Overview P EGASUS : ultra-low A ST designed to study stability - - PowerPoint PPT Presentation

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

INITIAL OPERATION

OF THE UPGRADED

PEGASUS ST EXPERIMENT

Raymond.J. Fonck University of Wisconsin-Madison for the PEGASUS team:

• D. Battaglia
• M. Bongard
• S. Burke
• N. Eideitis
• B. Ford
• G. Garstka
• M. Kozar
• B. Lewicki
• E. Unterberg
• G. Winz

presented to the 10th International ST Workshop Kyoto, Japan

• Sept. 29- Oct. 1, 2004

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

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 limited access to higher Ip/ITF
• Path to high Ip/ITF and β via suppression of instabilities
• After fire: Lab rebuilt with significant upgrades
• Advancing the experiment mission by improving plasma control

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

NSTX, MAST START

MEDUSA CDX-U, HIT, TST-M, Globus-M, ETE

PEGASUS

TS-3,4 Spheromaks

1.0 1.2 1.4 1.6 1.8 2.0

Ip/ITF

}

qψ=6 Aspect Ratio

“tokamak-spheromak

• verlap region”

100 10 1 0.1

Ip/Itf = figure of merit for access to low-A physics Pegasus is an extremely low-aspect ratio facility exploring quasi-spherical high-pressure plasmas with the goal of minimizing the central column while maintaining good confinement and stability

Mission: Explore plasma limits as A→1

Original Pegasus Goals:

• Stability and confinement at high Ip/ITF
• Extension of tokamak studies
• Limits on βt and Ip/ITF (kink) as A→1
• Overlap between the tokamak and the spheromak

Planned Future Emphases:

• Support ST program movement to next

stages

• EBW tests for heating & CD (w/PPPL)
• Noninductive startup tests
• Novel divertor design tests (w/UT)
• CT fueling tests (w/UCD)
• Diagnostics
• High-pressure gas puff for deep fueling

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Ohmic Trim Coils RF Heating Antenna Vacuum Vessel Centerstack:

Exposing Ohmic Heating Solenoid (NHMFL)

Equilibrium Field Coils Plasma Limiters Toroidal Field Coils

PEGASUS is University-Scale, Mid-Sized ST

Experimental Parameters Parameter Achieved Phase II Goals A 1.15-1.3 1.12-1.3 R (m) 0.2-0.38 0.2-0.45 Ip (MA) 0.16 0.30 IN (MA/m-T) 6-8 15-20 RBt (T-m) 0.03 0.1 κ 1.4−3.7 1.4−3.7 τshot (s) 0.02 0.05 ne (1019 m-3) 1-5

10

βt (%) 20 > 40 PHHFW (MW) 0.2 1.0

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

A < 1.3 → Ready Ohmic Access to High βt

30 25 20 15 10 5

β

t (%)

10 8 6 4 2

Conventional Tokamaks START βN = 6 β

N

= 3 . 5 = Pegasus data

IN = Ip/(aBt )

• βt up to 25% and IN up to 6.5 achieved ohmically
• Low field → high IN and βt

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Toroidal field utilization exhibits a “soft limit” around unity

• Maximum Ip ITF
• Soft limit due to two factors:
• Large, internal 2/1, 3/2 tearing modes degrade plasma
• Low shear over most of plasma, high resistivity
• Reduced Volt-sec as TF decreases

0.16 0.12 0.08 0.04 0.00

Plasma Current (MA)

0.12 0.08 0.04 0.00

TF Rod Current (MA) Ip=Itf

0.16

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Two factors contributed 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
• fixed sine-wave loop voltage
• 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
• Ip/ITF 1 ⇒ q0 1.5 - 2

Reduced effective Volt-seconds as TF ↓

• reduced toroidal field → delayed startup
• delayed startup + fixed sine Vloop waveform → reduced effective V-s
• kA
• Gauss

