the pegasus toroidal experiment: recent results and future plans

PPEGASUSEGASUSToroidal ExperimentToroidal Experiment

University ofUniversity ofWisconsin-MadisonWisconsin-Madison

15th International Spherical Torus

Workshop

Oct 22-24, 2009

Madison, WI USA

TThe PEGASUS Toroidal Experiment:he PEGASUS Toroidal Experiment:Recent Results and Future PlansRecent Results and Future Plans

Aaron J. Reddfor the PEGASUS Research Team

Work supported by U.S. DOE Grant DE-FG02-96ER54375

Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

• Determining limits to IN, t

• High IN, t accessed through j(R) manipulation and/or fast TF ramps

• Tokamak-spheromak overlap

• Peeling modes observed

• Driven by high (j||/B)edge

• Can explore ELM physics

• Non-inductive startup via point current sources• DC helicity injection

• Target plasmas couple to outer-PF induction & Ohmic solenoid drive

PPEGASUS is studying low-A physics andEGASUS is studying low-A physics and

developing non-solenoidal startup techniquesdeveloping non-solenoidal startup techniques

N = 6 5

4

3

2

NSTX

t (%)

IN (MA/mT)

10

20

30

40

50

02 4 60 8

Pegasus

PPEGASUS Overview OutlineEGASUS Overview Outline

• The PEGASUS Toroidal Experiment

• Non-solenoidal startup

• Peeling mode studies

• Future: Guns & RF startup & growth

• Summary


PPegasus is a Compact Ultralow-A STegasus is a Compact Ultralow-A ST


High-stress Ohmic heating solenoid

Experimental ParametersParameter To DateAR(m)Ip (MA)IN (MA/m-T)li

κτshot (s)βt (%)PHHFW (MW)

1.15 – 1.30.2 – 0.45

≤ .216 – 12

0.2 – 0.51.4 – 3.7≤ 0.025

≤ 250.2

HHigh igh TT at A≈1 accessible at high I at A≈1 accessible at high INN


• High IN requires current drive and current profile control

• Developing startup and current drive techniques, to support overall ST program and enable high- studies

• Future devices need non-solenoidal current drive anyway, PEGASUS simply needs current drive solutions now

TF Ramps

Gun PI

START, NSTX



• Non-solenoidal startup studies

– Plasma gun system description

– Limits on the driven toroidal current



• Summary


LLocal Plasma Current Sources + Helical Vacuum ocal Plasma Current Sources + Helical Vacuum Field Give Simple DC Helicity Injection SchemeField Give Simple DC Helicity Injection Scheme


• Current is injected into the existing helical magnetic field

• High Iinj & modest B filaments merge into current sheet

• High Iinj & low B current-driven B overwhelms vacuum Bz

– Relaxation via MHD activity to tokamak-like Taylor state w/ high toroidal current multiplication

Reduced Bz

BT=10 mT, Bz = 5 mT

MMagnetic helicity injection is current driveagnetic helicity injection is current drive


Magnetic helicity: linkage between magnetic fluxes

K is conserved in magnetized plasmas, decaying on resistive timescales.

In tokamaks, K is proportional to the product ITFIp.Increases in K correspond to increases in Ip.

Driving current on open field lines is helicity injection

DDC helicity injection startup on PEGASUS utilizes C helicity injection startup on PEGASUS utilizes

localized washer-gun current sourceslocalized washer-gun current sources


• Plasma gun(s) biased relative to anode:

– Helicity injection rate:

Vinj - injector voltage

BN - normal B field at gun aperture

Ainj - injector area

• Plasma guns have geometric flexibility

• Gun-based system can be scaled to larger devices, such as NSTX

Ý K inj 2VinjBN Ainj

Anode

3 plasma guns

Plasma streams

Divertorinjection

Midplaneinjecton

DDriven helical filaments can relax to an riven helical filaments can relax to an

axisymmetric tokamak-like stateaxisymmetric tokamak-like state


• Driven helical filaments are strongly unstable– Relax into the axisymmetric tokamak-like state

• Tokamak-like equilibrium satisfies a set of conditions– Radial force balance

– Helicity/power balance

– Kink stability: edge q > 3

– Taylor relaxation current limit

• Max Ip is determined by helicity injection rate and the Taylor relaxation limit, related to magnetic geometry– Scales with ITF, Ibias, and the width w of the driven layer

II

pp > 170 kA non-solenoidal startup > 170 kA non-solenoidal startup achieved with < 4 kA injected currentachieved with < 4 kA injected current


KFIT reconstructionfor #45321, a similar150 kA plasma,near Ip peak.

