J. Santos

Reflectometry-based plasma position feedback control demonstration at ASDEX Upgrade

In fusion experiments, real-time feedback control of the plasma position plays a vital role for machine protection and disruption avoidance. This control task is presently performed using magnetic measurements that, in future long pulse tokamak devices of the ITER class, may be affected by drifting integrators or radiation induced voltages in the magnetic pickup coils. These effects could have an impact on the magnetic equilibrium reconstruction, causing potential losses of position control and, consequently, leading to premature discharge termination or plasma-facing component damage. Frequency modulated continuous wave O-mode reflectometry, a non-magnetic dependent technique used to measure the density profile, was proposed to backup or complement the standard magnetic-based control in such devices. This new control scheme has just been successfully demonstrated for the first time on the ASDEX Upgrade (AUG) tokamak. The location of the plasma boundary, used in the control of the plasma column position, was tracked in real-time (RT) using dedicated algorithms and a new approach that combines the reflectometry edge profile and a scaled line integrated density measurement from interferometry. Although feasibility studies on the viability of this method had been previously conducted at AUG, the capabilities required to produce this on-line demonstration were only incorporated in the diagnostic after a recent upgrade of its data acquisition and processing hardware. The results herein presented show the first successful demonstration of the reflectometry plasma position application as proposed for ITER.

H-Mode Studies with Microwave Reflectometry on ASDEX Upgrade

The microwave reflectometry system on ASDEX Upgrade measures density profiles (in broadband swept operation) and plasma fluctuations (fixed-frequency operation) both at the high-field side (HFS) and low-field side (LFS). Densities up to can be probed. We analyse the evolution of turbulence at the L-H transition and during the ELMy phase of H-mode discharges. The detailed density profile evolution during type I ELMs is resolved and profile oscillations associated with ELM precursors are studied.

Assessment of density profile automatic evaluation from broadband reflectometry data

Automatic evaluation of electron density profiles from frequency modulated continuous wave (FM–CW) broadband microwave reflectometry on ASDEX Upgrade has been achieved in all plasma regimes combining a high performance system and advanced analysis tools. The results presented demonstrate the capability of the system to resolve the abrupt gradient changes occurring in the edge region during the L–H transition and in the presence of edge localized modes (ELMs). The temporal evolution of the density gradient could be obtained extending from the ohmic and L phases to the ELMy H phase. A new data analysis method that uses sets of consecutive sweeps to extract the group delay is also presented and its use versus single sweep analysis is discussed. It is shown that local perturbations may appear and disappear during the sweep time reinforcing the need to obtain redundant data in very short time intervals and apply rejection/averaging techniques.

Plasma position measurements from ordinary FM-CW reflectometry on ASDEX Upgrade

Microwave reflectometry has been proposed as an alternative/backup approach to the usual magnetic systems in long pulse operation on the International Thermonuclear Experimental Reactor (ITER) for plasma position and shape control. This new application of reflectometry must be fully demonstrated in present machines in view of its importance for ITER. First experiments performed in ASDEX Upgrade with frequency modulated continuous wave with reflectometry diagnostic showed that radial variations of the plasma column occurring during preprogrammed horizontal as well as vertical plasma displacements could be followed with sufficient accuracy (⩽1 cm) in phases with approximately constant density. In this article we determine the plasma edge position simultaneously in the inner and outer midplane, of ohmic L-mode density limit and ELMy H-mode discharges in ASDEX Upgrade, both with significant density variations. It is shown that if normalized density layers are selected, at a certain percentage of the average line density, the results from reflectometry are in good agreement with the corresponding data, separatrix position, derived from the magnetics. Further work aiming at the full demonstration of plasma position and shape measurements from reflectometry is discussed.

