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Possible further steps for upgrading the GDT device

Possible further steps for upgrading the GDT device. T.D. Akhmetov , A.A. Ivanov, and V.V. Prikhodko. Budker Institute of Nuclear Physics, Novosibirsk, Russia. Outline. Current parameters of Gas Dynamic Trap (GDT) Why upgrade? to increase electron temperature and hot ion energy content

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Possible further steps for upgrading the GDT device

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  1. Possible further steps for upgrading the GDT device T.D. Akhmetov, A.A. Ivanov, and V.V. Prikhodko Budker Institute of Nuclear Physics, Novosibirsk, Russia

  2. Outline • Current parameters of Gas Dynamic Trap (GDT) • Why upgrade? • to increase electron temperature and hot ion energy content • to optimize magnetic field • to improve MHD stability • Possibilities • proceed from 5 to 20 ms neutral beam injection • adjust the present coil system • add coils to enhance magnetic field from 3.3 to 4.5 kGs

  3. GDT layout Length:7 m Magnetic field: centerup to 0.33T mirror up to 15 T Mirror ratio:up to 35 Injection duration: 5 ms NBI power: up to 5 MW Warm plasma: (2-3)1013 cm-3, 200 eV Hot ions (H+, D+): up to 5·1013cm-3, <E>≈10 keV

  4. Typical experimental scenario • Cusp and expander are not used • MHD stability is provided by a biased limiter • Gas puffing maintains warm plasma density NB injection Gas puffing Plasma source 0.5 3.5 8.5 t, ms

  5. Incident NBI power Injection energy Einj = 2225 keV

  6. D0 injection into D plasma Hot ion diamagnetism with D0 injection into D plasma (B0= 0.33 T, R = 32) dWf /dt  0.4 kJ/ms By the end of injection n 51013cm3andTe  180 eV  for deuterons ei  4 ms No steady state yet

  7. Electron temperature at the axis Te , eV No steady state yet t, ms Experiment: Wf and Te are not saturated at 5 ms NBI Proposal: extend injection up to 20 ms to increase Wf and Te “Optimistic” estimation without  limit: max(Wf) 0.4 kJ/ms 20 ms  8 kJ

  8. Search for steady state Zero-order (space-averaged) numerical model includes • kinetic equation for hot ion distribution function fhi(e) • particle balance equations for warm ions and electrons nwi, ne • energy balance equations for electrons and warm ions Twi, Te • NB injection, gas puffing, and axial gas-dynamic plasma losses The model was adjusted to reproduce Te(t) and Wf(t) for 5 ms injection in the current experiments.

  9. Numerical simulation for 5 ms injection Calculation: ne = 1014 cm3, Pinj = 4 MW Experiment Te , eV t, ms

  10. Increase of injection pulse length Our simple numerical model gives qualitative agreement with experiments for small and large gas puffing for 5 ms NBI. Now the model is developed to better account for cold halo plasma and balance of neutral gas in order to proceed to 20 ms regime. 60% already and storage of hot ions will be limited soon by ballooning instability. Therefore, extension of the injection pulse together with magnetic field increase should allow accumulation of significantly greater hot-ion energy content which in turn should allow for greater Te.

  11. Outline • Current parameters of Gas Dynamic Trap (GDT) • Why upgrade? • to increase electron temperature and hot ion energy content • to optimize magnetic field • to improve MHD stability • Possibilities • proceed from 5 to 20 ms neutral beam injection • adjust the present coil system • add coils to enhance magnetic field from 3.3 to 4.5 kGs

  12. Plasma  near the turning point Estimation from magnetic field depression: max  0.6 Hot ion density estimation near the turning point < e > = 10keV n51013 cm3 Value of  is close to the ballooning instability limitin GDT (crit ~ 0.70.8) and will limit hot ion accumulation and electron heating. Can we decrease  near the turning point keeping the same or even larger Wf ? Since   phi /B2, to increase Wf   phidV , one hasto increase B or reduce hot-ion pressure near the turning point.

  13. Length of hot-ion turning region Let us change angle by and calculate the shift of the turning point • Hot-ion pressure near the turning region can be reduced by increasing the volume of this region, i.e. its length. • Either angular spread of hot-ion D.F. must be increased or magnetic field gradient must be reduced near the turning point. • Angular spread cannot be increased much, • Magnetic field gradient dB/dz(zs) can be increased by correction of currents in the coils or their positions near the turning point.

