Nov 2, 2016 - Injection Campaign. Highlights ... Initial HFS injector campaign in progress. â Development to .... System automation. â Intra-shot beam ...
Overview of Non-Solenoidal Startup Studies in the Pegasus ST M.W. Bongard, J.L. Barr, G.M. Bodner, M.G. Burke, R.J. Fonck, J.L. Pachicano, J.M. Perry, J.A. Reusch, N.J. Richner, C. Rodriguez Sanchez, D.J. Schlossberg
58th Annual Meeting of the APS Division of Plasma Physics
University of Wisconsin-Madison
2 November 2016 San Jose, CA
PEGASUS Toroidal Experiment
Layout 8.5” x 11”
8’ W x 4’ H
Title Banner Technology and Diagnostic Development
Local Helicity Injection (LHI) Provides Robust Non-Solenoidal Startup on the PEGASUS ST
Three HFS Injection Systems Implemented and Tested Since April 2016
Power Balance Model Provides Predictive Tool for Ip(t)
NIMROD Describes Current Helical Current Stream Reconnection as Drive Mechanism
LFS Local Helicity Injection Produces Core Te > 100 eV
Ip > 0.15 MA Achieved Via HFS Injection To Date
LHI Startup Scenarios Grow From Helical Current Streams to Quality, High Ip Plasmas
Multi-Year Technology Development has Produced Robust, High Performance Current Injectors
Analytic Formulation of Power Balance Model Elements Allow Partitioning of Energy Flow
Current Stream Interaction Manifests as Edge-Localized MHD Burst
Te (R, t) Remains Peaked for LFS Injection Geometry and Minimal VIND
HFS Helicity Injection Provides Non-Solenoidal Sustainment at High IN
High-Field-Side (Divertor) Injection Experiments Provide Confinement Tests & Higher Ip
Large-Ainj Injector Design Provides Enhanced Performance, Simplified Geometry
EquilibriumCalibrated Inductance Model Improves Estimates of NonSolenoidal VIND
ReconnectionDriven Ion Heating Gives Ti > Te During LHI
Technical Challenges Arise for LHI Startup With HFS Injection Geometry
LHI Provides Access to High-T at A ~ 1 with Non-Solenoidal Sustainment and Anomalous Ion Heating
Hierarchy of Physics Models Provide a Predictive Understanding for LHI Startup
Thomson Scattering Enhancements to Measure Te and ne Profiles During LHI
0-D Power Balance Model Used to Explore Projections for NSTX-U Startup
Different MHD Activity Observed Between LFS and HFS Injection Geometry
Poloidal Field Shaping Facilitates Relaxation at Full Toroidal Field (Binj = 0.23 T)
Progress in NonSolenoidal Startup on Pegasus
M.W. Bongard, APS-DPP 2016
Progress Toward Predictive Models of LHI
2016 Helicity Injection Campaign Highlights
Non-Solenoidal Startup via Local Helicity Injection
Reprints
Non-Solenoidal Startup via Local Helicity Injection
Technology and Diagnostic Development
Progress Toward Predictive Models of LHI
2016 Helicity Injection Campaign Highlights
Local Helicity Injection (LHI) Provides Robust Non-Solenoidal Startup on the PEGASUS ST Ip ≤ 0.18 MA via LHI (Iinj = 5 kA)
Helicity Injectors
LFS System HFS System
Anode
Shaped W Cathode/Anode
Mo PMI Suppression Guard Rings
VINJ
IINJ
+
Mo Cathode/Anode Shield
IArc
Mo Arc Cathode
Mo / BN Washer Stack
Varc
D2 gas feed
M.W. Bongard, APS-DPP 2016
+
Plasma Parameters Ip ≤ 0.23 MA shot ≤ 0.025 s BT 0.15 T A 1.15–1.3 R 0.2–0.45 m a ≤ 0.4 m κ 1.4–3.7
Injector Parameters Iinj ≤ 14 kA Iinj ≤ 4 kA Vinj ≤ 2.5 kV Ninj ≤ 4 Ainj = 2-4 cm2 Iarc ≤ 4 kA Varc ≤ 0.5 kV
LHI Startup Scenarios Grow From Helical Current Streams to Quality, High Ip Plasmas Three-Injector Array
LHI
OH (H-mode)
Injector Shutoff
Null Formation
Relaxation
Unstable injected current streams M.W. Bongard, APS-DPP 2016
Reconnect, relax to Tokamak-like state
Subsequent OH-Driven Tokamak
High-Field-Side (Divertor) Injection Experiments Provide Confinement Tests & Higher Ip •
Initial HFS injector campaign in progress – Development to minimize PMI as BTF increases
•
Configuration minimizes VIND
•
3-4x increase in HI drive: Veff ~ AinjVinj/Rinj
•
Test reconnection mechanisms at higher Ip, BTF
•
Injectors at longer pulse, high-BTF
NSTX-U Projected Performance Ohmic L-mode
R-R Stochastic
Fixed Te=150 eV
Fixed Te = 75 eV
: AinjVinj/Rinj [V-m]
