P. Bilkova

Global and pedestal confinement in JET with a Be/W metallic wall

Type I ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type I ELMs occur is reduced and below to the so-called critical type I–type III transition temperature reported in JET-C experiments. Furthermore, the confinement factor H98(y,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions Ploss/Pthr,08 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H98(y,2) = 0.8 is obtained, compared to H98(y,2) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H98(y,2) ≤ 1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling–ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 µs in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 < (measured pped)/(predicted pped,EPED) < 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions.

New chlorine‐substituted liquid crystals possessing frustrated TGBA and SmQ phases

Several series of chlorine‐substituted rod‐like liquid crystalline compounds have been synthesised containing a different number of lactate units in the chiral chain. Sequences of phases and phase transition temperatures were determined by differential scanning calorimetry, polarising optical microscopy and X‐ray diffraction studies. Depending on the molecule structure, the materials possess a rich variety of mesophases, i.e. the paraelectric chiral smectic A (SmA*), ferroelectric chiral smectic C (SmC*) and antiferroelectric smectic C (SmCA*) phases. Additionally, some compounds with one and two lactate groups exhibit the frustrated antiferroelectric twist grain boundary (TGBA) and smectic Q (SmQ) phases, respectively. The existence of the SmQ phase ordered in two directions was confirmed by X‐ray diffraction on a non‐oriented sample. The effects of the non‐chiral chain length and number of the chiral lactate groups on the mesomorphic properties, spontaneous polarisation, spontaneous tilt angle and complex permittivity have been studied and discussed.

RF magnetron sputtering of polymers

Abstract RF sputtering of polymeric materials such as polytetrafluoroethylene (PTFE) and polyethylene (PE) has been studied. Threshold powers of 220 and 80 W for the self-sputtering glow discharge mode were found for PTFE and PE, respectively. FTIR absorption data using polarized light disclosed a structural anisotropy of the sputtered PTFE films. A simple structural model is proposed. The first results obtained by FTIR spectroscopy of sputtered PE films are presented.

Status of the COMPASS tokamak and characterization of the first H-mode

This paper summarizes the status of the COMPASS tokamak, its comprehensive diagnostic equipment and plasma scenarios as a baseline for the future studies. The former COMPASS-D tokamak was in operation at UKAEA Culham, UK in 1992–2002. Later, the device was transferred to the Institute of Plasma Physics of the Academy of Sciences of the Czech Republic (IPP AS CR), where it was installed during 2006–2011. Since 2012 the device has been in a full operation with Type-I and Type-III ELMy H-modes as a base scenario. This enables together with the ITER-like plasma shape and flexible NBI heating system (two injectors enabling co- or balanced injection) to perform ITER relevant studies in different parameter range to the other tokamaks (ASDEX-Upgrade, DIII-D, JET) and to contribute to the ITER scallings. In addition to the description of the device, current status and the main diagnostic equipment, the paper focuses on the characterization of the Ohmic as well as NBI-assisted H-modes. Moreover, Edge Localized Modes (ELMs) are categorized based on their frequency dependence on power density flowing across separatrix. The filamentary structure of ELMs is studied and the parallel heat flux in individual filaments is measured by probes on the outer mid-plane and in the divertor. The measurements are supported by observation of ELM and inter-ELM filaments by an ultra-fast camera.

Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

We synthesised a series of rod‐like mesogens with a (S)‐2‐methylbutyl‐(S)‐lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free‐standing films, by Grandjean‐Cano texture and by NMR studies on a deuterium‐labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20–30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H‐NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.

Electrical properties of plasma-polymerized C:H films

Abstract Ac electrical properties of the dielectric part—hard hydrocarbon plasma polymer C:H of the composite Ag C:H prepared by an unbalanced magnetron were investigated. Capacitance and dielectric losses tg δ of the C:H film sandwiched between the two Al thin film electrodes and measured without breaking vacuum at a single frequency (1592 Hz) decreased considerably with time. In addition ac conductivity of the C:H film within 10 min after its deposition was measured in the frequency range 10 – 104 Hz. An explanation in terms of post deposition trapped free radical reactions resulting in their decay is given. This is supported by FTIR absorption measurements.

Laser system for high resolution Thomson scattering diagnostics on the COMPASS tokamak.

A new Thomson scattering diagnostic has been designed and is currently being installed on the COMPASS tokamak in IPP Prague in the Czech Republic. The requirements for this system are very stringent with approximately 3 mm spatial resolution at the plasma edge. A critical part of this diagnostic is the laser source. To achieve the specified parameters, a multilaser solution is utilized. Two 30 Hz 1.5 J Nd:YAG laser systems, used at the fundamental wavelength of 1064 nm, are located outside the tokamak area at a distance of 20 m from the tokamak. The design of the laser beam transport path is presented. The approach leading to a final choice of optimal focusing optics is given. As well as the beam path to the tokamak, a test path of the same optical length was built. Performance tests of the laser system carried out using the test path are described.

The role of the density profile in the ASDEX-Upgrade pedestal structure

Experimental evidence for the impact of a region of high density localised in the high-field side scrape-off layer (the HFSHD) on plasma confinement is shown in various dedicated experiments on ASD ...

Progress of development of Thomson scattering diagnostic system on COMPASS.

A new Thomson scattering diagnostic system has been designed and is being built now on the COMPASS tokamak at the Institute of Plasma Physics ASCR in Prague (IPP Prague) in the Czech Republic. This contribution focuses on design, development, and installation of the light collection and detection system. High spatial resolution of 3 mm will be achieved by a combination of design of collection optics and connected polychromators. Imaging characteristics of both core and edge plasma collection objectives are described and fiber backplane design is presented. Several calibration procedures are discussed. The operational deployment of the Thomson scattering diagnostic is planned by the end of 2010.

Profile measurements of the electron temperature on the ASDEX Upgrade, COMPASS, and ISTTOK tokamak using Thomson scattering, triple, and ball-pen probes

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (ΦBPP) and the floating potential (Vfl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula Te = (ΦBPP - Vfl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.