Stationary, High Bootstrap Fraction Plasmas in DIII-D Without Inductive Current Control

Transcription

1 Stationary, High Bootstrap Fraction Plasmas in DIII-D Without Inductive Current Control P. A. Politzer, 1 A. W. Hyatt, 1 T. C. Luce, 1 F. W. Perkins, 4 R. Prater, 1 A. D. Turnbull, 1 D. P. Brennan, 5 J. R. Ferron, 1 C. M. Greenfield, 1 R. J. Jayakumar, 2 R. J. LaHaye, 1 E. A. Lazarus, 3 C. C. Petty, 1 and M. R. Wade 3 1General Atomics, San Diego, California, USA 2Lawrence Livermore National Laboratory, Livermore, California, USA 3Oak Ridge National Laboratory, Oak Ridge Tennessee, USA 4Princeton Plasma Physics Laboratory, Princeton New Jersey, USA 5Massachusetts Institute of Technology, Cambridge, Massachusetts, USA politzer@fusion.gat.com 2th IAEA Fusion Energy Conference Vilamoura, Portugal 1-6 November 24 Paper EX/P2-7 QTYUIOP P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 1

2 Summary & Observations: Fully noninductive, high performance (normalized), high bootstrap fraction, stationary current plasmas have been sustained in DIII-D without transformer assistance. I p 625 ka, β p β N 3.3, H 89P 3, f bs.8, q The total current evolves as expected for a bootstrap-current-dominated plasma; current sustainment is favored by high density, high β p, and low l i. For a range of initial values, the plasma current evolves toward a single value. As the current profile evolves toward lower l i confinement gradually improves (principally particles). This correlates with a change in ELM character (become less grassy) and reduced Mirnov activity. Lower l i may be leading to higher stability limits (with a wall). P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 2a

3 The current and stored energy are limited by a relaxation oscillation involving repetitive growth and collapse of an ITB at large minor radius. Higher power increases the frequency of these events. The collapse is due to an ideal MHD mode, with peeling-ballooning characteristics. The ITB collapse changes the equilibrium and can lead to undesirable contact with the wall. In a fusion plasma it will lead to fluctuations in fusion power. Without transformer control, the total current is very sensitive to minor changes in boundary conditions. Use of the transformer as a control tool may be necessary even for steady-state operation. The stationarity of the current on the magnetic time scale, the recovery of the plasma from ITB collapse events, and the tendency with time toward more stable behavior is promising for future steady-state operation. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 2b

4 Context: Steady-state, advanced tokamak, burning plasmas require ~1% bootstrap current. However very little is known about the characteristics and behavior of fully noninductive, high bootstrap fraction plasmas. ADVANCED TOKAMAK NONLINEAR TRANSPORT COUPLINGS transformer source of poloidal flux auxiliary current drive auxiliary heating auxiliary angular momentum external internal J cd α-particle heating Σ V loop J ohm J bs bootstrap current p, T, n, v profiles fast heat & momentum transport loop (blue) conductivity profile magnetic flux diffusion heat, particle, & momentum fluxes B pol transport coefficients turbulent & neoclassical profiles and transport coefficients couple red & blue loops slow magnetic flux transport loop (red) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 3a

5 The properties of small tokamaks are entirely determined by external drives (green lines). In present tokamaks, the internal heat and momentum feedback loop (blue) competes with external sources, but the total current and current profile remain dominated by external drives. In a steady-state burning plasma all external controls will be weak and the plasma will assume entirely self-consistent profiles of J, n, T, and V. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 3b

6 Key Questions: What are the self-consistent n, T, J, and V profiles that the plasma wants? What are the beta limits associated with these profiles and what are the beta-limiting processes? What is the maximum current that can be sustained? Are these limits consistent with the expected fusion power production? Are these conditions dynamically stable? (I.e., if perturbed, does the plasma return to the same state?) What control methods are needed and what controls are possible? P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 4

7 Slow Evolution (on the Current Profile Evolution Time Scale) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 5

8 Best Case (So Far): Fully noninductive plasma. Current is stationary (on average). limit cycle behavior (relaxation oscillations) at high heating power (on transport time scale).. Confinement gradually improves (on magnetic time scale). P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 6a

9 I p (MA). d/dt (transformer current) = density (12 m 3) PNB (MW) neutrons/s (x 1 14) Dα (arb) (8ms smooth) (8ms smooth) Te() (kev) Ti() (kev) t (s) β (%) t (s) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 6b

10 Long-term (Slow) Profile Evolution: Central (ρ <.6) density and electron temperature rise. Ion temperature and toroidal rotation increase across entire plasma. Pressure peaking is low, and doesn't change during pulse. Current profile broadens l i decreases, q & q min rise. Improvement is correlated with a change in the character of MHD fluctuations as seen on the Mirnov loops. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 7a

