M. Manso

Reflectometry techniques for density profile measurements on fusion plasmas

Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

ELM pace making and mitigation by pellet injection in ASDEX upgrade

In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges. Injection of small size cryogenic deuterium pellets (~(1.4?mm)2 ? 0.2?mm ? 2.5 ? 1019?D) at rates up to 83?Hz imposed persisting ELM control without significant fuelling, enabling for investigations well inside the type-I ELM regime. The approach turned out to meet all required operational features. ELM pace making was realized with the driving frequency ranging from 1 to 2.8 times the intrinsic ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement by pellet pace making causes much less confinement reduction than by engineering means like heating, gas bleeding or plasma shaping. Confinement reduction is observed in contrast to the typical for engineering parameters. Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts while their frequency was increased. No significant differences were found in the ELM dynamics with the available spatial and temporal resolution. By breaking the close correlation of ELM frequency and plasma parameters, pace making allows the establishment of fELM as a free parameter giving enhanced operational headroom for tailoring H-mode scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several successful applications in different scenarios. It seems that further reduction of the pellet mass could be possible, eventually resulting in less confinement reduction as well.

Integrated Exhaust Scenarios with Actively Controlled ELMs

An integrated radiative high performance scenario has been established at ASDEX Upgrade based on simultaneous feedback control of the average divertor neutral particle and power flux in combination with a high, pellet induced frequency of edge localized modes (ELMs). This approach is fully compatible with the present tungsten wall coating covering about 65% of the plasma facing components and is intended for application in the envisaged full-tungsten experiment. In these experiments, divertor recycling and effective divertor temperature (derived from thermoelectric currents) were tuned by acting on fuel gas puff and argon injection rates. The ELM frequency (f(ELM)) was kept high by repetitive injection of small cryogenic deuterium pellets to avoid the radiative instabilities seen at low f(ELM) and high radiated power, and to control the ELM energy. No confinement loss is observed in this radiative type-I ELMy scenario with relatively flat density profiles. In contrast, similar type-III ELM scenarios achieved in hydrogen show a confinement loss of 25% as compared to the type-I phase. In parallel to pellets, alternative ELM trigger techniques have been investigated as well. Fast vertical plasma oscillations are able to synchronize the ELM frequency to values higher and lower than the intrinsic f(ELM), but remain to be tested in the integrated scenario. Supersonic gas injection showed better fuelling efficiencies than usual gas puffing but instantaneous ELM release has not been achieved. A particular experimental challenge for AUG conditions is to obtain a high pace making frequency, to establish scalings of confinement and energy loss as a function of controlled ELM frequency.

Review of data processing techniques for density profile evaluation from broadband FM-CW reflectometry on ASDEX Upgrade

Broadband FM-CW reflectometry is foreseen to be used on ITER to measure the electron density profile, both in the main and divertor plasmas. These measurements should provide crucial information for machine control and operation, in complement to magnetic diagnostics, and require the automatic evaluation of the density profile in all relevant plasma scenarios. This is a complex task due to the effects of plasma turbulence on the probing microwaves. A major and continued effort both on hardware development and data processing has been made in the ASDEX Upgrade broadband reflectometry diagnostic since 1991, which has led to noticeable improvements of density profile measurements. Here we review the evolution of the data processing techniques for profile evaluation. We first describe the standard methods that have been applied in ASDEX Upgrade (as well as in other machines) and discuss their advantages and limitations. Secondly, the novel techniques implemented in ASDEX Upgrade are presented and the resulting improvements on profile evaluation are discussed. It is shown that these are due to the ability to use both the a priori knowledge about profile measurements and the advanced features of the ASDEX Upgrade reflectometry diagnostic.

Frequency control of type-I ELMs by magnetic triggering in ASDEX Upgrade

Magnetic triggering of edge localized modes (ELMs) was reported first from TCV in ohmic plasmas showing type-III ELMs. This method, showing successful locking of the ELM frequency to an imposed vertical plasma oscillation, has now also been demonstrated in the ITER-relevant type-I ELM regime in ASDEX Upgrade. Our experiments showed the ELM frequency becoming identical to the driving frequency in steady state for an applied motion of only about twice the value caused by an intrinsic ELM event. Triggered ELMs still showing clear type-I features were found when the plasma down-shift velocity reached its maximum, corresponding to the lowest edge current value. This is the opposite of the behaviour expected from the peeling–ballooning nature attributed to the ELM boundary and to TCV observations. The reason for this behaviour is not yet clear.

