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The present COMPASS tokamak at the Institute of Plasma Physics in Prague is equipped with the 2-mm interferometer, which gives a possibility to measure line-average electron densities up to 1.2×1020 m−3. A high magnetic field tokamak COMPASS-U will be designed and built to replace COMPASS. Higher line-average plasma densities in COMPASS-U of about 5×1020 m−3 will require a new design of the interferometer. This will provide measurements in a wide density range and will allow using a real-time gas puff density feedback. A solution based on the solid-state technology is proposed. The system will use two microwave transceivers with close sub-millimeter wavelengths and will fulfill the principle of the unambiguous measurement. An assessment of signal corrections corresponding to non-linearity effects caused by plasma refractive index is performed. The ray-tracing FIESTA-8 code was used to model the propagation of the probing waves through COMPASS-U plasmas.

The fast microwave reflectometer system on the COMPASS tokamak consists of an O-mode polarized K-band (18 GHz–26 GHz), Ka-band (26 GHz–40 GHz), and a part of U-band (40 GHz–54 GHz). The plasma density profile from the edge plasma area is measured using a fast sweeping rate up of to 6 µs of the probing wave. The processing of the reflected signal is realized by the heterodyne detection configuration based on the I/Q modulator. Two different methods of dynamic calibration of the required linear sweep frequency, together with static frequency and dispersion calibration, were used. The electron density profile was reconstructed by a spectrogram-based method with four sweeps on average. The system has the capability to measure the mid-plane low-field side electron density profile in the density range from 4 × 1018 m−3 to 3.6 × 1019 m−3. Experimental results obtained on COMPASS discharges are presented to demonstrate the performance of the diagnostics.

The paper presents the results of research and development of two types of phase meters for heterodyne two-wave “unambiguous” interferometer designed to measure the plasma density in tokamak COMPASS. The aim of this work is the creation of a phase meter capable of measuring fast phase changes of each probing wave, and their phase difference. Plasma is probed by two microwave signals with different wavelengths propagating along identical paths to meet each other. Each analyzed signal acquires a certain phase shift j1 and j2, and the “unambiguous” interferometer determines the difference (j1 – j2). The phase measure-ent is carried out by using three-channel phase meters: a phase meter based on the pairs of the AD8302 and a phase meter based on field programmable gate arrays (FPGA). The proposed circuit decisions and algorithms of signal processing allow to measure phases in the range 0…360° in real time.

The COMPASS tokamak, a divertor device with clear H-mode and ITER-relevant geometry, has been re-installed in IPP Prague after its transport from CCFE in UK. The first plasma was achieved in December 2008. Many new diagnostic tools with both high temporal and spatial resolutions have been developed to address the scientific programme of COMPASS focused on H-mode physics and pedestal investigations. In the paper, an overview of existing and in near future planned diagnostics (magnetic, spectroscopy, microwave, probe and beam/particle) on COMPASS is given including their basic technical specifications and achievements.

The COMPASS-D tokamak, originally operated in UKAEA Culham, UK, is being reinstalled in the Institute of Plasma Physics (IPP), AS CR. COMPASS-D was designed as a flexible tokamak in the 1980s mainly to explore MHD physics. Its operation (with D-shaped vessel) began in UKAEA in 1992. In 2001 COMPASS-D had been mothballed and later offered to the Institute of Plasma Physics (IPP Prague) AS CR. The re-installation of the COMPASS-D tokamak in IPP Prague had started in 2006. Due to the low power available from the local grid, the two flywheel-generators (47 MVA, 45 MJ, 85 Hz, 6.3 kV) are installed as storage of the required energy. Four AC/DC thyristor rectifiers provide the desired current profiles in toroidal and poloidal coil systems. The start-up circuit, which provides a loop voltage necessary for the plasma generation, is based on fully solid-state switchers and two temporarily inserted resistors. This paper describes the design of the new power supply system and shows details of the commissioning procedures and installation. We also present some results of a comprehensive analysis of the circuit operation performed in Matlab Simulink.

The Compass-D tokamak is being planned to move from UKAEA Culham (England) be and reinstalled in IPP Prague (Czech Republic). An overview of Compass-D energetics is described in short. The new power sources have to be designed. Alternative solutions of power sources have been investigated. Ignitrons and mechanical breakers can be replaced by modern high-power semiconductors. With respect to the need of accumulated energy, new technologies of energy storage – supercapacitors and high-speed flywheels, not applied up to now in similar fusion projects, are characterized in comparison with traditional ones – capacitors and big motor-flywheel generators. It is shown that these technologies enhance possibilities in arrangement of power supplies for tokamaks as well as for other high-power demanding experiments with pulse length from fraction to tens of seconds.

