arXiv subjects

Simppa Äkäslompolo

Publications and source records attributed to Simppa Äkäslompolo.

11 recordsLinked to original sources

arXiv2021

ASCOT simulations of 14 MeV neutron rates in W7-X: effect of magnetic configuration

Neutron production rates in fusion devices are determined not only by the kinetic profiles but also the fast ion slowing-down distributions. In this work, we investigate the effect of magnetic configuration on neutron production rates in future deuterium plasmas in the Wendelstein 7-X (W7-X) stellarator. The neutral beam injection, beam and triton slowing-down distributions, and the fusion reactivity are simulated with the ASCOT suite of codes. The results indicate that the magnetic configuration has only a small effect on the production of 2.45 MeV neutrons from thermonuclear and beam-target fusion. The 14.1 MeV neutron production rates were found to be between $1.49 \times 10^{12}$ $\mathrm{s}^{-1}$ and $1.67 \times 10^{12}$ $\mathrm{s}^{-1}$, which is estimated to be sufficient for a time-resolved detection using a scintillating fiber detector, although only in high-performance discharges.

physics.plasm-ph↗

arXiv2021

Serpent neutronics model of Wendelstein 7-X for 14.1 MeV neutrons

In this work, a Serpent 2 neutronics model of the Wendelstein 7-X (W7-X) stellarator is prepared, and an response function for the Scintillating-Fibre neutron detector (SciFi) is calculated using the model. The neutronics model includes the simplified geometry for the key components of the stellarator itself as well as the torus hall. The objective of the model is to assess the 14.1 MeV neutron flux from deuteron-triton fusions in W7-X, where the neutrons are modelled only until they have slowed down to 1 MeV energy. The key messages of this article are: demonstration of unstructured mesh geometry usage for stellarators, W7-X in particular; technical documentation of the model and first insights in fast neutron behaviour in W7-X, especially related to the SciFi: the model indicates that the superconducting coils are the strongest scatterers and block neutrons from large parts of the plasma. The back-scattering from e.g. massive steel support structures is found to be small. The SciFi will detect neutrons from an extended plasma volume in contrast to having an effective line-of-sight.

physics.comp-ph↗

arXiv2019

Synthetic fast ion diagnostics in tokamaks: comparing the Monte Carlo test particle code ASCOT against experiments

Measuring fast ions, most notably fusion alphas, in ITER and future reactors remains an issue that still lacks an adequate solution. Numerical simulations are invaluable in testing the potential and limitations of various proposed diagnostics. However, the validity of the numerical tools first has to be checked against results from existing tokamaks. In this contribution, a variety of synthetic diagnostics for fast ions (collective Thomson scattering, neutral particle analyzer, neutron camera, infrared measurements, fast ion loss detector and activation probe) from the orbit-following Monte Carlo code ASCOT are compared to measurements from several tokamaks (ASDEX Upgrade, DIII-D and JET). Within the limitations of physics included in the numerical model and availability of input data from experiments, the agreement between synthetic data and measurements is found to be quite good.

physics.plasm-ph↗

arXiv2019

High-performance orbit-following code ASCOT5 for Monte Carlo simulations in fusion plasmas

We present a novel implementation of a Monte Carlo particle-following code for solving the distribution function of minority species in fusion plasmas, called ASCOT5, and verify it using theoretical results for neoclassical transport. The code has been developed from ground up with an OpenMP-MPI hybrid paradigm to take full advantage of current and next generation many-core CPUs with multithreading and SIMD operations. Up to 6-fold increase in performance is demonstrated compared to a previous version of the code which only utilizes MPI. The physics model of the code is comprehensively validated against existing theoretical work, and it is shown to faithfully reproduce neoclassical diffusion across three different collisionality regimes. In simulations for realistic tokamak plasmas, including complex non-axisymmetric geometry, ASCOT5 is verified to reproduce results from the previous version ASCOT4.

physics.plasm-ph↗

arXiv2015

Biot Savart Law integrator BioSaw

This contribution documents the methods used in the BioSaw code. The code is inteded to be a flexible tool for calculating magnetic fields due to coils in magnetic confinement fusion devices. It assumes the conductors are infinitesimally thin and can be described as either point sequences or circular coils. The code can calculate both the magnetic field as well as the vector potential due to the coils. The fields can be reduced very near the coils to avoid singular behaviour caused by the thin conductor approximation.

physics.plasm-ph↗

arXiv2015

Monte Carlo method and High Performance Computing for solving Fokker-Planck equation of minority plasma particles

This paper explains how to obtain the distribution function of minority ions in tokamak plasmas using the Monte Carlo method. Since the emphasis is on energetic ions, the guiding-center transformation is outlined, including also the transformation of the collision operator. Even within the guiding-center formalism, the fast particle simulations can still be very CPU intensive and, therefore, we introduce the reader also to the world of high-performance computing. The paper is concluded with a few examples where the presented method has been applied.

