A. Sardella

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 ...

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 ...

Upgrade of the RFX microwave frequency modulated reflectometer to ultrafast sweeping rate

The swept homodyne reflectometer installed on the RFX experiment is composed of five units which cover the frequency range 26.5–110 GHz. This diagnostic system is based on solid state IMPATT sources and can be operated in the O or X mode. During the first operation, at a sweep rate of 0.1 GHz/μs, the typical density fluctuations present in the reversed field pinch configuration usually prevented correct profile reconstruction, which was possible only in some special cases. To increase the sweep speed and overcome this limit, a new IMPATT driver has been built and IF amplifiers, control, and acquisition systems have been modified accordingly. The new configuration has been extensively tested in the 34–38 GHz range with a modulation rate exceeding 4 GHz/μs on many different plasma conditions. Measurements on plasma showed that both the phase and reflected power level can be correctly recovered from the IF signals. These results indicated the criteria necessary to extend the fast sweep capability to the full...

The time‐of‐flight neutral particle analyzer and its calibration system for the RFX experiment

A simple and low cost time‐of‐flight neutral particle analyzer has been operated on the RFX experiment. Simplifications have been introduced with respect to already existing similar devices, obtaining a low cost and reliable system. Good performance has been reached for hydrogen plasmas producing charge exchange H neutrals whose energy is lower than 3000 eV. Moreover a simple ion beam system has been projected and built to calibrate the electron multipliers used on the time‐of‐flight analyzer.

Thermal characterization of high strip current microchannel plate photomultipliers

We have investigated the thermal behavior of a set of 40 mm, high strip current, microchannel plate (MCP) photomultipliers developed for a diagnostic system of a fusion plasma by laser scattering. These detectors include a V-stack of very low resistivity (5 X 106 (Omega) m) microchannel plates and must be operated with a pulsed high voltage power supply under strict control of the temperature. Therefore the resistance of the V-stacks has been studied as a function of the MCP temperature and of the applied voltage by applying short pulses of high voltage to the detectors in a temperature comtrolled environment. At low applied voltage (approximately 1 V) a resistance of 2.05 X 106 (Omega) was measured for one of the V-stacks, with a resistance coefficient of -1.47 X 104 (Omega) /degrees C. At high voltage the resistance was found to be lower by about a factor three and showed a linear decrease with both the MCP temperature and the applied high voltage. From these measurements the detector thermal capacitance and thermal conductivity were found to be 0.79 J/degrees C and 1.85 X W/degrees C respectively. The measured parameters are used in a simplified thermal model of the detectors for the construction of the pulsed power supply control system necessary to operate them in safe conditions.

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.

First results in reflectometric plasma density measurements on RFX

A five‐band, high‐sweep‐speed reflectometer system for the RFX reverse field pinch experiment has been developed. To control the high‐noise environment we improved the intermediate frequency amplifying and conditioning system. We checked the modified system using a new in situ calibration method that pointed out the improvements. This new method is a suitable one for calibrating the reflectometer, because it allows one to place a mirror in a well‐defined position, thus giving a good radial (<2 mm) and angular resolution (<10−3 rad). Preliminary results on maximum density time evolution are presented.

Short Recovery Time, Multianode, Microchannel Plate Photomultiplier For Plasma Diagnostics.

A multianode MCP photomultiplier with a recovery time two orders of magnitude shorter than in conventional MCP devices has been developed for plasma diagnostics by laser scattering. The detector has an S20 photocathode, a low resistivity Z--plate electron multiplier and a 10 x 10 square array anode configuration. The Z-plate has a diameter of 25 mm, a resistance of 8.8 MO (at 20 °C) and is made of MCPs rated for operation at a DC strip current density in excess of 100 pA/cm2. Once inside the vacuum tube the Z-plate must be operated with pulsed high voltage, to avoid overheating. For this reason this PMT is suitable for the detection of fast bursts of light pulses, occurring at a repetition rate sufficiently low to permit the heat dissipation between successive bursts. The thermal behaviour of the detector has been investigated. First the thermal coefficient of the resistance was measured giving a value of -6.00 ± 0.04 x 104 0/°C (around 20 °C); then the temperature increase of the Z-plate due to a single high voltage square pulse was measured as a function of the pulse energy. The heat capacity and the thermal time constant were found to be 0.93 J/°C and 175 s respectively. The PMT electrical behaviour has also been studied in pulse current mode. At 900 V/plate and with 25 ns FWHM input light pulses, the maximum output charge density was found to be 8.2 x 10-10 C/cm2 in linear regime and 4.9 x 10-9 C/cm2 in saturated gain mode. The Z-plate recovers from the maximum linear pulse in about 50 las.

Gain saturation model of microchannel plate devices: recent advances

A previous model of microchannel plate (MCP) devices operating in conditions of gain saturation has been extended to include charge diffusion along the microchannel during the gain recovery process. To this purpose the set of independent recharging circuits previously associated to each MCP dynode has been replaced by a distributed parameter electrical network that represents the entire microchannel consistently with the structure of the microchannel wall as described in the literature. The model obtained in this way, unlike the previous one, takes into account the interaction between dynodes during the gain recovery and is also consistent with the operation of MCP devices in conditions of very fast gating. As for the previous model the gain and voltage along the channel are described by a pair of coupled, nonlinear differential equations whose numerical solutions are computed in conditions of a steady-state input current. Simplified analytical solutions for short pulse operations are also derived and discussed.

Numerical and analytical models of gain saturation in microchannel plate devices

We present and compared two different approaches for modelling microchannel plate (MCP) devices in regime of gain saturation. In our numerical model an MCP is described as a ladder network of interacting R and C lumped elements. The Kirchhoff equations of the network are coupled to a gain equation describing the amplification of input pulses as they progress into the microchannels. This non-linear system can be solved numerically and can be included into a best-fit algorithm capable of determining the model parameters from experimental data. An alternative analytical model was developed assuming a simplified network and describing pulse amplification and wall charge replenishment with a pair of differential equations. In this way, simpler analytical equations are found that describe an MCP in a broad range of operating conditions. Measurements on a Z-stack MCP photomultiplier showed that the numerical model provides a fairly accurate description of the MCP in pulse mode. The analytical model, although less accurate, is more suited to best-fit algorithms, allowing a remarkable reduction of computer time and of convergence problems.