H-mode and Non-Solenoidal Startup inthe Pegasus Ultralow-A Tokamak

R. J. Fonck for the Pegasus Group

26th IAEA Fusion EnergyConference
17-22 October 2016

University ofWisconsin-Madison

Kyoto International ConferenceCenterKyoto, JapanPEGASUS
Toroidal ExperimentPegasus ST at A~1 Facilitates AT Science
Studies at Small Scale
LHI Phase

High-stress Ohmic Heating Solenoid

OH Phase

H-mode2mTime [ms]•Local DC
Helicity
InjectorsA
R (m)Ip (MA)
BT (T)
Δtshot (s)RJF IAEA FEC2016 - 2Local Helicity Injection (LHI) for ST Startup–Inject current streams in plasma edge

Divertor Coils

1.15 – 1.3
0.2 – 0.45
≤ 0.25< 0.2≤ 0.025•

H-mode Physics at Ultralow-A–•

H-mode and ELM characteristics

Access to high IN > 10–Tokamak stability limits at A ~ 1

H-mode Readily Accessed at A ~ 1Limited L

Limited H
Fast visible imaging, Δt ~ 30 μs•

Low BT at A ~ 1 -> low H-mode PLH––•

Standard H-mode features––––RJF IAEA FEC2016 - 3

POH >> PITPA08 ~ BT0.80 ne0.72 S0.94
Limited or Diverted topology

Quiescent edge
Reduced HaImproved tESome differences compared to high-A

Diverted H

Edge Pedestals, Increased Confinement• Simple probe access through thepedestal in H-phase2• Short pulse, low Te,edge

Jφ [kA/m ]Single-shot, !t = 6 "s-1000– Jϕ(R,t): multichannel Hall probe1,2

H-200

65673
65678

L0.52 0.54 0.56 0.58 0.60 0.62

R [m]– p(R): triple Langmuir probe

Multi-shot average• Confinement increases 2x– L: H98 ~ 0.5±0.2– H: H98 ~ 1.0±0.2

p [Pa]– Requires time-evolving reconstructions

2072914-7293472842-72876

H100-8L-6-4-20R-Redge [cm]Bongard et al., Rev. Sci. Instrum. 81, 10E105 (2010)
Bongard et al., Phys. Rev. Lett. 107, 035003 (2011)

RJF IAEA FEC2016 - 42

PLH Consistent with Global Scalings, but Low-A
DifferencesA ~ 1.2 Equilibria
25

LimitedDiverted

20q

Z [m]

15
10

Diverted

Limited
0.8

0.8

0.4

0.4
Z [m]

Normalized PLH vs. Density

0.0-0.4-0.4

500.6

0.70.8ψN0.9

1.0-0.8
0.0

0.070914
t=24.025 ms

73580
t=25.055 ms-0.80.4
0.8 0.0R [m]0.40.8R [m]• Follows ITPA ne scaling• Magnetic topology independence– FM3: min ne ~ 1×1018 m-3– Diverted, limited edge q(y) similar– FM3 model:(PLHLIM/PLHDIV) ~ (qyLIM/qyDIV)-7/9 ~ 1

RJF IAEA FEC2016 - 5

Fundamenski et al., Nucl. Fusion 52, 062003 (2012)

At Low A, PLH ≫ PITPA08
Multi-Machine PLH / PITPA08 Comparison

Maingi et al., Nucl. Fusion 50, 064010 (2010)
Martin et al., J. Phys.: Conf. Ser. 123, 012033 (2008)
3 Wesson, Tokamaks, 4th ed. (2011), p. 630
12

RJF IAEA FEC2016 - 6

A ~ 1: ELMs n-numbers tend lower
Quiescent•Large (Type I) ELM

Small (Type III) ELM

Small (“Type III”):– Ubiquitous, lessperturbing– At POH ~ PLH– Low-n: A-dependent
structure• Peg., NSTX: n ≤ 4• A ~ 3: n > 8•

Small (Type III)

Large (Type I)

Large (“Type I”)– Infrequent, violent– At POH >> PLH– Intermediate-n: lower
range at low-A

RJF IAEA FEC2016 - 7Maingi, Nucl. Fusion 45, 1066 (2005)
Perez, Nucl. Fusion 44, 609 (2004)
Kass, Nucl. Fusion 38, 111 (1998)

Nonlinear ELM Precursors Observed• Magnetic signature of ELMs
have multiple n components– n = 8 grows continuously

