39 research outputs found
Sensitivity of detachment extent to magnetic configuration and external parameters
Divertor detachment may be essential to reduce heat loads to magnetic fusion tokamak reactor divertor surfaces. Yet in experiments it is difficult to control the extent of the detached, low pressure, plasma region. At maximum extent the front edge of the detached region reaches the X-point and can lead to degradation of core plasma properties. We define the 'detachment window' in a given position control variable C (for example, the upstream plasma density) as the range in C within which the front location can be stably held at any position from the target to the X-point; increased detachment window corresponds to better control. We extend a 1D analytic model [1] to determine the detachment window for the following control variables: the upstream plasma density, the impurity concentration and the power entering the scrape-off layer (SOL). We find that variations in magnetic configuration can have strong effects; increasing the ratio of the total magnetic field at the X-point to that at the target,B x/B t, (total flux expansion, as in the super-x divertor configuration) strongly increases the detachment window for all control variables studied, thus strongly improving detachment front control and the capability of the divertor plasma to passively accommodate transients while still staying detached. Increasing flux tube length and thus volume in the divertor, through poloidal flux expansion (as in the snowflake or x-divertor configurations) or length of the divertor, also increases the detachment window, but less than the total flux expansion does. The sensitivity of the detachment front location, z h, to each control variable, C, defined as , depends on the magnetic configuration. The size of the radiating volume and the total divertor radiation increase â (B x/B t) 2 and â B x/B t, respectively, but not by increasing divertor poloidal flux expansion or field line length. We believe this model is applicable more generally to any thermal fronts in flux tubes with varying magnetic field, and similar sources and sinks, such as detachment fronts in stellarator divertors and solar prominences in coronal loops
An approach to explore the eddy currents of the new type divertor for EAST device using ANSYS code
An effective method for eddy current calculation has been developed for EASTâs new divertor by using ANSYS. A 3D model of a double null divertor for the EAST device was built to evaluate eddy currents and electromagnetic (EM) forces on these components. The main input to the model is the plasma current and poloidal field coil currents, which are loaded into the model using experimental data measured from the EAST discharges. These currents generate magnetic fields that match those producing an EAST discharge, and the time variation of these fields produces the eddy currents in the divertors, along with from the resulting EM forces. In addition, the first 10 time steps were discussed for the eddy current generation and changing trend. It indicates that a static analysis before a transient mode start can solve the eddy current origination in the initial time steps. With this method, the EM transient response of EASTâs new divertor can be predicted based on ANSYS simulations. Furthermore, the method is also an effective approach to estimate the EM results for the in-vessel components of a fusion reactor during a disruption.National Basic Research Program of China (973 Program) (Grant 2013GB10200
Development and simulation of multi-diagnostic Bayesian analysis for 2D inference of divertor plasma characteristics
We present results of the design, implementation and testing of a Bayesian multi-diagnostic inference system which combines various divertor diagnostics to infer the 2D fields of electron temperature T e, density n e and deuterium neutral density n 0 in the divertor. The system was tested using synthetic diagnostic measurements derived from SOLPS-ITER fluid code predictions of the MAST-U Super-X divertor which include appropriate added noise. Two SOLPS-ITER simulations in different states of detachment, taken from a scan of the nitrogen seeding rate, were used as test-cases. Taken across both test-cases, the median absolute fractional errors in the inferred electron temperature and density estimates were 10.3% and 10.1% respectively. Differences between the inferred fields and the test-cases were well explained by solution uncertainty estimates derived from posterior sampling. This work represents a step toward a larger goal of obtaining a quantitative, 2D description of the divertor plasma state directly from experimental data, which could be used to gain better understanding of divertor physics phenomena
The role of particle, energy and momentum losses in 1D simulations of divertor detachment
A new 1D divertor plasma code, SD1D, has been used to examine the role of recombination, radiation, and momentum exchange in detachment. Neither momentum or power losses by themselves are found to be sufficient to produce a reduction in target ion flux in detachment (flux rollover); radiative power losses are required to a) limit and reduce the ionization source and b) access low-target temperature, T_target, conditions for volumetric momentum losses. Recombination is found to play little role at flux rollover, but as T_target drops to temperatures around 1eV, it becomes a strong ion sink. In the case where radiative losses are dominated by hydrogen, the detachment threshold is identified as a minimum gradient of the energy cost per ionisation with respect to T_target. This is also linked to thresholds in T_target and in the ratio of upstream pressure to power flux. A system of determining the detached condition is developed such that the divertor solution at a given T_target (or lack of one) is determined by the simultaneous solution of two equations for target ion current - one dependent on power losses and the other on momentum. Depending on the detailed momentum and power loss dependence on temperature there are regions of T_target where there is no solution and the plasma 'jumps' from high to low T_target states. The novel analysis methods developed here provide an intuitive way to understand complex detachment phenomena, and can potentially be used to predict how changes in the seeding impurity used or recycling aspects of the divertor can be utilised to modify the development of detachment
Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations
Plasma-molecular interactions generate molecular ions which react with the
plasma and contribute to detachment through molecular activated recombination
(MAR), reducing the ion target flux, and molecular activated dissociation
(MAD), both of which create excited atoms. Hydrogenic emission from these atoms
have been detected experimentally in detached TCV, JET and MAST-U deuterium
plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER
simulations for deuterium indicating a molecular ion density () that was
insufficient to lead to significant hydrogenic emission, which was attributed
to underestimates of the molecular charge exchange rate () for deuterium (obtained by rescaling the hydrogen rates by their
isotope mass).
