1 Progress in Spherical Torus Research Roger Raman University of Washington, Seattle and the NSTX National Research Team ICC Workshop May 2003, Seattle, WA

2 STs bridge a gap between certain ICC concepts and a tokamak Spherical tokamaks present both the strong toroidicity effects of compact tori (notably magnetic shear) and the good stability properties provided by the external toroidal field of conventional tokamaks. Ludwig, INPE

3 Reduction of Aspect Ratio leads to a new physics regime requiring new enabling systems High Large bootstrap current (> 70%) High dielectric constant  ~ , requires RF waves at high harmonics of  i New methods to initiate current without reliance on inboard coils Increased divertor and wall loading

4 Validation of high  and bootstrap current fraction physics and solenoid-free plasma startup and sustainment would lead to attractive reactor designs ARIES ST parameters R 3.2 m a2 m  3.4 I p 29 MA F BS 96% P CD 28 MW B T 2.1 T  T 50% P fusion 2980 MW Najmabadi et al, Fus. Eng. Desig. 65 (2003) 143

5 Capability to build low cost machines will speed up fusion development path Most of the current driven by the plasma Small auxiliary current drive needed Low values of B T leads to cheaper machines to enable construction of intermediate designs to demonstrate a level of scientific understanding that allows for extrapolation with confidence

6 ST Program has machines that make a connection with tokamaks as well as with spheromaks NSTX & MAST: PoP Experiments Pegasus: Very Low Aspect ratio experiment, RF current drive to develop sustainment and startup HIT-II: CHI for Startup and sustainment CDX-U: Li-Wall development, RF for sustainment Other STs also contribute to these studies (TST-2, ETE, Globus-M, HIST)

7 Low cost has allowed construction of many small machines M. Peng, ORNL

8 Results from the large STs

9 NSTX has shown high  capability at 1 MA  T = 35% determined by magnetic analysis B T = 0.3T, A = 1.4,  = 2.0,  = 0.8  N ~ 6 also obtained on MAST High beta discharges on both machines had no disruption, limited by operational constraints D. Gates, PPPL NSTX

10 Substantial bootstrap current fraction achieved in NSTX Goal is to control profiles of both pressure & current to maximize stability and bootstrap current contribution J.Menard, PPPL & S.Sabbagh, Columbia V loop ~ 0.1V for ~ 0.3s I NBI /I p = 0.18 I bootstrap /I p = 0.42 I non-ind /I p = 0.6

11 0.1MA of HHFW current drive inferred by circuit analysis HHFW are compressional fast Alfvén waves,  ~ k  V A ~ (6-12)  D For NSTX  ~ 4% - 38%, choose  / k 11 ~ V te 2 discharges with similar n e (r), T e (r)  V not caused by dI i /dt H.R. Wilson, PPPL, P. Ryan, ORNL

12 CHI has generated substantial toroidal current in NSTX Goal is to control discharge evolution to promote reconnection of toroidal current into closed flux surfaces

13 NSTX MAST MAST & NSTX show good confinement (Tau-E > 100ms), in general agreement with ITER scaling (IPB98y2), while extending the range of R/a in database Little difference in L and H mode and no power degradation on MAST Similar results for Tau-E from MAST and NSTX at high TF MAST has obtained H-modes in a ohmic plasma MAST & NSTX Teams

14 Natural divertor plasmas may offer an alternate configuration for high performance discharges In the ‘Natural Divertor’ configuration, inboard limited plasmas in an ST have an expanded outer SOL and reduced, evenly spread contact on the centre limiter. MAST Natural Divertor shot #4573 exhibited H- mode features: ELM-free periods with spontaneous density rise between ELMs. Formation of the Natural Divertor at low R/a: As aspect ratio A = R/a reduces, exhaust plume expands and is diverted from the centre post  M. Gryaznevich, MAST

15 Large STs have similar cross-section and main plasma parameters as conventional aspect ratio tokamaks of similar size MAST and ASDEX-Upgrade in Europe NSTX and DIII-D in USA Recent results: NSTX:  t >30%;  N ~ 6 >  N (no-wall) ;  pol ~ 1, H pby2 ~1.8, T e ~ 3.5keV; MAST:  N ~ 6 >  N (no-wall) ;  pol ~ 2, H pby2 ~ 2; T i, T e ~ 3keV; G > 2 MASTNSTX R (m) A (m) k2.4 (3)2.5 Ip (MA)1.2 (2)1.5 (1) BT (T) PAUX (MW)3 NBI (5) 1 EC (1.5) 5 NBI (5) 4 FW (6) Tpulse (s)< 0.65 (5)0.55 (5) Red shows design values MAST and NSTX

