25 research outputs found
Development and Validation of a Tokamak Skin Effect Transformer model
A control oriented, lumped parameter model for the tokamak transformer
including the slow flux penetration in the plasma (skin effect transformer
model) is presented. The model does not require detailed or explicit
information about plasma profiles or geometry. Instead, this information is
lumped in system variables, parameters and inputs. The model has an exact
mathematical structure built from energy and flux conservation theorems,
predicting the evolution and non linear interaction of the plasma current and
internal inductance as functions of the primary coil currents, plasma
resistance, non-inductive current drive and the loop voltage at a specific
location inside the plasma (equilibrium loop voltage). Loop voltage profile in
the plasma is substituted by a three-point discretization, and ordinary
differential equations are used to predict the equilibrium loop voltage as
function of the boundary and resistive loop voltages. This provides a model for
equilibrium loop voltage evolution, which is reminiscent of the skin effect.
The order and parameters of this differential equation are determined
empirically using system identification techniques. Fast plasma current
modulation experiments with Random Binary Signals (RBS) have been conducted in
the TCV tokamak to generate the required data for the analysis. Plasma current
was modulated in Ohmic conditions between 200kA and 300kA with 30ms rise time,
several times faster than its time constant L/R\approx200ms. The model explains
the most salient features of the plasma current transients without requiring
detailed or explicit information about resistivity profiles. This proves that
lumped parameter modeling approach can be used to predict the time evolution of
bulk plasma properties such as plasma inductance or current with reasonable
accuracy; at least in Ohmic conditions without external heating and current
drive sources
A model of multi-pass absorption of external EC radiation at initial stage of discharge in ITER
A model is developed for multi-pass absorption of external electron cyclotron radiation (ECR) in tokamaks, which is used at initial stage of discharge to overcome the impurity radiation barrier (burn-through). Model is based on a semi-analytical solution of the ECR transport problem in the case of multiple reflection of radiation from the wall of the vacuum chamber. We estimate the efficiency of absorption of injected radiation for typical values of the electron temperature and density at the initial stage of discharge in ITER