Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
The Path to High Field Utilizationin the PEGASUS Toroidal Experiment
G.D. Garstka for the Pegasus Team
2003 Innovative Confinement Concepts Workshop
Seattle, Washington
May 29, 2003
Work supported by U.S. D.O.E. Grant DE-FG02-96ER54375
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Overview
• Pegasus: ultra-low A ST designed to study stability limits as A→1 and Ip/Itf >1
• High βt and Ip=Itf achieved ohmically
• Low-order tearing modes and ideal kinks limit access to higher Ip/Itf
• Path to high Ip/Itf requires suppression of instabilities
• Upgrades will advance the mission by increasing q during startup and improving plasma control
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
NSTX,MAST
START MEDUSA
CDX-U,HIT,
TST-M,Globus-M,
ETE
PEGASUS
TS-3,4Spheromaks
Aspect Ratio1.0 1.2 1.4 1.6 1.8 2.0
100
10
1
0.1
I p/I
TF
}“tokamak-spheromakoverlap region”
Ip/Itf → figure of merit for access to low-A physics
qψ=6
Mission: Explore plasma limits as A→1
Pegasus is an extremely low-aspect ratio facility exploring quasi-sphericalhigh-pressure plasmas with the goal of minimizing the central column whilemaintaining good confinement and stability
• Stability and confinement at high Ip/Itf
• Limits on βt and Ip/Itf (kink) as A→1
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Pegasus facility produces ultralow-A plasmas
Achieved Parameters:
A 1.12-1.3
R 0.2-0.45 m
Ip 0.16 MA
RBt ² 0.03 T-m
κ 1.4 - 3.7
∆tpulse 0.01-0.03 s
<ne> 1-5x1019m-3
βt ² 20%
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
• High-βt (Ohmic): βt > 10%
• High-βN (Ohmic): βN > 4
• Large Ip/ITF: Ip/ITF ~ 1
Equilibrium reconstructions show low-A characteristics
• Natural κ: κ > 2
• High field windup: high qψ at low TF
• Paramagnetism: F/Fvac ~ 1.5 on axis(εβp < 1)
Fit ResultsIp 154 kA
R0 0.34 m
a 0.29 m
A 1.15
κ 1.33
F0 0.03 T-m
βt 18%
W 570 J
li 0.54
q0 1.0
q95 4.3-1.0
-0.5
0.0
0.5
1.0
Z (
m)
1.00.50.0
R (m)
0.6
0.4
0.2
0.0
MA
/m2
<j>ψ
1500
1000
500
0P
a
p
543210
1.00.80.60.40.20.0ψΝ
q
Flux PlotSample Reconstruction
Low-A Characteristics
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
A < 1.3 → ready ohmic access to high βt
30
25
20
15
10
5
0
β t (%)
1086420IN = Ip/(aBt )
ConventionalTokamaks
START-NBI(Sykes-EPS 01)
β N =
6
β N = 3.5= Pegasus data
• βt up to 20% and IN up to 6.5 achieved ohmically
• Low field → high IN and βt
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Toroidal field utilization exhibits a “soft limit” around unity
• Maximum Ip Å I tf
• Limit not disruptive or abrupt- Ip saturates or rolls over
0.16
0.12
0.08
0.04
0.00
Pla
sma
Cur
rent
(M
A)
0.120.080.040.00TF Rod Current (MA)
Ip=Itf
0.16
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Two factors contribute to the Ip/Itf = 1 soft limit
Large resistive MHD instabilities degrade plasma as TF ↓• low Bt and fast dIp/dt → early appearance of low-order q=m/n
• high resistivity early
• ultra-low A → low central shear
⇒ Result: rapid growth of tearing
modes and large saturated island widths
- Most common modes: m/n=2/1, 3/2
- Leads to decreased CE, Ip
Reduced available Volt-seconds as TF ↓• reduced toroidal field → delayed startup
• delayed startup + fixed sine Vloop waveform → reduced available V-s
• contributes to drop in Ip with reduced Itf
160
140
120
100
80
60
40
20
0
kA
-4
-2
0
2
4
Gauss
Ip
δB
10000
8000
6000
4000
2000
0
Fre
quen
cy (
Hz)
0.0220.0200.0180.0160.0140.012
2/1
3/2
2/1
Time (s)
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Centercolumn
Image Contours:Measured Reconstructed
Tangential PHC SXR image
Measured q-profile indicates low central shear
⇒
Reconstructed q-profile
• 2D soft x-ray camera gives q-profile- Measures constant-intensity surfaces
- Used as internal constraint on equilibrium
- Useful as q-profile diagnostic
• Measured q-profile ⇒ zero central shear- Typical of low-A
