Integrated Models for Plasma/Material Interaction during Loss of Plasma Confinement

A comprehensive computer package, High Energy Interaction with General Heterogeneous ‘&i.rget&ystems (HEIGHTS), has been developed to evaluate the damage incurred on plasma-facing materials during loss of plasma confinement. The HEIGHTS package consists of several integrated computer models that follow the start of a plasma disruption at the scrape-off layer (SOL) through the transport of the eroded debris and splashed target materials to nearby locations as a result of the energy deposited. The package includes new models to study turbulent plasma behavior in the SOL and predicts the plasma parameters and conditions at the divertor plate. Full two-dimensional comprehensive radiation magnetohydrodynamic models are coupled w-ith target thermodynamics and liquid hydrodynamics to evaluate the integrated response of plasma-facing materials. A brief description of the HEIGHTS package and its capabilities are given in this work with emphasis on turbulent plasma behavior in the SOL during disruptions. I. Brief description of HEIGHTS package Three key factors significantly influence the overall response and erosion of plasma-facing components (PFCS) as a result of the intense deposited energy during plasma instabilities. These are (a) the characteristics of particle-energy flow (i.e., particle type, kinetic energy, energy content, deposition time, and location) from the SOL to the diverter plate, (b) the characteristics of the vapor cloud that develop from the initial phase of energy deposition on target materials and its turbulent hydrodynamics, and (c) the generated-photon radiation and its transport in the vapor-cloud and nearby regions. The HEIGHTS package consists of several integrated models that describe and follow the start of a plasma disruption at the SOL through the transport of the eroded debris and splashed target matenkds, after the end of a disruption, to nearby locations as a result of the intense energy deposited. The characteristics of particle-energy flow from the core plasma to the -SOL during plasma instability events are studied with the recently developed SOLAS model, in which an analytical solution is derived for plasma particle distribution functions in SOL by using a modified form of the Fokker-Planck equation [1]. The dynamics of target thermal evolution, surface erosion due to vaporization, vapor-cloud formation and shielding effects, magnetohydrodynamic (MHD) expansion, vapor turbulent instabilities, and loss of confinement are studied with the comprehensive A*THERMAL-S code [2]. Most of the incident plasma kinetic energy during .—me ..,. — ., -$.. ,., , .. >.,, . . . . ... ..s. /-’.. . . . . ,-,, ,.-, . . . ,. _ . . ..-. . , .“— —.. -. . . . a disruption, however, will quickly be transformed into photon radiation if the vapor cloud is to be well confined by the magnetic field. The resulting photon radiation from the continuous plasma heating of the vapor cloud and the transport of the emitted radiation are very important and complicated problems. Vapor radiation in a closed divertor design can cause significant ‘ erosion of nearby components. For such analysis, the PhD and the SUPERATOM codes [3] are used; each is coupled to the A*THERMAL-S code. The PhD code calculates detailed deposition of the emitted photon radiation from the vapor cloud to nearby components. The SUPERATOM code calculates the atomic physics data of different target materials needed for calculation of the resulting radiation. The behavior and erosion of the free metallic surface of a liquid layer subject to various internal and external forces during the disruption are studied with the SPLASH code [4]. In addition, the SPLASH code calculates the explosive erosion and the characteristics of brittle destruction erosion of carbon-based materials (CBMS). Macroscopic erosion of metallic and crubon-based materials, can significantly exceed conventional erosion from surface vaporization by orders of magnitude. The redeposition of the eroded debris and splattered materials are analyzed with the DRDEP code [3]. Redeposited debris is of major concern for plasma contamination, for safety (dust inventory hazard), and for successful and prolonged plasma operations following plasma instability events. Tritium behavior and containment in the generated dust and eroded debris of plasma-facing materials (PFMs), as a result of various plasma instabilities, are being analyzed and evaluated with the TRICS and TRAP codes. Detailed models and initial results of these codes are described elsewhere and will not be discussed here [5]. The emphasis in this work is devoted to understanding turbulent plasma behavior in the SOL during a disruption. The analytical SOLAS code has been developed to predict the disrupting plasma parameters at the divertor plate. These parameters are then used as an input to the remaining HEIGHTS package to determine the overall response and lifetime of PFCS to plasma instabilities. III. Plasma behavior in scrape-off layer (SOLAS code) During various plasma instability events, the loss of confinement will cause the majority of the core particle flux to mrive at the SOL with a relatively high temperature T = TO, where TOis the core plasma temperature (TO= 10-20 keV) prior to a dismption. This is in con~ast to the normal operation scenario in which the escaping particles from the core plasma to a collisional SOL have a relatively lower temperature, T <1 keV. Because of the high temperature of the escaping particles during plasma instability events, the SOL plasma becomes collisionless and requires different treatment than that during normal operation. In the SOLAS code, the electrons in the SOL are considered to be composed of three different populations based .on their origin. Hot electrons arriving from the tokamak core