[en] Compact directional couplers-polarimeters were developed for the circular corrugated waveguide transmission lines of the joint IFP-ENEA Electron Cyclotron Resonance Heating experiment at 140 GHz, 2 MW on the Frascati Tokamak Upgrade in Frascati.A linear array of cutoff holes was drilled in the mirrors of the quasi-optical miter bends. The radiated pattern preserves the symmetry and polarization of the waveguide mode in the plane of the array. The direction of propagation is preserved too.Two pairs of standard gain horns with detectors are placed in the plane of the array along the propagation axes of incident and reflected radiation to detect both linear components. The whole assembly is enclosed in a shielded anechoic box of suitable geometry

[en] A significant class of disruptive discharges in tokamaks is characterized by an irreversible but rather slow current decay occurring while the electron temperature is still high. For definitiveness we refer to typical JET cases, where the observed rate of current decay is of the order of 2-3MA/s and the electron temperature T_e measured by the ECE polychromator may decay as fast as 6keV/s in the center and 1.5 keV/s at the position of the q=2 surface. The influx of impurities is moderate and Z_eff ∼6. The Ohmic resistance R_Ohm calculated with the proper plasma geometry and the ECE profiles is of the order of 1 μΩ plasma during the disruptive decay and these observations cannot be reconciled with the model of a choking effect of the plasma caused solely by a sudden inflow of impurities which may drive a radiative collapse of the plasma temperature to a few eV on a ms time scale. The discharge evolution is conveniently described by the trajectories on the plane of the parameters l_i (internal inductance) and q(a) (safety factor). Inspection of the path followed shows that the soft disruption phase often traces backwards the current ramp up phase, where an accelerated peaking up of the current profile may be associated to the effects of observed enhancement of MHD activity, as measured by the poloidal and toroidal magnetic pick-up coils. The observed irreversible process suggests that the effective resistance R_eff of the plasma may be due indeed to temperature independent MHD effects. The problem is discussed comparing the difference between the effective and the Ohmic resistances with the expected value of a model based on the coupling of unstable tearing modes with resonant surfaces close together, around the q=2 surface where a saturated background mode is assumed. The time scales of the process observed require that the power input to the plasma be evaluated considering the full coupling with the external active and passive circuit elements since it is incorrect to assume the trapping of the poloidal flux between the plasma and the vacuum vessel, which in JET has a toroidal time constant of 0.03 s. (author) 3 refs., 6 figs

[en] The measure of residual power of electron cyclotron (EC) waves propagating in FTU tokamak during ECRH experiments at 140 GHz is provided by three low-gain probes facing the tokamak walls. One probe is located in the same poloidal section of the EC launching system; the other two are toroidally displaced by 60 and 90 deg., respectively. A model for the estimate of the e.m. power coupled to the probes has been developed. Several discharges with two gyrotrons at 140 GHz, delivering 0.5 MW each for up to 0.5 s, and with different polarizations have been analyzed. Calculations of the power deposition and e.m. residual power are found to be in very good agreement with the experimental measurements

[en] A quasi-optical system has been designed to couple the power coming from a gyrotron with astigmatic gaussian beam output, into an oversized corrugated waveguide (HE11 mode). The fraction of the power injected in the transmission line can be controlled by means of a wire grid beam splitter. Polarization control is provided by two rotating corrugated mirrors of electrical depth λ/4 and λ/8 respectively

[en] A multichannel heterodyne radiometer in the frequency range 50--75 GHz which has been used to measure the second-harmonic cyclotron emission of the Thor Tokomak plasma is described. Real-time frequency switching, analog output averaging, autocalibration, data-acquisition, and graphical display are all functions performed under control of an Apple II personal computer. Typical experimental results are presented to illustrate the measuring capabilities of the instrument