⇒
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
• High Ip plasmas often disrupt
• q95 = 5 observed preceding disruption
- li=0.5 at this time
• DCON analysis ⇒ unstable to n=1 external kink
- m=5 most unstable mode
External kink stability also limits field utilization
160
120
80
40
0
Ip (
kA)
-200
0
200 dB/dt (T
/s)
8.0
7.0
6.0
5.0
q 95
8
6
4
2
0
0.0230.0220.0210.0200.0190.018
Time (s)
Signal A
mplitude (T
/s)
q95=5
Fre
e B
ound
ary
Ene
rgy
(AU
)
100
10
0
DCONResult
Core Mirnov Signal
Plasma Current
Mode Amplitude
ψN
0.0 0.2 0.4 0.6 0.8 1.0
Rea
l u1
1.0
0.8
0.6
0.4
0.2
0.0
m=5
m=4m=3m=2m=1
Poloidal mode eigenfunctions
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
The path to high field utilization: avoid early MHD
• High field utilization (Ip > Itf) requires suppression of tearing modes
- High βt, edge kink accessed at high Ip/Itf
• Approaches to increase Ip/Itf
- Transiently increase q during startup= via transient Itf waveform
- Manipulate current profile= variable dIp/dt= shape control and separatrix operation= transient Itf changes
- Reduce η before low-order rationals appear= Vloop control= radial position control
+ R small early ⇒ large J(r)= RF heating (HHFW)
+ requires well-controlled edge
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
x10
-3B~ Bpo
l
6
4
2
0 86420Shear @ rational surface
Pegasus Data
Mode amplitude reduced by manipulation of shear and q0
• Discharge tailoring → plasmas with reduced MHD activity- Increased W, Ip
• Increased shear, increased q0 ⇒ delay tearing onset
• MHD amplitude decreases with increasing shear
⇒ Access higher Ip/Itf via increased q0, Te, shear
Mode Amplitude
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
TSC: Fast TF rampdown is tool for q(r) and β manipulation
• Effect of fast ramp:- βt increases- qa drops as Bt, q0 falls more slowly (on τR time scale)- Toroidal flux conservation:
= edge poloidal currents driven, li drops= Ief increases to hold R, Ip increases due to induction and energy conservation
• Results of TSC PEGASUS simulation:- RBt: 0.06 to 0.03 T-m in 2 ms - Ip: up from 90 kA to 140 kA- βt increases from 5% to 16% - Φ up from 1 to 6 mWb
0 10 20 30Time (ms)
RB
t (T
-m)
0.06
0.05
0.04
0.03
0.02
TF Rampdown
Beta vs time J(r) evolution
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Shaping strongly affects edge q
1.351.301.251.201.15
Aspect Ratio
κ=1.25 κ=1.50 κ=1.75
δ=0.2
12
10
8
6
4
2
0
1.81.61.41.21.0
Elongation
δ=0 δ=0.25 δ=0.50
R0=0.45 m12
10
8
6
4
2
0
0.50.40.30.20.10.0
Triangularity
κ=1.25 κ=1.5 κ=1.75
R0=0.45 m
10
8
6
4
2
0
• κ, δ, A strongest effects
• Variation in q => variations in kink stability
• These plots from analytical expression
- Ip = 300 kA
- RBt = 0.028 T-m
qψ
qψ
qψ
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
New tools to access Ip > Itf
• Suppress tearing modes early in discharge evolution
= Transiently increase q during startup:
- Increased TF => high Itf, low inductance TF bundle
= Reduce resistivity before low-order rationals appear
- Maximize J => Vloop control, position & shape control
- Increase ohmic flux => new ohmic power supply
- Use HHFW system => position control, Vloop control
• Avoid edge kink modes at low field utilization
- Manipulate edge shear => divertor coils & PF shape control
- Decrease edge currents => loop voltage control
- Manipulate plasma shape => shape control
- Manipulate current profile => Vloop control, position control
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Active power supplies provide experimental flexibility
Ohmic IGCT H-Bridge Switch
ABB IGCT Presspack with IntegratedGate Unit - Steady State - 2.8kV@4kA
• New supplies: capacitor banks actively
switched by an H-bridge configuration
- IGCTs and IGBTs used
- Replaces old RLC supplies
using SCRs and ignitrons
- Cap banks situated outside building
• Enable coil current programming