16 Inboard fuelling Inboard midplane fuelling improves H- mode access (A Field) Inboard Gas Puff Outboard fuelling Initial estimates of fuelling efficiency ~ 50% 8-pellet gas gun injector (used on RTP, FOM); 3 pellet sizes: 0.5, 1, 2x10 20 atoms of deuterium Pellet velocity m/s Pellet injection provides internal fuelling in H-mode: K Axon, C Ribeiro, S Shibaev HSV frame showing PI Similar inboard fueling results on NSTX New fueling method (inboard mid-plane gas injection) developed on MAST MAST

17 Other results from large STs MAST uses a merging-compression method using the outer PF coils to generate solenoid-free startup current Power handling studies on MAST indicate that most of the heat load goes to the outboard divertor legs Unlike in conventional aspect ratio tokamaks, an empirical  N  4 l i limit has not been justified in STs. This means that reducing l i through profile control would allow further extension of the beta limits

18 Results from the medium sized STs

19 Pegasus explores extremely low-aspect ratio physics in high-  plasmas, with the goal of minimizing the central column while maintaining good confinement and stability  t up to 20% and I N up to 6.5 achieved ohmically ST - Spheromak Overlap region Pegasus parameters A R m Ip0.16 MA B T ~0.15 T  Pegasus Team

20 Toroidal field utilization exhibits a “soft limit” around unity Maximum I p ~ I tf in almost all cases Limit is not disruptive, I P saturates or rolls over Large resistive MHD instabilities degrade plasma as TF decreases Reduced available volt-seconds as TF is reduced I P /I tf is a figure of merit for access to low-A physics Pegasus Team

21 New method for CHI startup on HIT-II generates useful non-inductive current and improves the performance of inductive discharges New method “Transient CHI startup” developed on HIT-II Sequence of eleven discharges shows how CHI startup is robust and saves volt-seconds Sequence is initiated after Ti-wall conditioning. There is no wall conditioning between shots Method is applicable to a pre-charged transformer HIT-II Team HIT-II Parameters R/a 0.3/0.2  1.5 B t 0.5 T I p 265 kA

22 Under gas loaded wall conditions, good coupling to induction can be regained if CHI discharges can be initiated at lower neutral gas pressures Indicates need for good pre- ionization methods Method does not rely on any time changing poloidal field coil currents, therefore attractive for reactors where PF coils will be outside blanket structures Method to be tested on NSTX HIT-II Team

23 CDX-U studies role of Lithium PFCs on plasma operations and practical implementation issues Recycling & Fueling Impurity reduction Performance enhancement Radiation lossses, core Li accumulation Safety issues Motion of liquid during PF ramps, disruptions Heat/Li shield Tray temp. monitored TiC coated shield on CS Tray has a radius of 34cm, width of 10cm, depth of 64cm, & Temp. ~ 250C Electrical break between two halves, Liquid Li injected into both halves CDX-U Team CDX-U parameters R/a 0.34/0.24m Ip70 kA B t 0.2T

24 Initial Li experiments with tray 300C show improvement in plasma performance Plasma experiments started within hours of filling the tray Required five fold increase in gas fueling Immediate 30% increase in Ip 30% increase in discharge duration Virtual elimination of OII, CII radiation 2-3x reduction in D  emission No evidence for liquid Li ejection due to disruptions Plasma experiments started within hours of filling the tray CDX-U Team 0ne half of CDX-U is shown UCSD Li injector Center line

25 Ion heating by reconnection (during an IRE) observed by impurity ion Doppler in TST-2 Impurity temperature increases up to eV from eV T e and n e decrease TST-2 parameters R/a 0.36/0.23 B t 0.4 T I p 200 kA(design) Achieved parameters  T 5.7% (  N 2.7)  E 3 ms I p 90 kA B t 0.2 T TST-2 Team, Japan

26 Non-inductive current initiation by ECH in TST-2 ECH (2.45 GHz)  1 kA / 1 kW Low gas pressures  low collisionality Vertical field with positive curvature  trapped electrons TST-2 Team, Japan

27 ETE goals are to study Alfvén wave heating and edge plasma conditions Ip: kA, pulse up to 12ms Evidence for IREs Large currents induced in the walls Density ≤ 4E19 m -3, Te ≤ 160 eV New machine in the process of being fully commissioned ETE Parameters R/a 0.3/0.2   0.3 Bt T (<0.8T) Ip MA (design) ETE Team, Brazil

28 Summary Remarkable progress in ST physics and technology in just a few years PoP Experiments have been able to validate many of the predicted physics –  T ~ 35% achieved on NSTX –Good  E > 100 ms achieved by NSTX and MAST –Good progress with divertor power loading studies on MAST HIT-II has found a viable solution for plasma startup by CHI Liquid Li experiments on CDX-U are showing immediate plasma performance improvement Pegasus is finding a MHD limit as I p approaches I T, results may eventually relate to spheromaks Temperature increase during IREs seen on TST-2, current drive methods being developed