Raxis 0.33ma 0.28m 2.3li 0.48T 1.7%p 0.23

HHelicity Balance Provides One Limit on Current Ielicity Balance Provides One Limit on Current Ipp


• Far below relaxation current limit: max Ip occurs when dK/dt balances resistive decay

• Decay Vloop estimated with Vsurf

• Veff ≈ Vsurf indicates:

1. Injected helicity gets into the plasma

2. Maximum driven current limited by dK/dt

This study used only static-fielddivertor-gun discharges(no induction drive)

MMidplane-driven plasmas evolve inwardidplane-driven plasmas evolve inward


Estimated plasma evolution

Plasma guns

AnodeForms as small circular outboard plasma

As the helicity content increases,plasma expands into high-field region

Maintaining radial force balance may require vertical field ramps

TTaylor relaxation criteria also limits the aylor relaxation criteria also limits the

sustainable Isustainable Ipp for a given magnetic geometry for a given magnetic geometry


MMaximum Iaximum Ipp achieved when helicity and achieved when helicity and

relaxation limits are satisfied simultaneouslyrelaxation limits are satisfied simultaneously


Estimated plasma evolution

Plasma guns

Anode

These particular parameters require a Bv ramp, both for radial force balance and some induction.

Relaxation limit

Helicity limitIp max

Time

ITF = 288 kAVbias = 1kVVind = 1.5 VIinj = 4 kA w = dinj

L-mode e

SSufficient helicity injection is required to ufficient helicity injection is required to

drive the plasma up to the relaxation limitdrive the plasma up to the relaxation limit


K

DC Vbias Zinj

Max Ip increases with Vbias

R = 47 cm

120 V

900 V

Vbias =1200 V

Relaxation limit

Helicity Limited

High dK/dt with modest Ibias requires high impedance Zinj

EExperiments confirm relaxation limit xperiments confirm relaxation limit

scalings with Iscalings with ITFTF and I and Ibiasbias


• The relaxation limit Ip scales with (ITFIbias)1/2

• Experimental plasma currents follow these scalings:

EExperiments demonstrate dependence on xperiments demonstrate dependence on

the width of the driven current layerthe width of the driven current layer


• Relaxation current limit scales as w-1/2

• One-gun discharges had higher limits than corresponding three-gun cases, indicating the gun array was misaligned:

Anode

3 guns

w

GGun array was realigned, significantly un array was realigned, significantly

improving plasma performanceimproving plasma performance


• Changing the tilt of the gun array increased the max Ip by a factor of 1.5-1.7, implying a factor-of-3 change in w.

• In this configuration, have achieved Ip > 170 kA.Anode

3 guns

w

After alignmentBefore alignment

PPlasma gun startup provides a robust lasma gun startup provides a robust

target plasma for Ohmic handofftarget plasma for Ohmic handoff


• Handoff to Ohmic– Gun-driven target ~80 kA

– Most pre-OH current is captured by the OH drive

• Corresponding OH-only– Gun startup saved ~ 50% flux

• Will couple high-Ip targets to double-swing OH ramp

For more details, see D. Schlossberg’s talk at 2:15PM today





– Characterization of the modes

– Measuring JT(R) and p(R) to test theory


• Summary


OObserved Filaments are Similar to ELMsbserved Filaments are Similar to ELMs


• Filamentary, field-aligned structures – Present under conditions of high <jedge/B>

• PEGASUS: L-mode edge assumed– However, may still manifest same instability

Maingi, Phys. Plasmas 13, 092510 ,2006

NSTX

Scannell, Plas. Phys. Controlled Fus. 49, 2007

PEGASUS

MASTKirk, Plas. Phys. Controlled Fus. 49, 2007


NNear-unity A Maximizes Peeling Driveear-unity A Maximizes Peeling Drive

*: Thomas, Phys. Plasmas 12, 056123 2005

Device Jedge (MA/m2) Bφ,0 (T) Jedge/B

PEGASUS

~ 0.1 – 0.2 0.1 ~ 1

DIII-D* 1 – 2 2 0.5 – 1• PEGASUS operations at A → 1 lead to naturally high jedge/B

− Comparable to larger machines in H-mode

• However, source of peeling drive different – Large machines: H-mode p’ → jBS

– PEGASUS: Large dIp/dt (≤ 50 MA/s) → transient skin current

• Low-A geometry enables ITER-relevant research

AAccurate Stability Analysis Requires Local ccurate Stability Analysis Requires Local

Measurements of JMeasurements of JTT and p’ and p’


• Comparing experiment to peeling-mode theory requires accurate edge profiles:– Need both edge p(ψ) and j(ψ) profiles

• PEGASUS: measurements using probes– Hall-effect array constrains j(ψ)

– Langmuir array constrains p(ψ)

IInitial Internally Constrained Equilibriumnitial Internally Constrained Equilibrium