Reflectometry in conventional and advanced plasma scenarios on ASDEX Upgrade and perspectives for ITER

The discovery of advanced tokamak scenarios opened new possibilities for the realization of more compact machines with long pulse operation. This requires the simultaneous optimization of the plasma shape, and of the pressure, current and electric field profiles. In this complex process the diagnostics are crucial to understand the plasma physics, to validate the theoretical models and to measure parameters for control purposes. Here we present results obtained in ASDEX Upgrade showing the great potential of reflectometry for conventional and advanced tokamak investigation, derived from the diagnostic capability to perform a wide range of measurements with high spatial and temporal resolution, both at the edge and plasma core. The abrupt formation of sharp density gradients at the zones where internal and external transport barriers are formed, as well as the modifications of the turbulence parameters in those narrow plasma regions are shown. Density profile changes associated with fast events like ELMs and the L-H transition are resolved. Enhanced fluctuations that may be important for the transport across the edge barrier are also detected by reflectometry. The measurement of the radial profile of E x B velocity on ASDEX Upgrade is discussed and results are compared with v E×B profiles obtained in W7-AS stellarator. The first experimental demonstration that edge density profiles from reflectometry can provide accurate and reliable measurements of the temporal evolution of the plasma position is presented. This is essential to prove that reflectometry can be used as an alternative approach to supplement the magnetic measurements, for the control of plasma shape and position in long pulse operation, as it is proposed for HER. Finally new applications of reflectometry under development are discussed.

On the application of the Wigner–Ville distribution to broadband reflectometry

Abstract Herein we propose a data processing method to analyse broadband reflectometry signals based on the Wigner–Ville distribution (WVD). The method offers the advantage of good localization of the reflectometry signal energy in the time-frequency domain. Simulations are presented to test the ability of this technique to follow the average beat frequency that is the relevant quantity to invert the plasma profile. The method is compared with the short-time fourier transform (STFT) analysis both for simulated and for ASDEX Upgrade plasmas. It is shown that the WVD has potential advantages for the routine evaluation of density profiles. The method can also be useful to study fluctuations, namely when abrupt changes of turbulence occur at the L–H transition.

Characterisation of the epithermal neutron irradiation facility at the Portuguese research reactor using MCNP

The radiation field at the epithermal beamline and irradiation chamber installed at the Portuguese Research Reactor (RPI) at the Campus Tecnológico e Nuclear of Instituto Superior Técnico was characterised in the context of Prompt Gamma Neutron Activation Analysis (PGNAA) applications. Radiographic films, activation foils and thermoluminescence dosimeters were used to measure the neutron fluence and photon dose rates in the irradiation chamber. A fixed-source MCNPX model of the beamline and chamber was developed and compared to measurements in the first step towards planning a new irradiation chamber. The high photon background from the reactor results in the saturation of the detector and the current facility configuration yields an intrinsic insensitivity to various elements of interest for PGNAA. These will be addressed in future developments.

Application of a stratified plasma model to microwave reflectometry of density fluctuations

We discuss a method of the turbulence poloidal spectrum measurements, which utilizes conventional fixed frequency O-mode reflectometry. The method is extended to the case of arbitrary plasma density profiles employing the stratified model of radial density distribution. The technique is applied to estimate the poloidal spectrum, amplitude and rotation velocity of density fluctuations in the edge plasma during a standard H-mode discharge on the ASDEX Upgrade tokamak, making use of the available edge reflectometer channels.

Irradiation of electronic components and circuits at the Portuguese Research Reactor: Lessons learned

A program was started in 1999 in the Portuguese Research Reactor to test electronics components and circuits for the LHC facility at CERN, initially with a dedicated in-pool irradiation device used at a reactor power of 2 kW and later a facility built around a beam line with tailored neutron and gamma filters that could be used at 1 MW. Practice has shown the need to introduce several improvements to the irradiation procedures and facilities over the years. In this paper we review the lessons learned and the major improvements introduced.

Real-time reflectometry measurement validation in H-mode regimes for plasma position control.

It has been shown that in H-mode regimes, reflectometry electron density profiles and an estimate for the density at the separatrix can be jointly used to track the separatrix within the precision required for plasma position control on ITER. We present a method to automatically remove, from the position estimation procedure, measurements performed during collapse and recovery phases of edge localized modes (ELMs). Based on the rejection mechanism, the method also produces an estimate confidence value to be fed to the position feedback controller. Preliminary results show that the method improves the real-time experimental separatrix tracking capabilities and has the potential to eliminate the need for an external online source of ELM event signaling during control feedback operation.