  14. Hot-ion population in GDT For n~51013 cm3, Te~200 eV, Ei ~ 20 keV ion energy loss ms for H+ and 4.8 ms for D+ ion scattering ms for H+ Thus, scattering can be neglected during the whole plasma pulse length. In simple estimations we will neglect also deceleration of ions on electrons Hot-ion (neutral beam) distribution function is taken in the form 0  injection energy 0  pitch-angle of injection  angular width

  15. Hot-ion density and pressure distributions Peaking of density and pressure near the turning point relative to the central plane For << 0~1 In GDT 0=45 p(zs)/p(0) ~ 5.2 1/2 [degree]and for=5: p(zs)/p(0) ~ 2.3

  16. Reduction of pressure in the turning region Multiplier for the coil current 1.7 1.22 0.8 0.48 b(z) b(z) corrected z, cm z, cm turning point now p(z) corrected r/rB2 z, cm

  17. Effect of coil current correction • limit in the hot-ion turning region can be significantly improved by reducing the peak plasma pressure ~1.5 times using correction of the coil currents. It should increase the hot-ion energy content Wf possible for the given magnetic field strength.

  18. Outline • Current parameters of Gas Dynamic Trap (GDT) • Why upgrade? • to increase electron temperature and hot ion energy content • to optimize magnetic field • to improve MHD stability • Possibilities • proceed from 5 to 20 ms neutral beam injection • adjust the present coil system • add coils to enhance magnetic field from 3.3 to 4.5 kGs

  19. MHD flute stability criterion Assumptions:=8p/B2 << 1; axial symmetry; paraxial limit, a2/L2<<1 Plasma is stable if variation of potential energy of perturbations is positive For radially localized perturbations and for sharp boundary plasma (M.N.Rosenbluth, C.L.Longmire, 1957)  – field line curvature Advantages:  simplicity, clearness Disadvantages:  paraxial limit (fails in the turning region)  small  (fails in the turning region)  applicable only for small-scale modes or for p(r)= const and sharp boundary We will use this criterion as a starting point for estimations of MHD stability

  20. Optimal B(z) profile for GDT with p(z)=const For p(z)=const, |W| is minimal for[Bushkova, Mirnov, Ryutov, 1986] r, cm b(z)10 z, cm Magnetic field was originally optimized for p=const

  21. More realistic p(z) profile Now pressure is strongly anisotropic due to sloshing ions p pr''/rB2 r''/rB2 unfavorable curvature, r''<0 z,cm R=2, turning point for ions injected at 45 Magnetic field should be corrected to reduce unfavorable curvature. It will improve MHD stability.

  22. Corrected coil positions in GDT Minimization of potential energy W with p=p(B) for sloshing ions by shifting several coils reduces W by a factor of 2.7 compared to the present GDT system

  23. Corrected coil positions in GDT pr''/rB2 pGDT corrected GDT z,cm Relatively simple adjustment of coils can improve MHD stability

  24. Outline • Current parameters of Gas Dynamic Trap (GDT) • Why upgrade? • to increase electron temperature and hot ion energy content • to optimize magnetic field • to improve MHD stability • Possibilities • proceed from 5 to 20 ms neutral beam injection • adjust the present coil system • add coils to enhance magnetic field from 3.3 to 4.5 kGs

  25. Increase of magnetic field Additional coils from AMBAL-Mwith I=26.3 kA placed optimally to provide the same B(z) profile as in GDT, increase magnetic fieldin the central cell 1.36 times over the length 260<z<260 cm (turning points zt = 190 cm) up to 4.5 kGs. These coils can be fed by available capacitor storage of the GOL device. Increase of B will allow accumulation of hot-ion population with greater energy content Wf and further increase of Te

  26. Conclusions • 20 ms NBI together with magnetic field increase should provide steady state with significantly enhanced Wfast and Te • Proposed experiment with lengthening of hot-ion turning region may give additional information about  limit and increase Te • Adjustment of the present coil system may significantly improve MHD stability • Increase of central cell magnetic field by a factor of 1.36 is possible with available additional coils and capacitor storage A.A.Ivanov “Perspectives of development of magnetic mirror traps in Novosibirsk” Friday, July 9 12:10

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