• KFIT* using 5 point cubic spline basis set

• Better basis set is being implemented

*Sontag, A., et. al., Nuclear Fusion, 48, 095006 , 2008

Ip 157 kAR0 .30 ma .24 mA 1.2κ 2.2ℓi .25βp .10βt .02q0 6.1q95 17

PPeeling-mode studies are in progresseeling-mode studies are in progress


• Filamentary edge instabilities consistent with peeling modes– Nonlinear phase: explosive detachment and radial acceleration

• Direct comparison to theory requires accurate equilibria– Probes measure Bz(R,t) and pressure in the plasma edge

– Local equilibrium code KFIT is being modified, with basis functions that can capture the experimentally constrained profiles

• ITER-relevant physics accessible at low cost by operating at very low aspect ratio

• For more details, and progress toward testing the theory, see M. Bongard’s talk tomorrow at 10:40AM






– High-Ip non-solenoidal startup and growth

– Implementation of RF systems (EBW, HHFW)

– High-IN, high-T studies

• Summary


FFurther development of gun startup urther development of gun startup

needed to achieve high Ineeded to achieve high Ipp


• PEGASUS near-term goal is 0.3-0.4 MA– Allows characterization of confinement/dissipation in driven plasma

– Enables access to high Ip/ITF, high regimes

• This requires:– Increase TF: increase Taylor limit, improve confinement

– Increase gun current: increase Taylor limit

– Bigger/improved plasma guns: higher helicity injection rate

– Test augmenting the guns with shaped electrodes

• Outstanding issues:– What sets the bias impedance Zinj?

– Is the confinement/dissipation stochastic?

– What sets the width w of the driven region?

NNon-solenoidal startup with RF and/or on-solenoidal startup with RF and/or

helicity injection in PEGASUShelicity injection in PEGASUS


• Two RF systems to be available: HHFW and EBW

• HHFW: Two-strap antenna, 0.8 MW, 8-18 MHz

• EBW planned for next year: 0.5-1.0 MW @ 2.45 GHz– 2.45 GHz enabled by very low field at low-A

• Enables comparison of non-solenoidal startup scenarios:– Helicity injection + outer-PF induction

– EBW heating + outer-PF induction

– Helicity injection + EBW heating + outer-PF induction

• Enables non-solenoidal sustainment on PEGASUS:– RF heating, possible bootstrap overdrive

– Intermittent helicity injection

MMidplane-launched EBW damps near idplane-launched EBW damps near

magnetic axis and gives optimal heatingmagnetic axis and gives optimal heating


Ray-tracing and power depositioncalculations by S. Diemusing GENRAY and CQL3D.

PProposed PEGASUS Facility Modificationsroposed PEGASUS Facility Modifications


• Magnetic and power systems reconfiguration– Increase TF by factor of 2

– Activate divertor coils

– Deploy existing PCS

– Helicity injection supply development

• Internal hardware modifications– Install passive conducting plates

– Optimize gun geometry

– Install enhanced guns and/or electrodes

– Install internal radial position coils

• Improved core and edge diagnostics– Multi-point Thomson scattering

– Poloidal SXR array

– Ion spectroscopy: flows and Ti

– Visible brehmsstrahlung

Present Configuration

NNear-term planned physics campaignsear-term planned physics campaigns


• Continue and extend non-solenoidal current drive studies– Test understanding of relaxation current limit

– Study confinement/dissipation, bias impedance, driven layer width

– Use additional tools as they become available: RF, increased TF, etc

– Target is 0.3-0.4 MA non-solenoidal plasma current

• Rigorously test peeling-mode theory– Use current-profile and pressure-profile constraints

– Use divertor coils to add shear

• Explore non-solenoidal sustainment– Enables high-IN, high-T studies

PPEGASUS Summary: Creating High-IEGASUS Summary: Creating High-Ipp Non- Non-

Solenoidal Discharges & Studying Edge StabilitySolenoidal Discharges & Studying Edge Stability


• Exploration of high IN, t space facilitated by j(r) tools– Ip/ITF > 2, IN > 14 achieved; extend operation to high Ip, ne for high t

• Making progress with non-solenoidal startup– Ip ~ 170 kA using helicity injection and outer-PF rampup

– Using understanding of helicity balance and relaxation current limit to guide hardware and operational changes

• Adding RF systems: develop startup and sustainment scenarios

• Outstanding physics questions: edge, Zinj, confinement, etc.

– Ultimate goal is 0.3-0.4 MA non-solenoidal current

• Able to rigorously test Peeling-Ballooning theory– Edge measurements constrain equilibrium reconstructions

– Can compare stability calculations to experimental observations

the pegasus toroidal experiment: recent results and future plans

Documents

international st workshop

solenoidal current drive

current driveaaron

current drive techniques

overall st program

magnetic helicity injection

highb studiesfuture

high jbedgecan