[en] In the project of the New MWA diagnostic system at JET [L. Cupido et al,Fusion Engineering and Design,74(2005), E. De La Luna et al ,Recent developments of ECE diagnostics at JET, EC-13,2004], the pre-existing one channel Martin-Puplett interferometer (MPI) for ECE measurement needed a multichannel extension for oblique ECE, that was only possible with a complete redesign of the collection optics system. The new instrument [C. Sozzi et al,abstract EC-14,2006] is designed to analyze the incoming radiation from standard perpendicular view (two channels, from two different waveguides or O+X modes from the same one) and from oblique view of the ECE radiation at two different angles (four channels to measure the mostly-O and mostly-X modes separately) at ∼ 11 ^o and ∼ 23 ^o in the toroidal direction with respect to the perpendicular to the magnetic field. The new optical scheme [A. Simonetto et al,abstract EC-14,2006] needed a totally new mechanical layout of the system. The new mechanical layout must fulfil several critical constraints. All the six MIPs channels share the existing rotating mirror (helicoidally shaped, arranged on a wheel with diameter 300 mm) [M. Zerbini et al,HTPD,2004], and therefore the nearly 50 optical components must be positioned in a limited space. Moreover back-reflection on the rotating mirror requires precise alignment in off-normal directions. The polarization must be precisely controlled as well therefore each optical component (mirrors, beam splitters) requires an accurate machining and a precise spatial and angular positioning (the tight tolerances were estimated from electromagnetic simulations). The non-modularities have been concentrated in a small number of pieces (supports, positioners and wedges) and the number of non identical pieces has been minimized. A rigid layout was chosen for an alignment-free system, given the large number of components to be aligned and to endure the accidental vibrations undamaged. Most of components are placed on three layers, two perpendicular and one parallel to the axis of the rotating mirror. The layout of the main frame is therefore made of two optical planes in a '' T-shaped '' arrangement facing the rotating mirror. It can be removed without losing the correct alignment from the rotating mirror structure to gain access to the rotary reflector and the front fixed optics. This paper deals with the detailed description of the mechanical realization of the system. (author)

[en] The results of an ECRH experiment at 140 GHz, 0.5 MW, 15 ms, on the FTU tokamak are presented. The EC waves, generated by GYCOM, a gyrotron with Gaussian output, are transmitted to the plasma with an efficiency of approx. 93% by a hybrid system comprising both mirrors and corrugated waveguides. The waves, launched as an O-mode at the fundamental EC resonance from the low magnetic field side, are almost totally absorbed by the target plasma in a layer 4-8 cm wide, and cause an electron temperature increase of 0.8-2.5 keV. At electron densities above approx. 10²⁰ m^-3 the neutron production rate is also strongly enhanced (50-100%), the increase following the start of the ECRH pulse with a delay consistent with the e-i collision frequency. The ECRH power has been modulated to generate heat waves for confinement studies. (author). 2 refs, 5 figs

[en] In this paper the authors explore the possibility of using sections of overmoded smooth-wall circular waveguide instead of an overall corrugated waveguide transmission line to carry strong mm-wave power in ECRH experiments. To this end they carry out an exact analysis of the transitions from corrugated to smooth-wall waveguide and optimize the length and size of the smooth-wall waveguide to achieve minimum losses. The whole structure is described by a generalized scattering matrix whose elements are obtained using the exact quasi-TE and quasi-TM uncoupled modes in the lossy smooth-wall waveguide

[en] Arrays of thermopile detectors arranged on flat panels were used to measure stray radiation from high-power transmission lines at 140 GHz in the FTU tokamak electron cyclotron resonance heating experiment. These instruments are insensitive to polarization and to the direction of propagation within ±40 degree off normal incidence. They have a time constant of 120 ms, therefore amplification and synchronous detection are required for the measurement of short radiation pulses. No special measurement techniques are required with long pulses or cw radiation. The typical sensitivity is 2 mVcm²/mW in long pulse operation and 47 μVcm²/mW for 1 ms pulses. The detectors are sensitive to light and adequate filtering should be added to suppress it if necessary. These characteristics are adequate for the measurement of human exposure levels to electromagnetic radiation in the millimeter wave range. copyright 1996 American Institute of Physics

[en] Analytical fits for electron density profiles for the FTU tokamak are determined from experimental data