Mode ∂B/∂t• Modes show different timeevolutions

Mode Amplitude (t)– Simultaneously unstable modes– n = 6 fluctuates prior to crashTime [ms]RJF IAEA FEC2016 - 8

Time [ms]Complex Evolution of Jedge(R,t) Measured
During ELMs•

Challenge: nonlinear ELM dynamics at
Alfvénic timescales•Ubiquitous current hole formationobserved:– Type III: smaller, slower, no filament evident– Type I: larger, faster, evident filament– Qualitatively similar to JOREK– Probe access: simulation validation opportunities•

Complex, multi-modal collapse of edge
and filament formation

RJF IAEA FEC2016 - 9Large, Type I ELM nonlinear
Jedge(R,t) evolution

3D Edge Current Injection May Influence ELM Stability•

LHI system affects edge
plasma– Strong 3D edge current perturbation– Edge biasing to modify rotation– Similar to LHCD on EAST1•Jedge injection in H-mode
suggests ELM suppression– Low Iinj = ELM suppression– High Iinj = edge and discharge
degradation

1: Liang et al., Phys. Rev. Lett. 110, 235002 (2013)

RJF IAEA FEC2016 - 10

Three-Injector Array

Ip [MA]

Local Helicity Injection (LHI) Provides Robust
ST Startup without OH Solenoid
0.20Iinj

8OH (H-mode)

LHI

40.05
0.00

Unstable injectedcurrent streams

202530Time [ms]Reconnect, relax toTokamak-like state

3540

Vloop [V]0.10

RJF IAEA FEC2016 - 11120.150Subsequent
OH-Driven TokamakM nonlinear 0'RS'(K,$).

66221

35

Ohmic H-mode

30

or radial build.

ms]
40

H-mode startup.

Multi-Year Technology Development has Produced
Robust, High Performance Current Injectors!

– Jinj~1kA/cm2

1.6 cmVeff-VIR
R

VPF + VLpA

35

VINJ [V]

32

2

11Fig

RJF IAEA FEC2016 - 1230

– Mo PFCs

Gas Feed

1.0– Local limiter

Clean, High-Vinj
Operation
25– Cascaded shield rings

Shield Rings

0.5• PMI control: 1-2 cm from LCFS

20

Arc Source

Time [ms]Fig. 3-19. Concave injector with PMIvs. quiescent frustum shield design.

– Floating cathode shield

Injection
Cathode0.0

CathodeShield• High-voltage in SOL: Vinj > 1kV– Frustum cathode

Loop Voltage

Fig. 3-16. (a): 0-D model and !T($ );(b): 0-D LHI drive voltages.

Fig. 3-22. Maximum, average &XHY in conditioning.

• Washer-stack arc source:

Three-Injector
ArrayPhysics Models Provide a Predictive Understandingfor LHI Startup
Taylor Relaxation

1. Taylor relaxation, helicity
conservation

I p ≤ ITL ~– Steady-state maximum Ip limits

ITF I injw

Helicity Conservation

VLHI ≈Ainj Bϕ ,inj
ΨI p [VLHI +VIR +VIND ] = 0 ; I p ≤ ITL

2. 0-D power-balance Ip(t)34– VLHI for effective LHI current drive

Reconnecting LHI Current Stream

3. 3D Resistive MHD (NIMROD)– Physics of LHI current drive mechanism

Figure 1.14: Reconnection releases an axisymmetric current ring (shown near the centerof the figure) from the driven current channel plasma. Isosurfaces of = ±6 m 1 areshown at t = 2.95 ms in the 2-fluid computation, when Ip ' 7 kA. The central column
is shown for reference.

RJF IAEA FEC2016 - 13Vinj

D.J. Battaglia, et al. Nucl. Fusion 51 (2011) 073029.
N.W. Eidietis, Ph.D. Thesis, UW-Madison, 2007.
J. O’Bryan, Ph.D. Thesis, UW-Madison, 2014.
J. O’Bryan, C.R. Sovinec, Plasma Phys. Control. Fusion 56 064005 (2014)

Power Balance Model Provides
Predictive Tool for Ip(t)• Model reasonably recreates Ip(t)I p [VLHI +VIR +VIND ] = 0

0.20– VLHI: effective drive– VIND: analytic, from shape(t)– Taylor relaxation limit: Ip ≤ ITL0.15

Ip [MA]– VIR: resistive dissipation

0.0

κ: 1→2
δ: 0→ 0.5

R [m]

0.5

Iinj

202530Time [ms]System Voltages [V]

Z [m]