Centerstack Lower DIV Strike Plate
DIV injectors M.W. Bongard, APS-DPP 2016
Hierarchy of Physics Models Provide a Predictive Understanding for LHI Startup Taylor Relaxation
1. Taylor relaxation, helicity conservation
I p ITL ~
– Steady-state maximum Ip limits
ITF I inj w
Helicity Conservation
VLHI
Ainj B ,inj Vinj
I p VLHI VIR VIND 0 ; I p ITL
2. 0-D power-balance Ip(t) – VLHI for effective LHI current drive
Reconnecting LHI Current Stream
3. 3D Resistive MHD (NIMROD) – Physics of LHI current drive mechanism
M.W. Bongard, APS-DPP 2016
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)
Three HFS Injection Systems Implemented and Tested Since April 2016 • Two injectors at toroidally opposite positions in lower divertor region • Design point leverages high Ainj – 3–4 increase in VLHI over prior systems
Configuration 1
Configuration 2
– Ainj = 8 cm2 total – Vinj ≤ 1.2 kV – Iinj ≥ 8 kA total Configuration 3
• Systems vary Rinj, Zinj, local limiter geometry – Latest design incorporates floating, electropolished divertor shield plates M.W. Bongard, APS-DPP 2016
Multi-Year Technology Development has Produced Robust, High Performance Current Injectors Three-Injector Array
• Washer-stack arc source: – Jinj ~ 1kA/cm2
• High-voltage in SOL: Vinj > 1kV – Frustum cathode – Floating cathode shield
1.6 cm
Cathode Shield Injection Cathode Arc Source
• PMI control: 1-2 cm from LCFS
Shield Rings
– Cascaded shield rings – Local limiter – Mo, W PFCs
M.W. Bongard, APS-DPP 2016
Gas Feed
Clean, High-Vinj Operation
Large-Ainj Injector Design Provides Enhanced Performance, Simplified Geometry •
New injectors designed for HFS system – Doubled Ainj (2 cm2 4 cm2) – Compact design for lower divertor region
• Modular internal assembly – Permits in-vessel maintenance/repositioning – Exterior PFC components rapidly adjusted about common arc chamber / fueling system • Changes to Ainj, shield structures
– Integrated hypodermic gas feed alleviates field sensitivity from previous
• Refractory materials for resilience to harsh environment – W for high-Vinj cathode/anode – Mo for external shield assemblies M.W. Bongard, APS-DPP 2016
New: 4 cm2 Old: 2 cm2
Thomson Scattering Enhancements to Measure Te and ne Profiles During LHI • Improved timing / synchronization
Thomson Viewing Locations and A ~ 1 Plasma
– Higher realized laser power – Lowered beam scrape-off losses
• System automation – Intra-shot beam alignment – Data acquisition
• Stray light mitigation – Baffling, electronic gating
• Background signal reduction – Wire grid polarizers – High speed shutters M.W. Bongard, APS-DPP 2016
80 cm viewing region
Power Balance Model Provides Predictive Tool for Ip(t) • Model reasonably recreates Ip(t)
I p VLHI VIR VIND 0 – VLHI: effective drive – VIR: resistive dissipation – VIND: analytic, from shape(t) – Taylor relaxation limit: Ip ≤ ITL
• VIND dominates current drive with LFS mid-plane injection M.W. Bongard, APS-DPP 2016
System Voltages [V]
Shape Evolution
= 60eV
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)
Analytic Formulation of Power Balance Model Elements Allow Partitioning of Energy Flow I p VPF Vgeo VWm VIR VLHI 0
•
Recent Improvements
VIND Inductive Drive from Poloidal Fields
d VPF PF M V R20 BV t coils dt
R0
0 I p 1 Le ℓ i 1 BV p 4 R0 0 R 2 2 2 c( ) 1 0.98 2 0.49 4 1.47 6 1 M V (, ) 2 1 c( ) d( ) d( ) 0.25 1 0.84 1.44 2
Revised Lp, BZ models*
–
Moving plasma boundary
–
Neoclassical resistivity
LHI Drive
Ainj B ,inj VLHI Vinj eff
Resistive Dissipation
2 R0 VIR I p Rp I p Ap
Inductive Drive from Shape(t)
Plasma Magnetic Energy Change
dI p d dL Vgeo Le I p Le Ip e dt dt a( ) 11.81 2.05 ln 8 dt a( )(1 ) 2.0 9.25 1.21 Le 0 R0 b( ) 0.73 1 2 4 6 5 3.7 6 1 b( ) M.W. Bongard, APS-DPP 2016
–
VWm
1 d 1 2 Li I p I p dt 2
* S.P. Hirshman and G.H. Nielson 1986 Phys. Fluids 29 790 O. Mitarai and Y. Takase 2003 Fusion Sci. Technol. S. Ejima et al 1982 Nucl. Fusion 22 1313 J.A. Romero and JET-EFDA Contributors 2010 Nucl. Fusion 50 115002
C p2 ℓi Li 0Vp
