1. Overview of the TCV tokamak program: scientific progress and facility upgrades
OV/P-1
S. Coda for the TCV Team and the EUROfusion MST1 team1
Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), CH-1015 Lausanne, Switzerland
26th IAEA Fusion Energy Conference, Kyoto, Japan, October 2016
Introduction and highlights
ITER scenario development
Disruptions and runaway electrons
TCV is a medium-size European tokamak, known for extreme shaping versatility and high ECRH power (4 MW).
Now equipped with 1 MW NBI, upgrading soon to 2 MW. Also upgrading to variable closed divertor (essential for DEMO assessment) and 6 MW ECRH.
Devoted to advanced exhaust solutions for DEMO, ITER scenario development, fundamental scientific exploration and student training.
Ti =2.5 keV and vtor = 250 km/s reached with NBI.
Detachment documented in single-null (with varying flux expansion), snowflake-plus and -minus, super-X, X-divertor, X-point-target divertor configurations.
Detachment threshold independent of flux expansion.
Confirmed enhanced radiation region between two LFS SF- X-points predicted by emc3-eirene.
Narrow near-SOL heat-flux feature in limited plasmas disappears at high resistivity.
Disruptions mitigated by gas injection and ECRH.
Runaway electron generation (up to full runaway beam) and control demonstrated.
Integrated control of density, b, shape, q profile, NTMs.
If collisionality is clamped upwards by e.g., metal wall effects, how can pedestal height be increased to level compatible with Q=10?
Peeling-ballooning stability chart suggests an alternative path from above, i.e., increasing b in L-mode phase.
Strategy: maximize pressure in unfavorable configuration for L-H transition (ion B drift away from X-point), then switch configuration to induce transition.
Up to 50% increase in b over several ELM cycles.
Disruptive density limit as a fraction of Greenwald density increases with qedge and triangularity: at low current and positive d, Greenwald limit can be attained.
Initial, successful disruption mitigation experiments by massive gas injection and ECRH.
Runaway electron (RE) experiments in circular Ohmic plasmas: stationary, non-disruptive RE beam generated with line-averaged density ne < 3×1019 m-3 and toroidal electric field > 15 times the critical field.
RE mitigation by Ne or Ar gas injection only partially effective – limited throughput is possible reason.
In MGI-induced disruptions, robust RE beams are seen with ne < 2.5×1018 m-3.
Full current replacement by REs obtained, with pure RE-beam discharges for up to 650 ms (Te < 20 eV, current decay time >> bulk L/R time); stably controlled in current and discharge location.
RE mitigation by magnetic control: controlled shutdown through dynamically modified current reference, using double integrator control law.
Minimization of loop voltage important to reduce likelihood of deleterious MHD events.
G. Papp, EX/9-4 (Fri pm)
RE hierarchy: detectable (RE), strong (saturating horizontal HXR camera), very strong (saturating horizontal and vertical cameras)
I.T. Chapman, EX/3-6 (Wed pm)
Detachment in conventional and advanced divertor configurations
Documented detachment in a broad range of configurations to constrain modeling.
Extensive diagnostic set: 2 IR cameras, 114 wall-embedded Langmuir probes, fast reciprocating probe, tomographic bolometry, fast visible camera, SOL and divertor visible spectroscopic systems.
Detachment experiments performed in Ohmic plasmas with ion B drift away from X-point, which facilitates detachment and inhibits H-mode.
Before detachment, at 70% of detachment density, C III and Da fronts move towards X-point, though substantial radiation is still seen from outer leg at roll-over time.
Density shoulder forms after detachment in upstream SOL.
Recombination front moves little from target (a few cm), contrary to higher-density tokamaks.
Detachment stronger at higher current, independent of fueling and wall gap. Some hysteresis seen with density.
Surprising independence of detachment threshold on poloidal flux expansion (varied by 4x) or flaring, or total flux expansion (e.g., in super-X). However, reduced threshold density and increased detachment depth seen with increasing connection length (divertor leg height).
