Search

Limited available pellet velocities have so far restricted the refuelling performance of efficient launch schemes from the tokamak magnetic high field side (HFS). Although pellet injection during H mode has resulted in more peaked density profiles and enhanced performance with respect to gas puff refuelling, prompt particle and energy losses caused by pellet induced ELM bursts have still limited the extension of the operational area. Now, the preliminary version of a new optimized pellet injection set-up at ASDEX Upgrade allows for significantly higher injection speeds when launching pellets from the magnetic HFS. Intact pellets with velocities up to vP = 560 m/s were successfully injected instead of the vP = 240 m/s available with the previous set-up. The velocity increase results in a deeper pellet penetration and seems to follow the vP1/3 scaling derived from a multimachine study using conventional pellet launch from the torus outside. The inward shift of the pellet particle deposition profile with respect to the penetration depths turned out to be approximately the same for both launch velocities. The respectively achieved deeper particle deposition inside the plasma column reduced the particle and energy loss rates during the immediate post-pellet phase. Thus, further enhancement of tokamak operation in the high density regime seems feasible by means of high speed pellets launched from the torus inner side.

The ASDEX Upgrade broadband microwave reflectometry system uses frequency modulation of a continuous wave (FM-CW). With a monostatic antenna configuration optimised for eliminating spurious reflections, the system can probe the high and low field sides of the plasma (using both O- and X-modes) covering the density range from 0.3 to 6.76 × 10 19 m -3 . A combination of solid state fast tuneable microwave sources (Hyperabrupt Tuned Oscillators, HTO) with active and passive doublers are used. During the recent shutdown period several improvements were made. A new W band (75-110 GHz) channel was installed at the low field side to extend the density range to 15 × 10 19 m -3 . A broadband heterodyne detection was designed to improve detection, which is required due to high losses and low power available for this band. A similar detection scheme was implemented to all existing V band channels. Both systems can perform a full band sweep in 10 μs. Also a new Q (35-50 GHz) band O-mode channel with fixed frequency was installed for on-line fast monitoring the L-H transition based on the reduction of the plasma density fluctuations. The control system was upgraded with five new control boards for fixed frequency operation of the reflectometer. The new control software enables the operation in three different modes: broadband, fixed frequency and a mixed mode. A new dedicated data acquisition system came into operation to replace the existing CAMAC/VME configuration, enabling high sampling rate (1 GSPS). Advanced digital signal processing techniques were developed to obtain density profiles. With these new improvements the system is capable of measuring 720 electron density profiles per discharge in the range 0.3 to 15 × 10 19 m -3 , each in 20 μs with a separation of 40 μs.

A single channel microwave interferometer has been developed for a tokamak with an improved detection that uses an heterodyne configuration and is based on a differential quadrature phase concept. With the new detection very sensitive and accurate direct phase measurements are obtained due to the elimination of the spurious variations of the detected signals resulting from the nonideal behavior of the output mixers. As a result, the line integrated density along the vertical central chord of the vacuum chamber can be determined during the whole plasma discharge. Fast density variations occurring both at the formation phase of the plasma and during sawtooth cycles could also be measured.

We study the evolution of turbulence at several radial plasma locations in the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak, using microwave reflectometry. We analyze two types of scenarios: H-mode plasmas with improved performance and plasmas with electron cyclotron resonance heating modulation. Profound modifications of the turbulence spectra, occurring at specific plasma regions, have been found in both cases, indicating the relation between turbulence changes and confinement. The results demonstrate the ability of the reflectometry system on ASDEX Upgrade to make localized measurements. Mapping the radial distribution of turbulence can help us to understand the links between plasma rotation, shear, E×B flows, and the plasma turbulence behavior.

The reflectometry system installed in the tokamak ASDEX Upgrade has 11 channels primarily aimed at profile measurements and two dedicated channels operating at fixed frequency to study density fluctuations. The channels used for profile measurements can also be operated at fixed frequency. In this article we describe the potential of the system to study important characteristics of the plasma turbulence, from combined fixed frequency and profile measurements (to localize the reflecting layers). The numerical tools used to treat fixed frequency data are briefly described. Using a set of similar discharges and the ability of the microwave reflectometry system to probe simultaneously different layers with several channels, we obtain the time evolution of the integrated power spectra of the different signals versus the minor radius of the plasma. In plasma with H-mode edge and internal transport barrier we could identify a zone at the plasma core and another at the edge with reduced fluctuation levels, coinci...

The first Portuguese magnetic confinement experiment, the tokamak ISTTOK, has been in operation since 1993. This tokamak device is described and the main technological features, as well as the nove...