Equilibrium-Calibrated Inductance Model Improves Estimates of Non-Solenoidal VIND •
Maintaining radial force balance provides VIND –
Originally calculated via H-N formulae
•
Important to quantify contributions from shape, PF drive in LHI system design
•
Model equilibium database generated to test analytic formulae in realistic magnetic geometries – –
•
3
Poor partitioning of VIND between shape, VPF components found –
•
N = 331; 1.15 < A < 8; 1 0 1; 0.2 ℓ 0.75
However, total flux estimates in better agreement
Revised VIND model developed – –
Derived new coefficients in H-N formalism via fit to equilibrium database Weak dependence on , ℓ introduced
M.W. Bongard, APS-DPP 2016
S.P. Hirshman and G.H. Nielson 1986 Phys. Fluids 29 790
0-D Power Balance Model Used to Explore Projections for NSTX-U Startup •
•
Helicity dissipation (VIR) dependent on Te, realized electron confinement
Shape evolution for LFS LHI on NSTX-U
NSTX-U: Projected = 150 eV
Importance of VLHI, VIND depends on injector geometry, plasma growth scenario – Final plasma depends strongly on full time evolution
•
Injector geometry emphasizes different drive terms – LFS injection: VLHI early, VIND late – HFS: injection mainly VLHI
•
Need to explore plasma evolution with different dominant drive terms – Informs predictive model – Future: High Ip tests in both geometries
M.W. Bongard, APS-DPP 2016
LFS PEGASUS
HFS PEGASUS
NIMROD Describes Helical Current Stream Reconnection as Drive Mechanism
•
Divertor injection → minimal inductive drive
Divertor LHI Startup Shows suggestive commonality between experiment and NIMROD modeling
Current Ring
t = 2.93 ms
t = 2.91 ms
1. Streams follow field lines
2. Adjacent passes attract
M.W. Bongard, APS-DPP 2016
t = 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)
May 2016 PEGASUS High-speed Imaging
– Infers NIMROD streams in edge
Ip [kA]
Reconnection-drive edge ion heating Internal Bz Measurements
•
NIMROD
Any stochastic reconnection region may be localized to edge
External Bz Phase Correlation 0.4
Inferred Stream Location Z [m]
•
PEGASUS
~b[mT]
Magnetics localize coherent streams in edge
~b [T/s]
•
Ip [kA]
Current Stream Interaction Manifests as Edge-Localized MHD Burst
-0.4 0.0
M.W. Bongard, APS-DPP 2016
R [m]
0.85
Reconnection-Driven Ion Heating Gives Ti > Te During LHI
Continuous ion heating from reconnection between collinear current streams – –
No effect on current drive efficiency
Ip [kA]
•
Impurity Ti(0) ~ 100 – 500 eV > Te routinely observed during LHI
(a)
100 0
dBz/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)
16
18
20
22
24
26
Ion heating consistent with 2-fluid reconnection theory
HeII Ti [eV]
200 100
(c)
0 3%
(d)
Ti,⊥
Ti,||
5 - 70 kHz
2 1 0
(e)
0.02 %
0.2-0.4 MHz
0.01 0
25
26 Time [ms]
M.W. Bongard, APS-DPP 2016
27
28
Different MHD Activity Observed Between LFS and HFS Injection Geometry •
LFS (outboard) injection: – MHD initially continuous, large amplitude, n = 1 – Transitions to intermittent bursts later in the discharge – Burst spacing increase with Ip – Similar to NIMROD simulation
•
HFS (inboard) injection: – Continuous, large-amplitude n = 1 activity early on – Abrupt cut-off in large amplitude activity – Reduced n = 1 magnitude for remainder of discharge
• Differences suggest multiple current drive mechanisms present M.W. Bongard, APS-DPP 2016
LFS LHI
HFS LHI
LFS Local Helicity Injection Produces Core Te > 100 eV • Plasma shape grows inward from LFS injectors
Peaked Te(R) while Connected to Injectors 23.0 ms
24.2 ms
– Shape evolution generates VIND – VIND > VLHI during high-Ip phase
• Peaked Te(R) during drive phase (connected) – Not strongly stochastic – After disconnect radial compression drives skin current
• Core ne > 1019 m-3, Te ≥ 100 eV provides target for subsequent CD M.W. Bongard, APS-DPP 2016
Ip=0.08 MA
Connected
Ip=0.10 MA
Disconnected
Te (R, t) Remains Peaked for LFS Injection Geometry and Minimal VIND •
Plasmas with same LFS LHI system and static geometry evolution – Lower performance due to shape constraint •