Ip δB

• Frequency (Hz)
• 2/1

3/2 2/1 Time (s)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Tangential PHC SXR image Image Contours: Measured Reconstructed

⇒ ⇒

Center column

12 8 4 q 1.0 0.8 0.6 0.4 0.2 0.0 ψ

N

q ψ

N

• Measured q-profile indicates low central shear
• 2D soft x-ray camera gives q-profile
• Images soft x-rays
• Constant-intensity surfaces determined
• Mapped into flux space
• G-S equation with SXR constraints
• Iterate solution until convergence
• Measured q-profile ⇒ low central shear

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• Pegasus Data

δB/B (x10-3)

Mode Amplitude

• q-profile tailoring increases plasma performance
• Discharge tailoring → plasmas with reduced MHD activity, increase W and Ip
• Increased shear, increased q0 ⇒ delay tearing onset
• MHD amplitude decreases with increasing shear
• Increased toroidal field strength also reduces MHD activity
• Along Ip=Itf contour: δB ↑ as TF ↓
• At high TF effect of MHD minimal
• CE = 0.4
• At lower TF MHD amplitude increases
• CE increases
• Stored energy decreases

⇒ Access higher Ip/ITF, βt via increased q0, Te, shear

MHD affected by q-profile tailoring and TF strength

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• High Ip plasmas often disrupt
• q95 = 5 observed

preceding disruption

• i=0.5 at this time
• DCON analysis ⇒ unstable

to n=1 external kink

• m=5 most unstable mode
• Consistent with theory expectation

ψN

0.0 0.2 0.4 0.6 0.8 1.0

Real u1

1.0 0.8 0.6 0.4 0.2 0.0

m=5 m=4 m=3 m=2 m=1

Poloidal mode eigenfunctions via DCON 0.16 0.12 0.08 0.04 0.00

MA

0.024 0.020 0.016 0.012 8 7 6 5 4 8 6 4 2

AU

0.023 0.022 0.021 0.02

Time (s)

10 100

T/s

Plasma Current q95 Free- Boundary Energy (DCON) MHD Amplitude

High q95 external kink limit observed

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Planned path to access to high Ip/Itf, βt operation

• Suppression of large internal MHD modes
• Vary q(ψ)
• Lower η before q(ψ) approaches low-order rational mode surfaces
• Expand access to external kink modes studies
• Plasma time evolution, shape
• Edge conditions and edge currents
• Access to very high βt regime for stability analysis
• OH access and HHFW heating availability

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

New tools to access Ip > Itf

• Suppress tearing modes early in discharge evolution

= Transiently manipulate q during discharge:

• Increased TF at startup

=> high Itf, low inductance TF bundle

• Variable Ip and R0 control

=> coil-current-waveform control = 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

• Explore edge kink boundary at high field utilization
• Manipulate edge shear

=> divertor coils for separatrix & PF shape control

• Decrease edge currents

=> loop voltage control

• Manipulate plasma shape

=> shape control

• Manipulate current profile

=> Vloop control, position control

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Overview of PEGASUS Phase II Rebuild

• Power Systems Entirely Replaced
• PWM controlled H-Bridges allow for complete waveform control
• Coil currents increased significantly
• 6 MJ of electrolytic capacitors installed outside of experimental building
• New power buses installed
• Low-inductance Toroidal Field Centerstack Installed
• Provides increased, time-variable TF
• Lab Infrastructure Improved or Replaced
• Shielded conduits and cable trays installed
• New grounding system installed
• Control and Safety systems upgraded
• Bakeable gas system
• Upgraded AC, air, and water services installed
• Passive Stray field “flux catcher” installed for public safety

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Phase I laboratory layout (2002)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Laboratory before rebuild (October 2002)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Rebuilt laboratory (June 2004)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Power Systems consist of IGBT/IGCT Solid State H-Bridges