The electron density on COMPASS is measured using 2 mm microwave interferometer. Interferometer signal is used as an input for the feedback control loop, running under the MARTe real-time framework. Two different threads are used to calculate (fast 50 μs thread) and to control (slow 500 μs thread) the electron density. The interferometer measures a line averaged density along a measurement chord. This paper describes an approach to control the line-averaged electron density in a real-time loop, using a correction to the real plasma shape, the plasma position, and non-linear effects of the electron density measurement at high densities. Newly developed real-time electron density control give COMPASS the chance to control the electron density more accurately which is essential for parametric scans for diagnosticians, for physics experiments and also for achieving plasma scenarios with H-mode.

The present work is devoted to measure the absolute magnetic field produced by different coils in the EGYPTOR tokamak using a calibrated pickup coil. Scaling these measurements in different equations connected with the discharge currents from each supply system are performed. The pickup coil used in the present study is well calibrated with Helmholz coils at the IPP in Prague, Czech Republic. A 0.2% deviation has been found between an evaluation done in the present study and the calibration using Helmholz coils. Experimental measurements of the toroidal magnetic field are in good agreement with calculations to within 2%. Very low values of stray magnetic field components arising from TF and OH coils are recorded which proves that the compensation coils for these components are sufficient.

The EGYPTOR tokamak is a small device of rectangular cross section 25 × 20 cm. Its major (R) and minor (A) radius are 30 and 10 cm, respectively. The aspect ratio is 3. The present work is devoted to measure the absolute values of the discharge currents and to calibrate the different power systems used to supply different coils in the tokamak. Two different methods are applied to measure the discharge currents from different capacitor banks. The first is the direct method employing a calibrated resistance R=0.008 Ω; this resistance was prepared and calibrated in the IPP of the Czech Academy of Sciences, Czech Republic. The second is the indirect method using a calibrated Rogowski coil, fabricated and calibrated also in Prague (Czech Rogowski coil). Both methods are applied simultaneously. Details of each power system presently used are reported. Sensitivity calibration of two different Rogowski coils is also done, and comparison with the calculated sensitivity shows good agreement.

A correlation reflectometer in 8-mm wavelength range has been constructed and installed on CASTOR tokamak. The reflectometer can operate on three fixed frequencies (29, 33, and 35 GHz). This is universal correlation sine-cosine reflectometer, which can work in poloidal or toroidal correlation regime and in ordinary mode (O-mode) and/or extraordinary mode (X-mode). A computational method of a direct evaluation of the phase fluctuations as well as absolute value of the phase from sine-cosine data has been developed. Using this method the relative plasma density fluctuations in the CASTOR core plasma has been determined in the range of few percent under assumption of parabolic density distribution.

It has been shown on tokamak CASTOR by using an emissive Langmuir probe that positive plasma 'biasing' takes place in a radially very narrow layer just at the grill mouth. The effect scales linearly with the power applied. Apparently, this positive biasing results from the charge separation due to the escaping electrons, accelerated by the near field lower hybrid wave effects in this region, along the magnetic field lines (Fuchs V et al 1996 Phys. Plasmas 3 4023).

Theoretical models of EBW conversion mechanism are verified on Castor with simulated overdense plasma. Direct EBW-X conversion has been detected by a recently constructed ECE radiometer 17÷40 GHz with a perpendicularly positioned antenna. The crucial role of plasma edge density gradient has been confirmed.

Strong fluctuations of lower hybrid waves (LHW) amplitudes inside the plasma have been observed in experiments with different types of launchers. The fluctuation frequency is much higher than the frequency of plasma density fluctuations. This effect is explained by the presence of plasma fluctuations along the whole ray path. LHW spectrum broadening has been found by means of a double radiofrequency probe. This experimental result supports the assumption of spectral gap filling between the electron velocity distribution function and the spectrum launched into the plasma.

Magnetic and electrostatic fluctuations are measured in the CASTOR tokamak with a reasonable spatial and temporal resolution to better understand the nature of turbulent processes in tokamaks. We have found that the broadband magnetic fluctuations in the plasma core are nearly independent of the safety factor q(a). However, their level is dramatically reduced (by a factor of three) when the aspect ratio increases from 5.4 to 6.6. The character of electrostatic structures in the edge plasma was found to be quite similar to that observed in large tokamak experiments. Furthermore, the fluctuations of the longitudinal current have been identified in the scrape-off layer. These fluctuations might represent a link between the magnetic and electrostatic components of plasma turbulence in this region.