Density profile analysis during an ELM event in ASDEX Upgrade H-modes

This paper reports results on measurements of the density profiles. Here we analyse the behaviour of the electron density for a set of experiments in type I ELMy H-mode discharges in ASDEX Upgrade where the plasma current, plasma density, triangularity and input power were varied. Detailed measurements of the radial extent of the perturbation on the density profiles caused by the edge localized mode (ELM) crash (ELM affected depth), the velocity of the radial propagation of the perturbation as well as the width and gradient of the density pedestal are determined. The effect of a type I ELM event on the density profiles affects the outermost 20?40% of the plasma minor radius. At the scrape-off layer (SOL) the density profile broadens while in the pedestal region the density decreases resulting in a smaller density gradient. This change in the density profile defines a pivot point around which the density profile changes. The average radial velocity at the SOL is in the range 125?150?ms?1 and approximately constant for all the density layers far from the pivot point. The width of the density pedestal is approximately constant for all the ELMy H-mode discharges analysed, with values between 2 and 3.5?cm. These results are then compared with an analytical model where the width of the density is predominantly set by ionization (neutral penetration model). The width of the density profiles for L-mode discharges is included, since L- and H-mode have different particle transport. No agreement between the experimental results and the model is found.

Progress towards steady-state advanced scenarios in ASDEX Upgrade

Recent experiments at ASDEX Upgrade have concentrated on advanced scenarios, specifically ELMy H-mode discharges without an internal transport barrier which obtain a high βN = 3.5 in steady state. A highly shaped plasma configuration is used at 800 kA, with a triangularity δ = 0.42 and edge safety factor, q95, in the range 3.6-4.1. The product βNH89-P reaches up to 8.0 in plasmas approaching a stationary conditions, and βNH89-P{} = 11.5 is achieved transiently. These discharges are at high density, 80-90% of the Greenwald density limit. Discharges at the highest density show a strong reduction of the ELM activity, similar to type II activity, eliminating transient high heat loads on the divertor target. Finally, the non-inductively driven current approaches 70% of the total current making this regime a serious candidate for an advanced scenario in a reactor, and a benchmark for other advanced scenarios.

Identification of local Alfvén wave resonances with reflectometry as a diagnostic tool in tokamaks

Local Alfven wave (LAW) resonances are excited in tokamaks by an externally driven electro-magnetic field, below the ion cyclotron frequency. Based on TCABR (Tokamak Chauffage Alfven Bresilien) experiments and numerical calculations, it is shown that a combination of small power deposition in LAW resonances, swept by plasma density variation or scanned by varying generator frequencies, in combination with detection of the density fluctuations in the LAW resonances by reflectometry, can serve as a diagnostic tool for identification of the effective ion mass number Aef and q-profile in tokamaks. The idea is based on the simultaneous detection of the position of m = ±1 LAW resonances, which are excited by M/N = ±1/±2 antenna modes and m = 0 generated by the poloidal mode coupling effect in tokamaks. The m = 0 resonance depends only on the effective ion mass number and not on the q-profile, so that the mass number can be determined unambiguously. Then, we can determine q-factor at the position of m = ±1 LAW resonances. Using the multifluid ALTOK code, we identify mass number in TCABR experiments and demonstrate the possibility of applying this method in the Joint European Torus.

Characterization of Alfvén eigenmodes using NBI during current ramp-up in the ASDEX Upgrade tokamak

Alfv?n cascades (ACs) and beta-induced Alfv?n eigenmodes (BAEs) have been studied in the ASDEX Upgrade tokamak during the current ramp-up phase of neutral beam heated (NBI) discharges using principally reflectometry, but also soft x-ray (SXR) and electron cyclotron emission imaging (ECEI). ACs have been observed on the tokamak high-field side and low-field side in reflectometer signals even in the absence of a cutoff. Under this condition it is shown that the response is not due to an interferometry effect but due to backscatter. The radial structure of BAEs and ACs has been obtained by cross-correlating the reflectometer with SXR, ECEI and magnetic signals. The reflectometer signals reveal a variety of Alfv?n eigenmodes with different characteristics depending on the plasma heating scheme. Here, discharges with similar plasma parameters but varying NBI sources and/or additional electron cyclotron resonance heating were performed. It is shown that the bursting behaviour of ACs for qmin?<?2 depends on the NBI beam geometry. Also, a discrepancy in the n?=?2 AC minimum frequency of a Grand Cascade is explained by the linear gyro-kinetic code LIGKA simulations which include energetic particle effects.

Upgrade of the data acquisition and control system of the ASDEX upgrade microwave reflectometer

Microwave reflectometry is an important diagnostic to characterize the plasma electron density in fusion experiments. The broadband frequency swept microwave reflectometry system of the ASDEX Upgrade tokamak covers the frequency range of 16 to 100 GHz. In order to fully exploit the diagnostic capabilities it is mandatory to upgrade the data acquisition and control system. It should comply with shorter frequency sweep times and improved time resolution as well as accuracy allowing, for example, plasma position and shape measurements for control purposes, as it is foreseen for ITER. A new PCI-based system is under development based on a digitizer card with two channels, 12-bit resolution, and a waveform generator with one channel, 16-bit resolution. Both cards have 512 Mbytes of memory, a digital signal processor (DSP) for advanced processing modes, and a field programmable gate array (FPGA) for real-time algorithms and complex trigger managing modes. The system is able to support multiple cards (digitizers and signal generators) operating synchronously at a maximum rate of 210 MSamples/second.