Leonardo Giudicotti

SIMPLE ANALYTICAL MODEL OF GAIN SATURATION IN MICROCHANNEL PLATE DEVICES

We derive, discuss, and test against experimental data an analytical model of the gain saturation in microchannel plate (MCP) devices. By introducing a simple recharging circuit for each dynode, we extend the well‐known, unsaturated gain model of Eberhardt to a microchannel operating in condition of gain saturation and show that the amplification of a current pulse and the voltage drop along the channel can be described by a pair of coupled differential equations. Solutions of these equations are given in various conditions, including an approximate solution, valid in the case of weak saturation and a general solution in implicit form. The behavior of a microchannel operating in current mode is studied by finding the transient and steady‐state solutions obtained with an input step current wave form. Exact solutions are given for the charge gain of pulses with a short duration, compared to the dynode recharging time, and for the gain recovery of a microchannel after the amplification of a short pulse. The ...

Data analysis for a rotating quarter-wave, far-infrared Stokes polarimeter.

Data analysis techniques are reviewed and extended for the measurement of the Stokes vector of partially or completely polarized radiation by the rotating quarter-wave method. It is shown that the conventional technique, based on the Fourier analysis of the recorded signal, can be efficiently replaced by a weighted least-squares best fit, so that the different accuracy of the measured data can be taken into account to calculate the measurement errors of the Stokes vector elements. Measurement errors for the polarization index P and for the azimuth and ellipticity angles psi and chi of the radiation are also calculated by propagation error theory. For those cases in which the above technique gives a nonphysical Stokes vector (i.e., with a polarization degree of P>1) a constrained least-squares best fit is introduced, and it is shown that in this way a Stokes vector with P = 1 (rather than P<or=1) is always obtained. In addition an analysis technique useful to remove from the measured data systematic errors due to initial misalignment of the rotating quarter-wave axis is described. Examples of experimental Stokes vectors obtained by the above techniques during the characterization of components for a far-infrared polarimeter at lambda=118.8 microm for applications in plasma diagnostics are presented and discussed. Finally the problem of the experimental determination of physically consistent Mueller matrices (i.e., of Mueller matrices for which the transformed Stokes vector has always P<or=1) is discussed, and it is shown that for simple Mueller matrices of the ABCD type, whose elements can be determined by the measurement of a single Stokes vector, the imposed P<or=1 constraint gives a sufficient condition for physical consistency. On the other hand, the same constraint, when imposed to the set of four basic Stokes vectors conventionally measured for the determination of a full 16-element Mueller matrix, gives only a necessary but not a sufficient condition.

Analytical, steady-state model of gain saturation in channel electron multipliers

By using the transmission line modeling (TLM) technique we derive a simple model describing the saturation of the gain in channel electron multipliers and show that it generalizes and extends a previous steady-state model due to Shikhaliev. Then by introducing a physically consistent rational approximation of the non-linear gain equation we derive an exact,steady-state,analytical solution in which,contrary to other authors,empirical assumptions about the functional dependence of the internal voltage are not required. The model is then used to simulate a multianode microchannel plate (MCP) photomultiplier,showing that the computed gain in saturated conditions is qualitatively in agreement with published experimental data. Finally,we discuss the general validity of our model,we suggest possible measurements and comment existing data relevant for its validation. r 2002 Elsevier Science B.V. All rights reserved.

Development of a multipoint Thomson scattering system for a large reversed field pinch experiment

A multipoint Thomson scattering system is under development for the measurement of Te and ne spatial profiles in RFX, a large (a=0.48 m, R=2 m, I≤2 MA) reversed field pinch experiment. In this system, the beam from a low divergence ruby laser (E=15 J, Δt=30 ns) is focused to cross the plasma in the equatorial plane. The scattering signal from 30 locations along a plasma diameter is collected by means of high transmission fiber optic bundles. An effective viewing dump was obtained by engraving sharp poloidal grooves in the first wall graphite tiles. The scattering light spectra are analyzed by means of three spectrometers, each including an F/3.4, aberration corrected, holographic grating and a 40‐mm‐diam MCP photomultiplier with a 10×10 anode array. These detectors use a high strip current, V‐plate, electron multiplier with 105 electron gain and have a recovery time of less than 30 μs. In the 300‐channel data acquisition electronics, the plasma light background may be sampled 100 ns before the scattering ...

The multichord far infrared polarimeter of the RFX experiment

A multichannel far infrared (FIR) polarimeter has been installed on the reversed field experiment (RFX) to measure the poloidal magnetic field profile, BΘ. The polarimeter uses a FIR laser (λ=118.8 μm) whose beam is transmitted to the machine via a 31 m long, nitrogen filled beam line. Faraday rotation measurements are reported for five of six parallel chords crossing the 1 m diameter RFX plasma on a poloidal cross section. The density profile is known from a CO2 interferometer. Low noise deuterated L-alaine doped triglycene sulphate (DLATGS) detectors are used. An accuracy of about 0.2° is obtained in the measured Faraday rotation angle, which is sufficient to estimate the characteristics of the BΘ profile. Removal of the noise induced by mechanical vibrations allowed acceptable measurements of Faraday rotation angles.