-0.5

<Te> = 60eV

0.05

0.5

LHI Drive Limited

0.100.00

Shape 0.0
Evolution

TaylorLimited• VIND dominates current drive
RJF IAEA FEC2016 - 14

Eidietis et al., J. Fusion Energ. 26, 43 (2007)
S.P. Hirshman and G.H. Nielson 1986 Phys. Fluids 29 790
O. Mitarai and Y. Takase 2003 Fusion Sci. Technol.
Battaglia et al., Nucl. Fusion 51, 073029 (2011)35

NIMROD Describes Current Helical CurrentStream Reconnection as Drive Mechanism•Divertor injection = no inductive drive

Divertor LHI Startup Shows suggestive
commonality between experiment andNIMROD modeling

Current Ringt = 2.91 ms

1. Streams follow
field linest = 2.93 ms

2. Adjacent passes
attractt = 2.95 ms

3. Reconnection
pinches off current
rings

NIMROD Simulation[O’Bryan PhD 2014]

J. O’Bryan, et al., Physics of Plasmas, 19, 080701 (2012)
J. O’Bryan, C.R. Sovinec, Plasma Phys. Control. Fusion 56 064005 (2014)
RJF IAEA FEC2016 - 15May 2016 PEGASUSHigh-speed Imaging

Current Stream Interaction Manifests as
Edge-Localized MHD Burst
PEGASUSNIMRODIp [kA]Ip [kA]

6040200400~b[mT]– Infers NIMROD streams inedge~ḃ [T/s]

Magnetics localize
coherent streams in edge

Outboard Mirnov [T/s]•

Ip [kA]

80200
0-200-400
16171819202122

Time [ms]•Reconnection-drive edge
ion heating
Internal Bz Measurements

External Bz Phase Correlation
0.4

Any stochastic
reconnection region may
be localized to edge

InferredStream Location
Z [m]•-0.40.0RJF IAEA FEC2016 - 16

R [m]

0.85

Te(0) > 100 eV Suggests
Favorable Confinement Scaling?
Peaked Te(R) while Connected to Injectors•Te(0) ≳ 100 eV comparable toOhmic L-mode– With ne  1019 m-3, provides goodtarget plasmas for subsequent
sustainment– Not strongly stochastic across profile– Drive: VIND (across plasma), VLHI(edge)•Disconnected

Possible two zone confinement

Confinement estimates suggest
favorable scaling to NSTX-U– OH, stochastic models

Confinement Scaling Estimates

Plasma Current [MA]•Connected0.50.4Ohmic
L-mode0.30.2<Te>=30eV

0.1
0.0012345
Normalized Helicity Injection::Normalized
A V Helicity/R [V-m]Injection
inj inj

RJF IAEA FEC2016 - 17R-R
Stochastic

A.B. Rechester and M.N. Rosenbluth, PRL 40 (1) 1978
C.R. Sovinec and S.C. Prager, Phys. Plasmas 3 (3) 1996
Stoneking, et al., Phys. Plasmas 5 (4) 1998

inj

Reconnection-driven Ion HeatingGives Ti > Te During LHI

Continuous ion heating from reconnection
between collinear current streams––No effect on current drive efficiencyIp [kA]

•Impurity Ti(0) ~ 100 – 500 eV > Te routinelyobserved during LHI(a)1000dBz/dt[T/s]•Ion heating correlated with high frequency MHD
fluctuations, not with discrete reconnection
between helical streams

400(b)

0-400

Significant ion heating (~ few 0.1 MW)

161820222426HeIITi [eV]200

Ion heating consistent with
2-fluid reconnection theory
300

3%

Ti,⊥
Ti,||200(d)

Ti,⊥

Ti,||

5 - 70 kHz

21150

01000.02 %500.010(c)0250
He-II Ti [eV]

1000102030
1/2Iinj(Vinj)

RJF IAEA FEC2016 - 18[a.u.]4050(e)0.2-0.4 MHz

025

26
Time [ms]2728Understanding Confinement Scaling in LHI is
Critical for Predicting to NSTX-U and Beyond
NSTX-U: Confinement Scalings

1.0Rapid improvement with VLHI
under favorable scalings

<Te>=
100eV

0.8 ITER-97L
Ip [MA]•– Possible reduction in injector
requirements

Scaling

0.6

R-R
Stochastic

0.4
0.2

Veff

480.0

Ip [MA]

1.60.3 PEGASUS
1.2 NSTX-U
Projected
0.2 Projected
0.80.10.0

Veff-V-IR
VIR200.40.02AinjVinj2= 18-21 kVcm
AinjVinj ~ 4.5 kV-cm
4 Te = 60 eV Te= 60 eV
4VPF + VLp

