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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.

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.

The broadband reflectometry system on ASDEX Upgrade is now routinely providing electron density profiles with a minimum temporal spacing of 30 μs. The plasma is probed from zero density to ne=6.64×1019 m−3 at the low field side with combined O and X modes, and from ne=0.318×1019 to ne=6.64×1019 m−3 at the high field side using O mode. Here we analyze the performance of the diagnostic for profile measurements taking into account its design options and the experimental results obtained in a wide range of plasma regimes. We address several questions, in particular the measuring range of the antennas and the range of density gradients. It is shown that it is possible to obtain a detailed evolution of the density profile and density gradient even in the presence of turbulence, providing that sufficient data can be extracted from the plasma. The radial movements of the plasma can be measured with great accuracy, which is especially important for the use of reflectometry for plasma position control, as is propos...

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.

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 broadband microwave reflectometry system on ASDEX Upgrade has been recently upgraded with new channels. It has now 12 channels that probe simultaneously the high and low fields plasma, with ultrafast sweeping (20–100 μs). X mode (33–75 GHz) is used for the scrape off layer, and the O mode (16–110 GHz) is reflected from densities between 0.3×1019 and 1.5×1019 m−3. A fast sweeping heterodyne system was developed for the highest frequency channels (50–110 GHz) to cope with higher losses and lower incident power. Fixed frequency operation is employed for fluctuation measurements. A dedicated channel (33–55 GHz) operating in fixed frequency provides a signal to continuously monitor the level of density fluctuations (e.g., L–H transition), during the whole discharge. The diagnostic is fully operated by remote control. The acquisition system is based on specially developed VME boards with up to 1 Gsamp/s sampling rates. Automatic profile inversion was recently implemented using data validation and rejection ...

A reflectometry system has been developed for ASDEX Upgrade to measure the plasma profile from the scrape‐off layer until the bulk plasma, simultaneously at the high and low field sides. Unique features of the system are the ultrafast broadband frequency modulation of a continuous wave using solid state stable hyper abrupt tuned oscillators (down to 10 μs), high and low field side channels and fully remote control operation, via optical fiber links. Due to the special design of the transmission line, with decoupled in going and out going lines and one‐antenna configuration, the system is optimized for reception and spurious reflections are eliminated. The ultrafast operation guarantees that the effect of plasma turbulence is greatly reduced. Both features determine the high performance of the diagnostic. A dedicated data acquisition system handles the large amounts of data generated by the broadband operation. Recent developments include the operation of new channels and an automatic and accurate frequency calibration circuit. Also, advanced digital signal processing techniques were applied to obtain density profiles with high spatial and temporal (20 μs) resolutions under turbulent plasma regions, e.g., the scrape‐off layer. Experimental results are presented showing the great sensitivity of the diagnostic to plasma radial movements and its tolerance to vertical movements of the plasma. Density profiles measured in ELMy regimes illustrate the capabilities of the diagnostic to detect fast profile changes.