Density Profile Evolution during Dynamic Processes in ASDEX Upgrade

• Autor(es): I. Nunes ; J. Santos ; Francisco José Batista Salzedas ; M. Manso ; F. Serra ; G. Conway ; L. Horton ; Neuhauser ; W. Suttrop ; CFN team ; ASDEX team
• Data: 2004
• Identificador Persistente: http://hdl.handle.net/10216/368
• Origem: Repositório Aberto da Universidade do Porto
• Assunto(s): Ciências Físicas; Física; Mecânica clássica; Dinâmica de fluidos; Física do plasma
• Tipo de Documento: Documento de conferência
• Idioma: Português

Descrição

The impact of ELM activity and density limit disruptions on the plasma density as well as the use of reflectometry for plasma position and shape control are discussed here. Fast density measurements allow the definition of three characteristic phases of the ELM: precursor, collapse and recovery. At the precursor phase low frequency oscillations starting approximately 3 ms prior to the ELM collapse are observed in the density profiles. Fluctuation measurements show a continuous rise of the broadband fluctuations prior to the ELM collapse. In the collapse phase the crash of the density profile is observed. In the recovery phase, the density profile starts to recover its pre-ELM shape. A comparison between HFS and LFS density behaviour shows a delay on the density collapse, comparable to the ion parallel transport time indicating a ballooning character of the ELM. For a set of ASDEX Upgrade ELMy H-modes, where plasma parameters such as plasma current, plasma density, triangularity and input power were varied, the radial velocity, ELM particle losses and affected depth, density pedestal width and edge density gradient are determined and its dependence with the plasma parameters determined. A comparison between intrinsic and pellet triggered ELMs shows no relevant differences suggesting that the triggering mechanism for both ELM type cannot be distinguished by the density profile dynamics or the density fluctuations. The changes observed on the density profiles during density limit disruptions support the observation that the erosion of the temperature is due to convection and not by stochastization. The information about the density profiles also provides a powerful tool for plasma position and shape control, as proposed for ITER to complement the magnetic measurements. Provided that the density is constant within the magnetic flux surfaces, a scaling factor between the line average density and the density at the separatrix can be used to estimate the density at the separatrix. The separatrix position can then be found from the measured density profiles. Software tools developed to quantify the errors involved in reflectometry gap evolution were used in ASDEX Upgrade ELMy H-mode discharges. The estimated position of the separatrix is found within 1 cm of accuracy when compared tosimilar magnetic data.