11 .8 profiles at 1.97 s & 5.1 s ne (12 m 3) Te (kev). Jll MA/m Ωφ (krad/s) ρ 1 Ti (kev) ρ 4 2 q ρ.8 1. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 7b

12 Parameter Improvement Over 2.5 s: parameter ±.5 s s ratio n e (1^2) T e (kev) T i (kev) H 89P H 98y p()/ p β p β N W (MJ) q min l i τ E (ms) Energy and particle confinement improve slowly (on current profile relaxation time scale). Mainly better particle confinement. Current profile broadens. β N H 89 ~ 8-1, but with q 95 ~ 1, β N H/q2 is a factor of 4-5 below ITER baseline. Similar operation at q 95 ~ 5 requires β N ~ 6 (2x current, same f bs, β p ). P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 8

13 Improving Confinement & Broadening Current Profile Leads to Higher Bootstrap Current 3.5 6*l i β p, β N.6 Ip Ibs+Inbcd 2. q, qmin 4*l i 4.4 Ibs t (s) q profile monotonic or slightly reversed 3.2 (βn reaches 6 li) Inbcd current (MA) t (s) (fbs > 8% for >.5 s) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 9

14 Bootstrap & NBCD Calculated From Measured ne, Te, Ti, and Zeff Profiles Agree With Total Current Profile From EFIT.8 J (MA/m2).6 Jbs+Jnbcd.4.2 Jbs J Jnbcd ρ fbs.84 fnbcd.2 accounts for total within uncertainty of experiment and model. Bootstrap alignment =.71 BSA = 1 J = J B /B T dv n e T e dv n e T e J J boot J (measure of RF power needed to drive non-bootstrap current) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 1

15 Good and Poor Confinement Phases of these Discharges Correlate With Different Mirnov and Dα Signals 2 ms smooth β T (%) Poor confinement corresponds to small, rapid ELMs and large rms db/dt Good confinement corresponds to fewer, larger ELMs and smaller, more coherent db/dt..56 I p (MA) l i Detailed characterization of differences and determination of reason for transition to better confinement is work to be done t (s).55 P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 11a

16 4 poor confinement at 4. s good confinement at 4. s db/dt (T/s) db/dt (T/s) 5e15 Dα Dα ITB collapse time P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 11b

17 later transition P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 11c

18 earlier transition P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 11d

19 Parametric Variations P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 12

20 Higher Density Gives Higher Bootstrap Current density (12 m 3) Ip (MA).45 fbs t (s) The total noninductive current and the bootstrap fraction both depend strongly on the plasma density. Three discharges prepared identically except for differing initial (pre-l-h) density. With increasing density, the bootstrap fraction increases, and the rate of decay of the total current is reduced. Note also that variations in the total current are closely correlated with variations in the density, even on a short time scale. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 13

21 Current Converges to a Single Value From a Range of Starting Points current (MA) Ip (25ms avg) t (s) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 14

22 Relaxation Oscillation Limit Cycle of ITB, Energy and Current P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 15

23 Stored Energy and Current Are Limited by a Relaxation Oscillation Ip (MA) diamag (arb) With increased NB heating power, neither the stored energy nor the current increase significantly. Both W and I p show intervals of rapid increase, terminated by a very fast drop. The time between events is 1's of ms These events tend to become smaller as the discharge evolves. 4 PNB (MW) (4ms avg) t (s) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 16

24 Relaxation Oscillation on the Transport Time Scale Ip (MA) diamag Collapse is associated with a very large Dα pulse in the divertor region The Dα pulse is often, but not always preceeded by an ELM Dα 1 db/dt (T/s) t (s) 8 The MHD has not yet been examined in detail. ITB occurs at large minor radius. After collapse, profiles recover to their previous values. P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 17

25 Profiles Show Internal Transport Barrier Collapses and Recovers ne (12 m 3) 1 4 Te (kev). 12 J (MA/m2) khz 1 khz Ωφ (krad/s) ρ Ti (kev) ρ 3.9 s s 4.25 s 8 4 q ρ P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 18

26 During Recovery, Ion Thermal Conductivity Improves Throughout Plasma Core 1 ρ = Evaluate χ i with TRANSP using measured profiles. Average over ρ =.2 zones. χ i (m2/s) χ i improves uniformly from center outward t (s) P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 19

27 Initial Ideal MHD Analysis Re(ξ ψ) 1..5 q=3 q= m=4 m=3 m=2 m=6 m=5 m=7 m=8 GATO ideal stability analysis of indicates an unstable mode with peeling-ballooning character. (Sum of poloidal harmonics peaks at edge, even though largest single harmonic is at radius of max p'.) ψ P. A. Politzer IAEA 2th FEC 3Nov4 EX/P2-7 2