method not very robust. No native variable types or optimizers. Vacuum Field BCs Poloidal Flux BCs 32/39

33 Magnetic Compression Stability Maps Levitated without PFBCs 2x Levitation Current without PFBCs with PFBCs 33/39

34 Magnetic Compression Boundary Conditions 34/39

35 Magnetic Compression Sequence Hollow F (f /f =-0.006), initial axis is 14% kV over 30us compression +5us of relaxation. Initial temperature is 53 ev, final is 210 ev. 35/39

36 Magnetic Compression Nonlinear Stability n=1 amplitude is small until late into compression. growth rate too small to be dominant no sign of accelerating growth rate Could observed n=1 be due to offset in DC coils? n=0 n=1 36/39

37 Summary Research at General Fusion is steadily progressing for all components necessary to build a prototype system. No deal-breakers encountered yet. MTF compression is not fast enough to outrun ideal MHD instabilities: q-profile and FC geometry must be controlled. The ST-style geometry offers much better stability than the chalice geometry (but shaft current still necessary). Experiments and some VAC simulations die from n=1 instability. There is ongoing work to reproduce this behaviour in Nimrod. Improving the poloidal flux boundary condition code to use native variable types and optimizers. Magnetic compression E-field BC source functioning well Must test n=1 displacement in externally applied fields 37/39

38 Hiking Mt. Sanitas, Boulder 38/39

39 PCS10 Mirnov Probe Configuration 6 probes in poloidal slice at 320 location 6 probes equally spaced toroidaly at r=52mm /39