ABB IGCT 2.8kV@4kA Steady-State ~ 50 cm long

System Toroidal Field Ohmic Heating Equilibrium Field Phase I

• 60 turns
• Quasi-DC
• 150 kA-t max
• Half-sine

Waveform

• 40 KA at 10kV
• Monolithic coil set
• 2 Resonant banks
• Waveform constrained

by startup concerns

• No divertor

Phase II

• 12 turns
• Time-variable
• up to 450 kA-t
• 8 IGBT Bridges
• Programmable
• 48 KA at 2.7kV
• 12 IGCT Bridges
• Independent coils
• 20 IGBT Bridges
• Evolution free from

startup constraints

• Divertor installed
• Many thanks to the HIT Group for their assistance!

Example

ITF (KA)

0.08

Time (s)

• Vloop

8

Phase I Phase II

Time (s)

.035

• Ief (KA)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• Benefits
• Tailor the current waveform to match the

needs of the desired plasma evolution

• PWM controlled modern IGCT/IGBT

semiconductors

• More reliability and control with less overall

stored energy

• H-Bridge regeneration mode minimizes heating
• f critical coil sets
• Fault detection and interruption capability
• HIT group: CAMAC based, optically isolated

PWM controller

Pulse Width Modulated (PWM) H-Bridges

IGBT H-Bridge (2 of up to 28) 900V, 4kA at up to 5kHz Insulated Gate Bipolar Transistor

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

PEGASUS Centerstack Assembly and TF Waveform

• 60 Turn Bundle

12 Turn Bundle

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Toroidal Field Joint Area is Critical

Cross-Sectional Drawing (Top)

Contact Finger Wedge Reactor Bottom Wedge Driver Wedge Top Coil Leg G-10 Support Plates 12-Turn Bundle Plastigage™ Pressure Paper

Fit-Check Diagnostics

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Toroidal Field Upper Joint Assembly

Bare TF Assembly Fully Assembled TF Joint

Copper Finger Wedge Reactor Wedge Set Screws Coolant Channel

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• Initially using PF power supplies for OH ⇒ limited Volt-s and Vloop
• 1st plasma in late May
• 2 month campaign in Summer 2004
• Transient suppression and PS stabilization
• New facility tests and systems shakedown
• Effects of wall currents with new waveforms
• Low power startup studies
• Stabilizing operations
• New power systems stabilized and working as desired
• Robust to major failures
• Recent upgrades to enhance operations
• Major grounding change to stabilize PS
• New diagnostics installed
• Single plasma gun installed for tests of CD and fueling
• Late Sep - Early Oct: Installation of first High-V OH power supplies
• Upcoming campaign in Fall-Winter 2004-2005: Use New Tools
• Commission new OH system for high-power ops
• Tearing mode suppression
• Access to Ip/Itf > 1, low-q, high IN, high βt regime
• Characterize ext kink limits
• Introduce separatrix
• Use gun for startup assist
• Install more guns

Present Status - Coming into full power ops

Recent Plasma

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• 20
• 15
• 10
• 5

5 10 15 20 Coil Current (kA) 0.04 0.03 0.0 2 0.0 1 0.00 Shot 24239 OH EF123 EF678 EF45 (Bias) TF 30x1

• 3

20 10 Poloidal Field (R=0.25 m) 40 30 20 10 ms 1.2 1.0 0.8 0.6 0.4 0.2 0.0 A.U. 40 30 20 10 x10-3 4 2

• 2
• 4

Vloop (V) 0.04 0.03 0.02 0.01 0.00 s 100 80 60 40 20 Ip (KA)

Vloop Coil Currents Poloidal Flux

Typical waveforms for ohmic operations

Gas Puffs EC PI Ip

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Ip/Itf space expanded

0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00

Plasma Current (MA)

0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00

TF Rod Current (MA) Ip = ITF Pegasus Phase I Aug-Sep 2004

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Plasma guns being tested for startup and fueling

