154 research outputs found
Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging
Tangentially viewing fast camera footage of the low-field side snowflake
minus divertor in TCV is analysed across a four point scan in which the
proximity of the two X-points is varied systematically. The motion of
structures observed in the post- processed movie shows two distinct regions of
the camera frame exhibiting differing patterns. One type of motion in the outer
scrape-off layer remains present throughout the scan whilst the other, apparent
in the inner scrape-off layer between the two nulls, becomes increasingly
significant as the X-points contract towards one another. The spatial structure
of the fluctuations in both regions is shown to conform to the equilibrium
magnetic field. When the X-point gap is wide the fluctuations measured in the
region between the X-points show a similar structure to the fluctuations
observed above the null region, remaining coherent for multiple toroidal turns
of the magnetic field and indicating a physical connectivity of the
fluctuations between the upstream and downstream regions. When the X-point gap
is small the fluctuations in the inner scrape-off layer between the nulls are
decorrelated from fluctuations upstream, indicating local production of
filamentary structures. The motion of filaments in the inter-null region
differs, with filaments showing a dominantly poloidal motion along magnetic
flux surfaces when the X-point gap is large, compared to a dominantly radial
motion across flux-surfaces when the gap is small. This demonstrates an
enhancement to cross-field tranport between the nulls of the TCV low-field-side
snowflake minus when the gap between the nulls is small.Comment: Accepted for publication in Plasma Physics and Controlled Fusio
Runaway electron synchrotron radiation in a vertically translated plasma
Synchrotron radiation observed from runaway electrons (REs) in tokamaks
depends upon the position and size of the RE beam, the RE energy and pitch
distributions, as well as the location of the observer. We show that
experimental synchrotron images of a vertically moving runaway electron beam
sweeping past the detector in the TCV tokamak agree well with predictions from
the synthetic synchrotron diagnostic Soft. This experimental validation lends
confidence to the theory underlying the synthetic diagnostics which are used
for benchmarking theoretical models of and probing runaway dynamics. We present
a comparison of synchrotron measurements in TCV with predictions of kinetic
theory for runaway dynamics in uniform magnetic fields. We find that to explain
the detected synchrotron emission, significant non-collisional pitch angle
scattering as well as radial transport of REs would be needed. Such effects
could be caused by the presence of magnetic perturbations, which should be
further investigated in future TCV experiments.Comment: 7 pages, 4 figures. Accepted for publication in Nuclear Fusio
Performance assessment of a tightly baffled, long-legged divertor configuration in TCV with SOLPS-ITER
Numerical simulations explore the possibility to test the tightly baffled,
long-legged divertor (TBLLD) concept in a future upgrade of the Tokamak \`a
configuration variable (TCV). The SOLPS-ITER code package is used to compare
the exhaust performance of several TBLLD configurations with existing unbaffled
and baffled TCV configurations. The TBLLDs feature a range of radial gaps
between the separatrix and the outer leg side walls. All considered TBLLDs are
predicted to lead to a denser and colder plasma in front of the targets and
improve the power handling by factors of 2-3 compared to the present, baffled
divertor and by up to a factor of 12 compared to the original, unbaffled
configuration. The improved TBLLD performance is mainly due to a better neutral
confinement with improved plasma-neutral interactions in the divertor region.
Both power handling capability and neutral confinement increases when reducing
the radial gap. The core compatibility of TBLLDs with nitrogen seeding is also
evaluated and the detachment window with acceptable core pollution for the
proposed TBLLDs is explored, showing a reduction of required upstream impurity
concentration up to 18% to achieve the detachment with thinner radial gap
Validation of SOLPS-ITER Simulations against the TCV-X21 Reference Case
This paper presents a quantitative validation of SOLPS-ITER simulations
against the TCV-X21 reference case and provides insights into the neutral
dynamics and ionization source distribution in this scenario. TCV-X21 is a
well-diagnosed diverted L-mode sheath-limited plasma scenario in both toroidal
field directions, designed specifically for the validation of turbulence codes
[D.S. Oliveira, T. Body, et al 2022 Nucl. Fusion 62 096001]. Despite the
optimization to reduce the impact of the neutral dynamics, the absence of
neutrals in previous turbulence simulations of TCV-X21 was identified as a
possible explanation for the disagreements with the experimental data in the
divertor region. This motivates the present study with SOLPS-ITER that includes
kinetic neutral dynamics via EIRENE. Five new observables are added to the
extensive, publicly available TCV-X21 dataset. These are three deuterium Balmer
lines in the divertor and neutral pressure in the common and private flux
regions. The quantitative agreement metric is combined with the conjugate
gradient method to approach the SOLPS-ITER input parameters that return the
best overall agreement with the experiment. A proof-of-principle of this method
results in a modest improvement in the level-of-agreement; shortcomings of the
method and how to improve it are discussed. Alternatively, a scan of the
particle and heat diffusion coefficients shows an improvement of 10.4% beyond
the agreement level achieved by the gradient method. The result is found for an
increased transport coefficient compared to what is usually used for TCV L-mode
plasmas, suggesting the need for accurate self-consistent turbulence models for
predictive boundary simulations. The simulations indicate that ~65% of the
total ionization occurs in the SOL, motivating the inclusion of neutrals in
future turbulence simulations towards improved agreement with the experiment
Real-time plasma state monitoring and supervisory control on TCV
In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation. In this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state arc modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule. These algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements. In TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECI I) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation.Peer reviewe
Role of the pedestal position on the pedestal performance in AUG, JET-ILW and TCV and implications for ITER
The role of the pedestal position on the pedestal performance has been investigated in AUG, JET-ILW and TCV. When the pedestal is peeling-ballooning (PB) limited, the three machines show a similar behaviour. The outward shift of the pedestal density relative to the pedestal temperature can lead to the outward shift of the pedestal pressure which, in turns, reduces the PB stability, degrades the pedestal confinement and reduces the pedestal width. Once the experimental density position is considered, the EPED model is able to correctly predict the pedestal height. An estimate of the impact of the density position on a ITER baseline scenario shows that the maximum reduction in the pedestal height is 10% while the reduction in the fusion power is between 10% and 40% depending on the assumptions for the core transport model usedIn other plasmas, where the pedestal density is shifted even more outwards relative to the pedestal temperature, the pedestal does not seem PB limited and a different behaviour is observed. The outward shift of the density is still empirically correlated with the pedestal degradation but no change in the pressure position is observed and the PB model is not able to correctly predict the pedestal height. On the other hand, the outward shift of the density leads to a significant increase of eta(e) and eta(i) (where eta(e,i) is the ratio of density to temperature scale lengths, eta(e,i) = L-eta e,L-i/L-Te,L-i) which leads to the increase of the growth rate of microinstabilities (mainly ETG and ITG) by 50%. This suggests that, in these plasmas, the increase in the turbulent transport due to the outward shift of the density might play an important role in the decrease of the pedestal performance.Peer reviewe
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