The paper presents the results of upgrading and description a two-wavelength “unambiguous” microwave interferometer for line integrated electron density measurements at the “COMPASS” tokamak. The probing signals in the frequency range of 131 and 133 GHz are generated by IMPATT diode oscillators. Double conversion heterodyne receivers are used for detection. Use of each oscillator is twofold: as a source for the probing signals and as a local oscillator for heterodyne detection. The plasma is probed with two separate waves of different wavelengths traveling along identical paths to meet each other. Each probing beam experiences a different phase shift φ1 and φ2 and the “unambiguous” interferometer determines the difference φ1-φ2. The phase measurement is realized by using the 3 pieces of phase detectors based on pairs of phase/gain evaluation boards (AD8302). Application of the phase detectors provides information on the average plasma concentration in real time and restore the values of plasma density in the case of phase jumps by an amount > 2π.

This article presents “Compass” TOKAMAK microwave diagnostics calibration and results accuracy enhancing experiment methodology. Main subject is microwave diagnostic system phase measurements description. Additional interest is mistakes, attendant to phase measurement. Emphasis is put on phase detectors measurements mistakes correction methodology; two different phase meter configurations are described. Phase detectors, based on AD8302 CPLD, results processing methodology are presented.

The electron density and temperature profiles measured by the Thomson scattering diagnostic on the COMPASS tokamak are used for estimation of electron kinetic energy, energy confinement time, and effective charge number Z(eff). Data are compared with the line-integrated electron density measured by a microwave interferometer in an ohmically heated plasma with a circular cross section. An error analysis of both electron temperature and density are performed by two methods-a constant chi-square boundaries method and a Monte Carlo simulation, determining asymmetrical error bars for the electron temperature.

(1993). Czech national consciousness in the Baroque era. History of European Ideas: Vol. 16, No. 4-6, pp. 935-941.

The plasma edge of the CASTOR tokamak ( R = 0.4 m, a = 0.1 m) has been investigated using a thermal lithium beam. The radial profiles of the electron density have been derived from the radiation profiles of the excited lithium atoms at λ = 671nm. Probe measurements have shown that the outward particle flux due to the turbulent density and potential fluctuations via an associated E × B drift, which could be responsible for the observed anomalous diffusion at the plasma edge in ohmic discharges, is poloidal strongly asymmetric. These results have been confirmed by measurements of the edge density profiles on top and at the bottom of the torus. A 45° toroidally movable sector limiter gives the possibility of varying the distance to the limiter continuously. The influence of the sector limiter position on the edge density profiles has been investigated and compared to measurements with a radially movable sector limiter far from the lithium beam (180° ). The additional heating with RF ( f = 1.25 GHz) produces in the CASTOR tokamak a lower hybrid current drive effect, which enhances the particle confinement time. This behaviour has been indicated by steeper edge density profiles during LHCD.

Understanding of the scrape-off layer (SOL) physics in tokamaks requires diagnostics with sufficient temporal and spatial resolution. This contribution describes results of experiments performed in the SOL of the CASTOR tokamak (R=40 cm, a = 6 cm) by means of a ring of 124 Langmuir probes surrounding the whole poloidal cross section. The individual probes measure either the ion saturation current of the floating potential with the spatial resolution up to 3 mm. Experiments are performed in a particular magnetic configuration, characterized by a long parallel connection length in the SOL, L_par ~q2piR. We report on measurements in discharges, where the edge electric field is modified by inserting a biased electrode into the edge plasma. In particular, a complex picture is observed, if the biased electrode is located inside the SOL. The poloidal distribution of the floating potential appears to be strongly non-uniform at biasing. The peaks of potential are observed at particular poloidal angles. This is interpreted as formation of a biased flux tube, which emanates from the electrode along the magnetic field lines and snakes q times around the torus. The resulting electric field in the SOL is 2-dimensional, having the radial as well as the poloidal component. It is demonstrated that the poloidal electric field E_pol convects the edge plasma radially due to the E_pol x B_T drift either inward or outward depending on its sign. The convective particle flux is by two orders of magnitude larger than the fluctuation-induced one and consequently dominates.