- Loop voltage control
- TF flat-top and fast rampdown
- Position and shape control
- Stray field compensation
- Active divertor control
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Ohmic heating system provides loop voltage control
• Old waveform: half-sine wave
- non-optimal waveform
- effective plasma time: 15 ms
- effective flux delivered: 40 mV-s
• New waveform: programmable
- can optimize Ip(t) waveform
= possible feedback on Ip
- effective plasma time: 30 ms
- effective flux delivered: 90 mV-s
Sample Loop Voltage Waveform
Plasma time(old waveform)
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Equilibrium field upgrade providesradial position and shape control
1 2
3
4
5
6
78Div
Div
• Upgraded EF system designed with maximum flexibility- 8 separate coils + 2 divertor coils- 5 independent 20 kA supplies
= bipolar divertor coils= 1,2,7,8 coils= 4 and 5 coils= 3 coil= 6 coil
• High current and fast response necessary to force field through conducting vessel wall (2 ms skin time)
Coil Locations
(3-6 for up/down control)
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
400
Toroidal Field: Increased field and fast rampdown
Sample Rampdown Waveform
• Previous system- 60 turns- 2.5 kA/turn- Limited by resistivity
= Itf ² 150 kA
• New system- 12 turns- > 40 kA/turn- Limited by switches
= Itf ² 500 kA
Comparison: old core vs new core
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Partial Assembly Showing Fingers (Top)
Toroidal Field Centerstack Assembly
Cross-Sectional Drawing (Top)
ContactFinger
WedgeReactor
BottomWedge
DriverWedge
SetScrewRing
SupportRing
TopCoilLeg
G-10SupportPlates
12-TurnBundle
Finished Assembly with Wedge Reactor (Top)
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
TF assembly in close quarters of small centerstack region
Pressure Paper
Plastigage™
Final FitInitial Fit
Pressure Paper
• New diagnostic tools aid joint assembly- Pressure sensitive paper
= Turns red when compressed= Indicates surface area in contact
- Plastigage™= Plastic wire compressed in joint= Width after compression indicates gap
• Pegasus joint assembly- Initial fit: good resistivity, but poor alignment & gap- Surfaces honed to improve alignment- Ag mesh inserted to distribute load- Final fit: good contact over finger area
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Experiment Status: Power Supplies and Lab Reconfiguration
• Power supplies- Capacitors relocated to outside vault- Capacitors in-house- Switches on order- Buswork all installed
• Lab reconfiguration- Control and DAS centralized in shielded room- All signals run in shielded conduit- New water and AC lines run
• New diagnostics- Pulse height analysis (Te)- Visible bremsstrahlung (ne(r))- 31-channel bolometer (Prad(r))- Upgraded 2D SXR camera (q(r))- Upgraded 270 GHz interferometer (ne)
• Status - May 2003- TF reassembly complete- Pumpdown imminent- Capacitors installed: June 2003- Power testing: Summer 2003- Plasma ops resume: Fall 2003
Cap Banks
Switchroom
Charge/Dump
Screen Room
Pegasus
TransmissionLines
Lab Configuration - 2002
Lab Configuration - 2003
Main Cable Tray
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
Photos of lab reconfiguration
Old Configuration New Configuration
View from NW corner View from NW corner
View from pit entrance View from E wall
Pegasus Toroidal ExperimentUniversity of Wisconsin-Madison
5
4
3
2
1
0
-1
-2
6543210
With Compensation
Active Coils
Passive Coils5
4
3
2
1
0
-1
-2
6543210
Without Compensation
All axes in meters
Equilibrium field must be compensated for public safety
• Increased EF will lead to large fields around the machine- 4 m above, and 6 m South, are public places- Uncompensated field can be as high as 50 Gauss- Possibly hazardous to pacemaker users
• Implement “compensation coils” to reduce this field- 8 passive coils on ceiling + 2 active coils at 3 m radius