0.5

NSTX-U: VLHI ~ VIND

System Voltages [V]

1.0Ip [MA]0.0V
1.5-VIR LHI-VIR
2VPFV+IND
VLp

Time [ms]Time [ms]Zeff = 2eff

eff

22Veff

VPF 0+ VLp

Zeff = 11.6
1.2 NSTX-U
Projected
0.8
Z =10.4
Z =20.0

AkVcm
AinjVinj= V18-21injVinj = 40 kV-cm Veff
eff
8 Te= LHI
860 eV Te = 150 eV4012 0 16 4 0 8 412 816 120 164

Time [ms]RJF IAEA FEC2016 - 190.1

System Voltages [V]Veff

Ip [MA]Ip [MA]

AinjVinj
Te = 60 eV

0.5– Confinement studies needed when
0.0
sustained by VLHI
00.1

System Voltages [V]1.0234Vnorm = NinjAinjVbias/Rinj [V-m]

0.3 PEGASUS
0.3 PEGASUS
0.2 Projected
0.2

0.0
2.0 2
~ 4.5 kV-cm
System Voltages [V]Will need VLHI ~ VIND

System Voltages [V]

•0.1

1.51

PEGASUS: VLHI < VIND

0.20.02.00Ip [MA]

Current projections: Ip~1 MA on
NSTX-U accessible
0.3 PEGASUSIp [MA]•<Te>=30eV

System Voltages [V]0.0-VIR

VPF + VLp

AinjVinj = 40

Te = 150-V4-VIRIR
0VPF V+ VLp
IND

0 100
20 40 60
08 201240 1660 80

Time [ms]Time [ms]Time [msDivertor Injection Experiments Provide
Confinement Test & Higher Ip
0.5– Development to minimize PMI as BTF increases

0.4•

Minimal VIND

0.3

•3-4x increase in HI drive: Veff ~ AinjVinj/Rinj•Reconnection mechanisms to higher Ip, BTF0.1•

Injectors at longer pulse, high-BTF

0.00Ip [MA]

Initial DIV injector campaign in progress•0.2

Projected Injector Performance
Previous DivInjectors
Ohmic
L-modeNew DivInjectors
R-R
Stochastic
Fixed
Te=30eV1234
Normalized Helicity Injection :AinjVinj/Rinj [V-m]CenterstackLower DIV
Strike Plate

DIV injectors

RJF IAEA FEC2016 - 205

Divertor Injection Provides Non-solenoidal
Sustainment at High IN• Constant geometry: minimal VIND

IN = 10.7• Low ITF ~ 0.6 Ip• IN > 10 accessible– Constant or ramped-down BTFVinj• Potential for high bT– Aided by anomalous ion heating

NeRJF IAEA FEC2016 - 21

TeTi_OVLHI Provides Access to High-bT at A ~ 1 with NonSolenoidal Sustainment and Anomalous Ion Heating
Initial Exploration of High-bT Space••

Equilibrium reconstructions estimate bT (~<P>/BT02)–Matches external magnetics, Ptot(0), and edge in Te(R)–Includes anomalous Ti(0)–Some caveats for these initial results
Assumes closed flux surfaces inboard of injectors•Role of SOL edge current•Magnetics-only reconstructions scaled via comparison tothose with kinetic constraints•

Need full kinetic profiles in futureHigh bT plasmas often terminated by disruption–••n = 1, low-m precursors

Expands accessible high IN, bT space for tokamak
stability studies at extreme toroidicity–Campaign underway to document, extend to higher Ip–

Improved LHI injector hardware to increase Ip, BTF
accessRJF IAEA FEC2016 - 22

Studies at A ~ 1 Support Development of theTokamak Concept•H-mode plasmas with pedestal diagnostic access– Standard characteristics: pedestal; low Dα; increased τe; H98– Strong PLH scaling with A– Insensitivity to magnetic topology– Transient current hole formation during ELMs
– ELM mode structure reflects strong peeling drive•

Local Helicity Injection provides non-solenoidal startup and sustainment– IP up to ~ 0.18 MA with Iinj ~ 5-6 kA– Taylor Relaxation, Helicity conservation, and resistive MHD modeling– Appears scalable to large scale, but questions on confinement and reconnection dynamics– Flexible geometry options for helicity and inductive drive tradeoffs, and engineering constraints•

Non-solenoidal Ip(t) via LHI enables access to stability tests at extreme
toroidicity– Sustained operation at high IN, high bTRJF IAEA FEC2016 - 23