Automatic laser beam alignment for a Thomson scattering system

A system for the automatic alignment of a pulsed ruby laser beam is under development for the Thomson scattering diagnostics of RFX, a large reversed‐field pinch machine now under construction. In this experiment the laser will be 11 m from the machine and the beam alignment at the 750×4‐mm scattering volume will be actively maintained to within 0.5 mm. The beam direction in space is measured in two reference planes fixed to the collection optics frame by means of two 80‐mm2 fast quadrant photodiodes. A double‐channel preamplifier is used for each quadrant in order to measure both the 30‐ns FWHM ruby pulse and a low‐power cw He–Ne beam propagating through the same optical path. Every time the main laser is fired, the relative direction of the two beams is determined so that the position of the He–Ne beam can be used for feedback control of the steering optics between plasma shots.

Far-infrared polarimetric characterization of metallic mirrors exposed to a tokamak plasma

We describe the polarimetric characterization in the far infrared (λ=118.8μm) of a set of metallic mirrors and of a corner cube retroreflector whose surface has been exposed to a tokamak plasma. The objective of the measurements was to investigate possible changes of the polarization state of the incident radiation due to plasma depositions and identify possible sources of errors for the measurement of the ITER q profile by far-infrared (FIR) polarimetry. Tests have been made on Mo and Cu mirrors exposed to the plasma in TEXTOR and Tore Supra. A corner cube retroreflector exposed in Tore Supra was also tested. The thickness and composition of the plasma depositions on these optical elements were known by previous surface analysis and visible ellipsometry studies. Our tests have been carried out in the FIR laboratory of the RFX experiment in Padova, Italy, and in the Laser Research Laboratory of the Department of Electrical and Electronic Engineering at University College Cork, Ireland, using a Stokes pola...

Nonlinear optical effects in Raman calibrations of a Thomson scattering system.

We present an investigation of the occurrence of stimulated Raman scattering and other nonlinear optical effects during Raman calibrations of Thomson scattering diagnostic systems for magnetic fusion plasmas. When these effects take place, the intensity of the Raman lines is unpredictable, and the calibrations are impossible. In this research Raman scattering from H(2) and D(2) at filling pressures up to 1 atm has been experimentally investigated using the Thomson scattering system of the ETA-BETA II reversed field pinch device. Stimulated Raman Stokes light has been observed at filling pressures above 230 and 500 mbars for H(2) and D(2), respectively, for input laser pulses of 8 J and 30 ns (FWHM) duration. Evidence has been found that the stimulated Raman light does not originate from the observed scattering volume but is detected as light diffused into the vacuum chamber. To explain these results, the Raman gain and the intensity of the stimulated Raman light are calculated, taking into account the multimode structure of the laser beam. We find that significant power conversion from the input laser beam to the Stokes wave takes place near the output window of the vacuum chamber. Part of this radiation is diffused back into the machine, and this part is detected as superimposed on the spontaneous Raman signal. Finally we discuss the Raman calibrations in RFX, a larger plasma device in which the Raman medium will be N(2) at a temperature up to 350°C, and show that a filling pressure of 100 mbar gives a sufficient calibration signal, avoiding any nonlinear effect.

Electronics for microchannel plate detectors of a Thomson scattering system

We describe electronics developed for the operation of multianode microchannel plate (MCP) photomultipliers in a multipoint Thomson scattering system. A pulsed, digitally programmable high voltage power supply with current feedback and a built‐in protection circuit has been designed to bias the detector in safe conditions, taking into account the nonlinear behavior of the MCP resistance. The 180‐V photocathode voltage is switched by a fast, current driven, optically coupled metal‐oxide‐semiconductor field‐effect transistor. Gating pulses with 10‐ns rise time and 50–500‐ns programmable duration are generated with negligible noise at the output anodes.

THOMSON SCATTERING MEASUREMENTS IN THE RFX REVERSED FIELD PINCH

The first systematic measurements of the electron temperature (Te) spatial profile have been obtained in the reversed field pinch experiment RFX with a single pulse Thomson scattering (TS) diagnostic. Scattered light from a ruby laser pulse (E⩽15 J, Δt=30 ns) is collected through three objectives from 10 positions along a diameter in the plasma equatorial plane, with a spatial resolution of 2.5 cm. Plasma discharges with current in the range 700–900 kA have been investigated finding evidence of a quite flat Te profile. Data dispersion significantly greater than experimental uncertainties provides an indication of remarkable plasma fluctuations. Results are in good agreement with Te measurements from other single chord spectroscopic diagnostics (SiLi detector and SXR double filter), showing a reliable operation down to an electron density ne=3×1019 m−3. Integration of this apparatus with a ND:YLF laser system for multipulse Thomson scattering measurements, sharing the same input optics, is under way.