In this work, we have performed new SOLPS-ITER simulations with the default
rate setup and a modified rate setup where ion isotope mass rescaling was
disabled. This increased the content by . By disabling
ion isotope mass rescaling: 1) the total ion sinks are more than doubled due to
the inclusion of MAR; 2) the additional MAR causes the ion target flux to
roll-over during detachment; 3) the total emission in the divertor
increases during deep detachment by roughly a factor four; 4) the neutral atom
density in the divertor is doubled due to MAD, leading to a 50\% increase in
neutral pressure; 5) total hydrogenic power loss is increased by up to 60\% due
to MAD. These differences result in an improved agreement between the
experiment and the simulations in terms of spectroscopic measurements, ion
source/sink inferences and the occurrence of an ion target flux roll-over
The role of particle, energy and momentum losses in 1D simulations of divertor detachment
A new 1D divertor plasma code, SD1D, has been used to examine the role of recombination, radiation, and momentum exchange in detachment. Neither momentum or power losses by themselves are found to be sufficient to produce a reduction in target ion flux in detachment (flux rollover); radiative power losses are required to a) limit and reduce the ionization source and b) access low-target temperature, T_target, conditions for volumetric momentum losses. Recombination is found to play little role at flux rollover, but as T_target drops to temperatures around 1eV, it becomes a strong ion sink. In the case where radiative losses are dominated by hydrogen, the detachment threshold is identified as a minimum gradient of the energy cost per ionisation with respect to T_target. This is also linked to thresholds in T_target and in the ratio of upstream pressure to power flux. A system of determining the detached condition is developed such that the divertor solution at a given T_target (or lack of one) is determined by the simultaneous solution of two equations for target ion current - one dependent on power losses and the other on momentum. Depending on the detailed momentum and power loss dependence on temperature there are regions of T_target where there is no solution and the plasma 'jumps' from high to low T_target states. The novel analysis methods developed here provide an intuitive way to understand complex detachment phenomena, and can potentially be used to predict how changes in the seeding impurity used or recycling aspects of the divertor can be utilised to modify the development of detachment
The physics of turbulence localised to the tokamak divertor volume
Fusion power plant designs based on magnetic confinement, such as the tokamak design, offer a promising route to sustainable fusion power but require robust exhaust solutions capable of tolerating intense heat and particle fluxes from the plasma at the core of the device. Turbulent plasma transport in the region where the interface between the plasma and the materials of the device is handled - called the divertor volume - is poorly understood, yet impacts several key factors ultimately affecting device performance. In this article a comprehensive study of the underlying physics of turbulence in the divertor volume is conducted using data collected in the final experimental campaign of the Mega Ampere Spherical Tokamak device, compared to high fidelity nonlinear simulations. The physics of the turbulence is shown to be strongly dependant on the geometry of the divertor volume - a potentially important result as the community looks to advanced divertor designs with complex geometry for future fusion power plants. These results lay the foundations of a first-principles physics basis for turbulent transport in the tokamak divertor, providing a critical step towards a predictive understanding of tokamak divertor plasma solutions
Design and implementation of a prototype infrared video bolometer (IRVB) in MAST Upgrade
A prototype infrared video bolometer (IRVB) was successfully deployed in the Mega Ampere Spherical Tokamak Upgrade (MAST Upgrade or MAST-U), the first deployment of such a diagnostic in a spherical tokamak. The IRVB was designed to study the radiation around the lower x-point, another first in tokamaks, and has the potential to estimate emissivity profiles with spatial resolution beyond what is achievable with resistive bolometry. The system was fully characterized prior to installation on MAST-U, and the results are summarized here. After installation, it was verified that the actual measurement geometry in the tokamak qualitatively matches the design; this is a particularly difficult process for bolometers and was done using specific features of the plasma itself. The installed IRVB measurements are consistent both with observations from other diagnostics, including magnetic reconstruction, visible light cameras, and resistive bolometry, as well as with the IRVB-designed view. Early results show that with conventional divertor geometry and only intrinsic impurities (for example, C and He), the progression of radiative detachment follows a similar path to that observed for large aspect ratio tokamaks: The peak of the radiation moves along the separatrix from the targets to the x-point and high-field side midplane with a toroidally symmetric structure that can eventually lead to strong effects on the core plasma inside the separatrix
Identification of the primary processes that lead to the drop in divertor target ion current at detachment in TCV
Using SOLPS-ITER we model a TCV conventional divertor discharge density ramp to understand the role of various processes in the loss of target ion current. We find that recombination is not a strong contributor to the rollover of the target ion current at detachment. In contrast, the divertor ion source appears to play a central role in magnitude (the source of most of the ion target current) and time, apparently dropping during the density ramps due to a drop in power available for ionization