Nitrogen seeding in LFS snowflake-minus confirms prediction by emc3-eirene of enhanced impurity radiation region between two X-points.
The TCV tokamak
R = 0.88 m, a = 0.25 m, BT = 1.53 T, Ip = 1 MA.
Carbon wall.
Flexible shaping.
4 MW ECRH (2nd + 3rd harmonic).
1 MW NBI (15-25 keV, D or H, tangential), installed in 2015 after vessel modifications.
NBCD observed in co- and counter-injection experiments.
Ti =2.5 keV and vtor = 250 km/s reached.
B. Esposito, EX/P8-27 (Fri pm)
Real-time plasma control
before detachment
after detachment
Highly modular digital PCS structure, using reflective- memory technology, facilitates steady addition of new CPU or I/O nodes (seven at present).
Generalized shape controller developed and successfully tested, based on r/t, sub-ms equilibrium reconstruction (rtliuqe) and SVD approach to optimize coil actuation.
Real-time MHD mode analysis technique tested successfully.
Integrated multi-parameter control is under development: profile simulator raptor used for off-line controller design and testing, and r/t plasma state estimation. Shape, b, density, q profile, and NTMs controlled simultaneously.
H. Anand, EX/P8-32 (Fri pm)
b and density control
F. Felici, EX/P8-33 (Fri pm)
Wall cleaning with ECRH (for JT-60SA)
SND
LFS SF-
Second-harmonic ECRH wall conditioning in He is a technique likely to be needed in JT-60SA.
Documented in TCV, varying toroidal, vertical and radial fields (Bpol~ %×Btor), He injection, ECRH power ( kW, extrapolating to 1-5 MW for JT-60SA).
Optimum Bv and BR minimize discharge onset time and maximize wall coverage and cleaning efficiency.
B. Geiger, EX/P8-30 (Fri pm)
B.P. Duval, EX/P8-29 (Fri pm)
H. Reimerdes, EX/2-3 (Wed am)
Ion current
BV+BR
BV only
Magnetic flux
D. Douai, EX/P8-31 (Fri pm)
SOL properties and heat flux profile
Power decay length lq increases with divertor leg length while spreading factor S is independent of it.
SOL density shoulder seen also in attached conditions with ion B drift towards X-point: correlates with blob size but not with connection length  role of turbulence likely, but mechanism unclear.
Limited L-mode regime (ITER start-up) exhibits double SOL lq scale length (ITER wall panel recently redesigned to accommodate enhanced flux). Now: first observation on TCV of disappearance of narrow feature at high normalized resistivity. Partial success of gbs nonlinear simulation in reproducing double scale length.
Facility upgrades
Heating
Second 1-MW NBI, oppositely injected, at higher energy (50-60 keV) for Alfvén wave studies [2019].
Two 1-MW dual-frequency (X2+X3) gyrotrons [2019].
Divertor
Closed divertor with variable baffles [2020].
Cryopumping and additional divertor coils possible.
Additional divertor diagnostics: Langmuir probes, pressure gauges, bolometers, IR camera, magnetic probes, Thomson scattering, spectroscopy.
Conclusions and outlook
A. Fasoli, FIP/P4-40 (Wed pm)
TCV is developing the physics basis for the evaluation of alternative divertor configurations for DEMO.
All divertor configurations proposed so far have been realized in TCV, and the associated exhaust physics is being extensively studied in attached and detached conditions.
The installation of a neutral beam injector has also made TCV a more direct contributor to ITER physics, exploring high-performance H-mode, maximized pedestal height, and MHD instability avoidance.
Disruption and runaway electron (RE) mitigation is a strong new area for TCV, with full RE beams created and controlled.
Divertor and ECRH+NBI power upgrades planned for
N. Vianello, EX/P8-26 (Fri pm)
Density scale length vs blob size, divertor collisionality, density, connection length
1H. Meyer, OV/P-12 (Mo pm)
B. Labit, EX/P8-25 (Fri pm)
P. Ricci, TH/P6-8 (Thu pm)
This work was supported in part by the Swiss National Science Foundation