Gun installed in lower divertor region Time-integrated plasma image Gun orifice

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Low Gun Current/Power --> Linear Current Scaling

• Current channel follows field line
• Maintains helical nature
• Current (A)
• Time (s)
• Current Amplification

Vbias = 50 V sn 24433 I_tor Igun_x5 Ratio

• Total toroidal current ~ 5 x gun current
• Ip/Ig constant

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Low Gun Current/Power --> Linear Current Scaling

• Current channel follows field line
• Maintains helical nature

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

HIgh Gun Current/Power --> Nonlinear Current Scaling

• Total toroidal current > 5 x gun current
• Ip/Ig increases
• Current channels merge/reconnect
• Generates extended plasma
• Current (A)
• Time (s)
• Current Amplification

Vbias = 400 V sn 24437 I_tor Igun_x5 Ratio

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

HIgh Gun Current/Power --> Nonlinear Current Scaling

• Current channels merge/reconnect
• Generates extended plasma

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• Early indications: gun startup compatible with OH

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

Mercier Criterion

• 2.5
• 2.0
• 1.5
• 1.0

ArcSinh DI (A.U.) x10-1 x10-1 ArcSinh DR (A.U)

1.5 0.0 0.5 1.0 0.0 0.2 0.4 0.6 0.8 1.0

ρ

GGJ Resistive Criterion

0.0 0.2 0.4 0.6 0.8 1.0

ψN

5 10 15 20 25 30

Newcomb Criterion, n=1 (A.U.)

q Profile

1 2 3 4

DI,DR > 0 => Unstable

Flux Plot

• Equilibria Parameters
• High-Ip/Itf, “zero”-β Equilibria

Plasma Limits Modeled at Ip/Itf ~ 3

• DCON Output
• Approaches Phase II Goals of PEGASUS

βN = 6 βN = 3.5

Increase Aux. Heating

PEGASUS START Conventional Tokamaks

Increase Ip/ITF

120 100 80 60 40 20 βt 20 15 10 5 IN = Ip/aBto [MA/(m T)]

• 1.0
• 0.5

0.0 0.5 1.0 1.0 0.5 0.0

R (m) Z (m)

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

EBW heating and current drive of interest for ST regime

• Plasma startup, sustainment
• Applicable to low-field, overdense plasmas
• Of interest to future NSTX development

Basic principles tested on W-7S and CDX

• Need to be tested at significant power levels

Pegasus good candidate for EBW development

• Low-cost 2.45 GHz technology
• Klystrons and waveguide available from PLT
• Need to demonstrate good target plasma control

Working with PPPL to develop best approach

• Modeling
• Hardware
• Experiments

Economical Tests of EBW Possible on PEGASUS

(a) EBW ray tracing calculations for a 250 kA PEGASUS equilibrium, central density of ne(0) = 1013 cm-3. (b) n|| along the ray path, showing the up- shift depends upon launch position. (c) Power deposition profiles corresponding to the rays in (a) and (b).

deposition profile n|| upshift

0.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 1.2 rho rho 20 40 60

• 4

80

• 2

10 12 2 4 6 8 10

1 1 2 2 3 3 4 4 5 5 6 6 6 5 4 2 3 2

(a) (b) (c) n|| MWm-3

PEGASUS Toroidal Experiment University of Wisconsin-Madison

10th STW Sept.29, 2004 - rjf

• Phase I ops finished Spring 2002
• Ip/Itf = 1.1
• βt = 25%
• Factors found limiting plasma current:

+ internal resistive modes + V-s limitations + external kinks

• Staged upgrades were proposed to suppress limiting mechanisms
• Fire initiated a “front-loading” of upgrades
• Upgrades are mostly completed
• New switching power supplies (final installation now)
• New capacitor banks
• New TF centerstack
• New control code
• New signal runs and screen room
• Phase II ops have begun
• Low power OH
• Plasma gun tests
• New diagnostic installations

PEGASUS now poised to exploit its unique niche