A significant step towards demonstrating the feasibility of coupling Lower Hybrid (LH) waves in ITER has been achieved in the latest LH current drive experiments in JET. The local electron density in front of the LH launcher was increased by injecting gas (D2 or CD4) from a dedicated gas injection module magnetically connected to the launcher. PLHCD=3MW was coupled with an average reflection coefficient of 5%, at a distance between the last closed flux surface and the launcher of 10cm, in plasmas with an internal transport barrier (ITB) and H‐mode edge, with type I and type III ELMs. Following a modification of the gas injection system, in order to optimise the gas localisation with respect to the LH launcher, injection of D2 proved to be more efficient than CD4. A D2 flux of 5−8×1021el/s provided good coupling conditions at a clearance of 10cm, while when using CD4, a flux of 12×1021el/s was required at 9cm. The plasma performance (neutron rate, H‐factor, ion temperature) was similar with D2 and CD4. An ...

Summary form only given. The magnetic fluctuations are monitored in radial and poloidal directions deep (upto r/a=0.4) into the CASTOR tokamak and their relationship with electrostatic fluctuations is studied. For performing the experiments CASTOR was operated with: R=0.40 m, a=0.085 m, B/sub T/=1 T, I/sub p/=12 kA and n~/sub e//spl les/10/sup 19/ m/sup -3/. A set of eight magnetic probes (all of them 4 mm in length and 4 mm in diameter), with all the probes registering the radial component of the magnetic field and radially spaced on each other, is constructed to study the radial magnetic field fluctuations (B~r). With this set the whole radial profile from r/a=1 to r/a=0.4 could be achieved for each shot. The radial correlation length and propagation velocity of B~r are computed. For further diagnosing the relation between the magnetic and electrostatic fluctuations a new experiment has been conceptualised. A rogowski coil has been constructed and a Langmuir probe is installed in the center of the coil. The rogowski coil has a diameter of 1 cm with 300 number of turns wound on a circular form and a cross section of every turn is 0.00314 cm/sup 2/. The rogowski coil registers the current flowing in the tokamak scrape-off-layer (SOL) region and at the edge. The sensitivity of the coil is sufficiently increased in order to observe the current filaments existing in the tokamak SOL and edge region.

The efficiency of ECRH experiment on the CASTOR tokamak at 18 GHz and 29 GHz is estimated. The power absorption during the O-X-EBW conversion process is determined numerically.

The COMPASS tokamak recently started operation at the Institute of Plasma Physics AS CR, v.v.i., Prague. A new 16-channel radiometer, operating alternatively in three frequency bands, has been designed and constructed. The system is prepared for detection of normal electron cyclotron emission (O1 or X2) or oblique electron Bernstein wave emission. The end-to-end calibration method includes all components that influence the antenna radiation pattern. A steady recalibration is possible using a noise generator connected to the radiometer input through a fast waveguide PIN-switch. Measurements of the antenna radiation characteristics (2D electric field) were performed in free space as well as in the tokamak chamber, showing the degradation effect of structures on the Gaussian beam shape. First plasma radiation temperature measurements from low-field circular plasmas are available.

Understanding of underlying physics in the edge plasma of tokamaks requires knowledge of the plasma density, potential, electron and ion temperature, ion flows and their fluctuations with a high spatial and temporal resolution. A family of electric probes, which have been designed and tested for this purpose in the CASTOR tokamak, is reviewed and examples of their performance are given. In particular, we focus on description of the 1D and 2D arrays of Langmuir probes for spatially resolved measurements of the edge turbulence, the Ball pen and emissive probes for direct measurements of the plasma potential, the optimized Gundestrup probe for measurements of parallel and perpendicular ion flow, and the tunnel probe for fast measurement of electron and ion temperatures. Additional information on individual diagnostics is available in the listed references. PACS 52.70.Ds

Micrometre sized tantalum droplets were injected into a tokamak plasma by a controllable arcing gun located behind the wall. The trajectories of the ablating particles were photo- graphed by a high speed camera. Various possible mechanisms which may explain the observed curvature of the particle paths are discussed. The migration of the ablated material in the tokamak was studied by post-mortem analysis of collector probes and limiters.

Les mesures du courant de plasma et de la tension montrent une phase avalanche avec croissance du courant exponentielle dans les premieres 200 μs et une phase de striction entre 200 et 400 μs. Le taux de croissance depend des champs magnetiques perpendiculaires externes. Etude de la relation entre densite de courant et densite d'electrons et du role de l'effet de peau

The authors present conclusive evidence of anomalous absorption of an intense RHCP wave at high magnetic fields omega ce/ omega >1, where omega ce and omega are the electron cyclotron frequency and the frequency of the electromagnetic wave.