physics.plasm-ph↗

arXiv2015

Adjoint Monte Carlo Simulation of Fusion Product Activation Probe Experiment in ASDEX Upgrade tokamak

The activation probe is a robust tool to measure flux of fusion products from a magnetically confined plasma. A carefully chosen solid sample is exposed to the flux, and the impinging ions transmute the material making it radioactive. Ultra-low level gamma-ray spectroscopy is used post mortem to measure the activity and, thus, the number of fusion products. This contribution presents the numerical analysis of the first measurement in the ASDEX Upgrade tokamak, which was also the first experiment to measure a single discharge. The ASCOT suite of codes was used to perform adjoint/reverse Monte Carlo calculations of the fusion products. The analysis facilitates, for the first time, a comparison of numerical and experimental values for absolutely calibrated flux. The results agree to within a factor of about two, which can be considered a quite good result considering the fact that all features of the plasma cannot be accounted in the simulations. Also an alternative to the present probe orientation was studied. The results suggest that a better optimized orientation could measure the flux from a significantly larger part of the plasma.

physics.plasm-ph↗

arXiv2015

Simulations of fast ion wall loads in ASDEX Upgrade in the presence of magnetic perturbations due to ELM mitigation coils

The effect of ASDEX Upgrade (AUG) ELM mitigation coils on fast ion wall loads was studied with the fast particle following Monte Carlo code ASCOT. Neutral beam injected (NBI) particles were simulated in two AUG discharges both in the presence and in the absence of the magnetic field perturbation induced by the eight newly installed in-vessel coils. In one of the discharges (#26476) beams were applied individually, making it a useful basis for investigating the effect of the coils on different beams. However, no ELM mitigation was observed in #26476, probably due to the low plasma density. Therefore, another discharge (#26895) demonstrating clear ELM mitigation was also studied. The magnetic perturbation due to the in-vessel coils has a significant effect on the fast particle confinement, but only when total magnetic field, $B_{tot}$, is low. When $B_{tot}$ was high, the perturbation did not increase the losses, but merely resulted in redistribution of the wall power loads. Hence, it seems to be possible to achieve ELM mitigation using in-vessel coils, while still avoiding increased fast ion losses, by simply using a strong $B_{tot}$. Preliminary comparisons between simulated and experimental Fast Ion Lost Detector (FILD) signals show a reasonable correspondence.

physics.plasm-ph↗

arXiv2015

Calculating the 3D magnetic field of ITER for European TBM studies

The magnetic perturbation due to the ferromagnetic test blanket modules (TBMs) may deteriorate fast ion confinement in ITER. This effect must be quantified by numerical studies in 3D. We have implemented a combined finite element method (FEM) -- Biot-Savart law integrator method (BSLIM) to calculate the ITER 3D magnetic field and vector potential in detail. Unavoidable geometry simplifications changed the mass of the TBMs and ferritic inserts (FIs) up to 26%. This has been compensated for by modifying the nonlinear ferromagnetic material properties accordingly. Despite the simplifications, the computation geometry and the calculated fields are highly detailed. The combination of careful FEM mesh design and using BSLIM enables the use of the fields unsmoothed for particle orbit-following simulations. The magnetic field was found to agree with earlier calculations and revealed finer details. The vector potential is intended to serve as input for plasma shielding calculations.

physics.plasm-ph↗

arXiv2015

Adjoint Monte Carlo calculation of charged plasma particle flux to wall

This manuscript describes an adjoint/reverse Monte Carlo method to calculate the flux of charged plasma particles to the wall of e.g. a tokamak. Two applications are described: a fusion product activation probe and a neutral beam injection prompt loss measurement with a fast ion loss diagnostic. In both cases, the collisions of the particles with the background plasma can be omitted.

physics.plasm-ph↗

arXiv2013

ASCOT: solving the kinetic equation of minority particle species in tokamak plasmas

A comprehensive description of methods, suitable for solving the kinetic equation for fast ions and impurity species in tokamak plasmas using Monte Carlo approach, is presented. The described methods include Hamiltonian orbit-following in particle and guiding center phase space, test particle or guiding center solution of the kinetic equation applying stochastic differential equations in the presence of Coulomb collisions, neoclassical tearing modes and Alfvén eigenmodes as electromagnetic perturbations relevant to fast ions, together with plasma flow and atomic reactions relevant to impurity studies. Applying the methods, a complete reimplementation of the well-established minority species code ASCOT is carried out as a response both to the increase in computing power during the last twenty years and to the weakly structured growth of the code, which has made implementation of additional models impractical. Also, a benchmark between the previous code and the reimplementation is accomplished, showing good agreement between the codes.

physics.plasm-ph↗