Te(R) > 85 eV with majority LFS LHI-drive 23.03 ms
28.03 ms
Ip=46.7 kA
Ip=48 kA
High R0, reduced Aplasma
– VIND ~ 0 < VLHI; Te(0) ~ 80 eV
•
Te(R) peaked while driven by outboard LHI Contrast-enhanced high-speed image and fast boundary reconstructions
M.W. Bongard, APS-DPP 2016
Technical Challenges Arise for LHI Startup With HFS Injection Geometry •
Initial relaxation to tokamak state – More difficult for low Rinj, high Binj – Magnetic geometry constrained by injector clearance requirements
•
•
Current source behavior at increased Binj Plasma-material interactions – PMI on injector surfaces • inhibits Vinj • can damage injectors
– PMI on machine surfaces • Impedes reproducibility • More severe for HFS injection M.W. Bongard, APS-DPP 2016
Above: LHI plasma before and after relaxation Below: example of PMI on injector (left), eventually leading to insulator failure (right)
Poloidal Field Shaping Facilitates Relaxation at Full Toroidal Field (Binj = 0.23 T) •
Milestone for HFS LHI system achieved
•
Technical challenge with HFS injectors: –
Lower Rinj → higher BTF with respect to LFS system •
–
BTF increased ~ 10× over previous experiments •
•
→ more Bz for injector clearance (~ Bz/BTF) → Relaxation at constant Iinj more difficult
Poloidal field shaping key to full-field relaxation –
Reduces midplane |B| and maintains injector clearance
–
Limited by Iinj-deformed streams contacting vessel
M.W. Bongard, APS-DPP 2016
Vacuum Field
Deformed Field
Ip > 0.15 MA Achieved Via HFS Injection To Date •
VLHI ~ 1 kV increased 2 over previous HFS LHI experiments
•
Most operations at low field: – Binj = 0.046-0.092 T •
(20-40% of Pegasus maximum)
– Reduced PMI, easier relaxation
•
Full BTF scenarios developed – Binj = 0.23 T, ITF = 0.288 MA – Ip ≈ 0.1 MA – PMI more prevalent at high BTF
•
Injector geometry variants addressing observed PMI – Improvements found in each iteration
M.W. Bongard, APS-DPP 2016
HFS Helicity Injection Provides Non-Solenoidal Sustainment at High IN Constant geometry: minimal VIND
•
Low ITF 0.6 Ip
•
IN > 10 accessible
Access to IN > 14, ne ~ 1x19 m-3 with HFS Injection, BTF Rampdown
Ip (kA)
•
– Constant or ramped-down BTF
Potential for high T
IN
•
Ne
M.W. Bongard, APS-DPP 2016
Te
Ti_OV
ne (1019 m-3)
– Aided by anomalous ion heating
LHI Provides Access to High-T at A ~ 1 with Non-Solenoidal Sustainment and Anomalous Ion Heating •
•
Equilibrium reconstructions with kinetic constraints used to determine ≡2 / –
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 to those with kinetic constraints
•
Need full kinetic profiles in future
High T plasmas often terminated by disruption –
•
•
n = 1, low-m precursors
Expands accessible high IN, T space for tokamak stability studies at extreme toroidicity –
Campaign underway to document, extend to higher Ip
–
Improving LHI injector hardware to increase Ip, BTF access
M.W. Bongard, APS-DPP 2016
Initial Exploration of High-T Space
Progress in Non-Solenoidal Startup on Pegasus • LHI provides high Ip, non-solenoidal tokamak startup
– Flexible injection geometry balances VLHI and VIND drive, engineering constraints
– Improved power balance model suggests technique is scalable to larger devices – Questions remain on confinement and reconnection dynamics • Thomson scattering: Peaked Te, ne suggest favorable realized confinement
• New high-field-side injector systems exploring strong VLHI limit – Injector operation and relaxation to tokamak demonstrated at full TF (Binj ~ 0.25 T) – Completely VLHI driven startup and sustainment realized – Non-solenoidal Ip(t) via LHI enables access to stability tests at extreme toroidicity • Sustained operation at high IN, high T
• Present campaign: – Optimize HFS injector implementation to mitigate PMI at high BTF – Develop high Ip scenarios to test scalings in LFS, HFS geometries – Design CHI system for comparison studies (with PPPL, U. Wash) M.W. Bongard, APS-DPP 2016
Reprints Reprints of this and other PEGASUS presentations are available online at http://pegasus.ep.wisc.edu/Technical_Reports