Progress in Spherical Torus Research

1
Progress in Spherical Torus Research

• Roger Raman
• University of Washington, Seattle
• and the NSTX National Research Team
• ICC Workshop
• 28-30 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 (gt 70)
• High dielectric constant ? 30-100, 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
• a 2 m
• ? 3.4
• Ip 29 MA
• FBS 96
• PCD 28 MW
• BT 2.1 T
• ?T 50
• Pfusion 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 BT 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
NSTX

• ?T 35 determined by magnetic analysis
• BT 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
10
Substantial bootstrap current fraction achieved
in NSTX
Vloop 0.1V for 0.3s INBI/Ip
0.18 Ibootstrap/Ip 0.42 Inon-ind/Ip
0.6

• Goal is to control profiles of both pressure
  current to maximize stability and bootstrap
  current contribution

J.Menard, PPPL S.Sabbagh, Columbia
11
0.1MA of HHFW current drive inferred by circuit
analysis

• HHFW are compressional fast Alfvén waves,
• ? k?VA (6-12) ?D
• For NSTX ? 4 - 38, choose ?/ k11 Vte

2 discharges with similar ne(r) , Te(r) ?V
not caused by dIi/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
MAST NSTX show good confinement (Tau-E gt
100ms), in general agreement with ITER scaling
(IPB98y2), while extending the range of R/a in
database
NSTX
MAST

• 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
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
?
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.
M. Gryaznevich, MAST
15
Large STs have similar cross-section and main
plasma parameters as conventional aspect ratio
tokamaks of similar size
Red shows design values
Recent results NSTX ?tgt30 ?N 6
gt?N(no-wall) ?pol 1, Hpby21.8, Te
3.5keV MAST ?N 6 gt?N(no-wall) ?pol 2,
Hpby2 2 Ti, Te 3keV G gt 2
NSTX and DIII-D in USA
MAST and ASDEX-Upgrade in Europe
MAST and NSTX
16
New fueling method (inboard mid-plane gas
injection) developed on MAST
Inboard midplane fuelling improves H-mode access
(A Field)
Pellet injection provides internal fuelling in
H-mode K Axon, C Ribeiro, S Shibaev
Outboard fuelling
8-pellet gas gun injector (used on RTP, FOM) 3
pellet sizes 0.5, 1, 2x1020 atoms of deuterium
Pellet velocity 300 - 1200m/s
Inboard fuelling
Similar inboard fueling results on NSTX
Initial estimates of fuelling efficiency 50
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 bN 4li limit has not been justified
  in STs. This means that reducing li 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
ST - Spheromak Overlap region
Pegasus parameters A 1.12 - 1.3 R 0.2 - 0.45
m Ip 0.16 MA BT 0.15 T ? 1.4 - 3.7

• ?t up to 20 and IN up to 6.5 achieved ohmically

Pegasus Team
20
Toroidal field utilization exhibits a soft
limit around unity

• Maximum Ip Itf in almost all cases
• Limit is not disruptive, IP saturates or rolls
  over
• Large resistive MHD instabilities degrade plasma
  as TF decreases
• Reduced available volt-seconds as TF is reduced

IP/Itf 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 Parameters
• R/a 0.3/0.2
• ? 1.5
• Bt 0.5 T
• Ip 265 kA

HIT-II Team
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
CDX-U parameters R/a 0.34/0.24m Ip 70 kA Bt 0.2T

• Recycling Fueling
• Impurity reduction
• Performance enhancement
• Radiation lossses, core Li accumulation
• Safety issues
• Motion of liquid during PF ramps, disruptions

TiC coated shield on CS
Heat/Li shield
Tray temp. monitored
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
24
Initial Li experiments with tray temp. _at_ 300C
show improvement in plasma performance
0ne half of CDX-U is shown

• 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

UCSD Li injector
Center line
Plasma experiments started within hours of
filling the tray
CDX-U Team
25
Ion heating by reconnection (during an IRE)
observed by impurity ion Doppler in TST-2

• Impurity temperature increases up to 400-550 eV
  from 50-100 eV
• Te and ne decrease

• TST-2 parameters
• R/a 0.36/0.23
• Bt 0.4 T
• Ip 200 kA(design)

Achieved parameters ?T 5.7 (?N 2.7) ?E 3
ms Ip 90 kA Bt 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
New machine in the process of being fully
commissioned ETE Parameters R/a 0.3/0.2 ? 1.6 -
1.8 ? 0.3 Bt 0.4 - 0.6T (lt0.8T) Ip 0.2 - 0.4MA
(design)

• Ip 40 - 60kA, pulse up to 12ms
• Evidence for IREs
• Large currents induced in the walls
• Density 4E19 m-3, Te 160 eV

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 gt 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 Ip approaches
  IT, results may eventually relate to spheromaks
• Temperature increase during IREs seen on TST-2,
  current drive methods being developed