NSTX ET1 intro

1
NSTX RWM Active Feedback Physics Design Quick
Recap
S. A. Sabbagh
NSTX Global Mode Stabilization Meeting December
5th, 2003 PPPL
2
Active control may sustain 68 margin above
bNno-wall
Ex-vessel RWM control coils
VALEN model of NSTX (cutaway view)
105
With-wall limit (bN wall)
Active feedback coil
104
103
Passive
102
Growth rate (s-1)
101
Active gain (V/G)
100
1.0
2.5
10-1
Cb º (bN bNNo-wall)/(bNwall bNNo-wall)

• External control coil design with realistic
  geometry
• Internal control coil design computed to reach
  DbN 94

Sensors
VALEN (J. Bialek)
(Equilibria used have bNno-wall 5.1 bNwall
6.9)
3
Exterior control coil can provide adequate
stabilizing field
10-2
Skin depth limit
Exterior coil feedback limit
Interior coil

• Initial system plan has 6.8kAturns (Applied
  Bedge 27G _at_ 54Hz)
• Exterior coil design decision based on time,
  budget, risk constraints balanced by performance

Realistic exterior coil
Applied Bedge(G) / control coil ampturn
10-3
Original exterior coil design
(bNbNNo-wall)/(bNwallbNNo-wall)
94
68
10-4
Driving Frequency (Hz)
4
Active mode control modeling shows mode
stabilization

• Mode control feedback algorithm simulation
• Ideal system response (no latency)
• Simulations including latency show 200 ms is
  maximum time delay allowing stabilization at
  highest bN

Start feedback
4
3
Coil current
2
1
Poloidal field from mode at sensor (G)
Control coil current (A)
0
-1
Field at sensor
-2
(bNbNNo-wall)/(bNwallbNNo-wall) 65
-3
Time (ms)
5
The external coil / SPA power supply approach
still a viable path forward

• The anticipated time delays are acceptable, but
  small margin
• Stabilization at the highest bN may be marginal
  (requires lt 200ms)
• Stabilization potential remains high with
  realistic coil model
• Present Labik coil model shows possible
  stabilization at Cb 68
• System performance with present sensors being
  evaluated
• See J. Bialeks talk (next)
• Consider far less capable solutions only as an
  extreme fall-back position
• For example, audio amps / external coil perhaps
  better than nothing
• Audio amps are not a SPA replacement!
• VALEN calculations underway to evaluate such a
  system