The paper describes the measurement of the electron mobility in semiconducting plates placed between two cylindrical antennas with diameterd−0·75λ0 by means of the microwave Faraday effect. This system enables (i) to measure the Faraday rotation in semiconducting plates of arbitrary transverse dimensions, greater or at least equal to the diameter of dielectric antennas, (ii) to evaluate the unknown value of the charge carrier mobility from the measured rotation in an unbounded solid state plasma slab approximation. The measurement of the Faraday rotation in n-type Ge plates are reported. The comparison of the experimental data with the theory shows good agreement.

Let me open with two caveats. First, given the very brief length dictated for this treatment of Czech and Slovak humanisms, I have chosen to concentrate on their main lines of development and on their primary — i.e., literary — aspect. Second, domestic humanism has received comparatively little attention from Czech and Slovak historians to date. The former have been occupied primarily with the great national-religious revolt, the Hussite Reformation, with which European humanism coalesces when it reaches Bohemia and Moravia. And Slovak historians are still doing basic research into much of their nation's past.

The plasma edge of the CASTOR tokamak was investigated using a thermal lithium beam and electrical probes. The density profiles in the scrape-off layer (SOL) obtained by means of the lithium beam technique clearly reflect the particular limiter configuration and allow the evaluation in terms of D⊥. These profiles are in good agreement with simultaneous Langmuir probe measurements at different radii. In discharges with additional lower hybrid current drive (OH/LHCD) steeper profiles are obtained, which indicates a reduced radial particle transport. This is supported by the evaluation of fluctuation measurements (density and poloidal electric field) in terms of the turbulent radial particle flux, and by a reduced temporal decay of the line-averaged density during RF. Thus, in effect LHCD improves the particle confinement.

The paper deals with the experimental study of the absorption of microwave energy in plasma in a strong magnetic field, omega ce/ omega >>1, where omega ce and omega are the electron cyclotron and wave frequency respectively. The microwave energy is delivered to the plasma by a cylindrical helical structure. The experiments have verified the possibility of efficient absorption of the microwave energy for ratios up to omega ce/ omega =14. It was found that the absorption is practically independent of the magnetic field, and for magnetic fields lying in the interval omega ce/ omega in (1, 14) is nearly constant.

Throughout the Revolution of 1848–1849 the national aspirations of the Hungarians generally evoked a negative response from both liberal and radical Czechs, although, on the whole, the radicals expressed less hostility than the liberals toward developments in Hungary. Of the two groups, the Czech radicals were the least interested in maintaining the territorial and political integrity of the Habsburg monarchy and the more revolutionary in their demands and expectations. Consequently they had less cause to criticize the Magyars. Indeed, after the Habsburgs declared war against Hungary and the Russians openly assisted them in subduing the Hungarian revolutionaries, the radicals openly expressed sympathy for the Magyar cause.

The paper deals with an experimental investigation of the cutoff conditions for RHCP and LHCP waves propagating in an axially magnetized plasma filling a circular waveguide. The results of an experiment carried out in the X-band are in a qualitative agreement with the linear theory.

One type of low-frequency oscillation measured in the plasma column generated by an electron cyclotron resonance device (Datlov, Musil, and Zacek, 1968) in a waveguide in a homogeneous magnetic field is shown to be an ion acoustic wave because of variation of the frequency as a function of the measured temperature of electrons, the length of column and the mass of ions.

This paper deals with the penetration of a strong electromagnetic wave into a dense inhomogeneous plasma generated by electron cyclotron absorption of this wave on a magnetic beach. It was experimentally found that the plasma density increases considerably after a certain time. This increase of plasma density is due to a passage of an electromagnetic wave through an evanescent region which appears in the inhomogeneous plasma in the region where the electron density is such that omega 02/ omega 2 approximately=1; omega is the frequency of the wave which generates and penetrates the plasma, omega 0 is the plasma frequency. The time dependences of the reflected power, spatial electron density distribution and transmitted power demonstrate the existence of the evanescent region and show the conditions for the free passage of the electromagnetic wave through this region. The experimental results are in a good qualitative agreement with the theory.

This paper describes the experimental verification of the linear transformation of the (left-hand circularly polarized) wave in a magnetoactive plasma, in which the electron density surpasses the critical value. The study is carried out for magnetic fields satisfying the condition omega ce/ omega

The possibility of formation of thin oxide films on silicon in a microwave magnetoactive oxygen plasma at pressures of 5*10-5-1 Torr is described, and the corresponding experimental results are presented.