Milic M. Pejovic

Microcontroller based system for electrical breakdown time delay measurement in gas-filled devices

This paper presents realization of a digital embedded system for measuring electrical breakdown time delay. The proposed system consists of three major parts: dc voltage supply, analog subsystem, and a digital subsystem. Any dc power source with the range from 100 to 1000 V can be used in this application. The analog subsystem should provide fast and accurate voltage switching on the testing device as well as transform the signals that represent the voltage pulse on the device and the device breakdown into the form suitable for detection by a digital subsystem. The insulated gate bipolar transistor IRG4PH40KD driven by TC429 MOSFET driver is used for high voltage switching on the device. The aim of a digital subsystem is to detect the signals from the analog subsystem and to measure the elapsed time between their occurrences. Moreover, the digital subsystem controls various parameters that influence time delay and provides fast data storage for a large number of measured data. For this propose, we used the PIC18F4550 microcontroller with a full-speed compatible universal serial bus (USB) engine. Operation of this system is verified on different commercial and custom made gas devices with different structure and breakdown mechanisms. The electrical breakdown time delay measurements have been carried out as a function of several parameters, which dominantly influence electrical breakdown time delay. The obtained results have been verified using statistical methods, and they show good agreement with the theory. The proposed system shows good repeatability, sensitivity, and stability for measuring the electrical breakdown time delay.

Influence of tube wall material type and tube temperature on the recombination processes of nitrogen ions and atoms in afterglow

The recombination of positive ions and atoms during an afterglow period, created during previous electrical breakdown and gas discharge, at the molybdenum glass and copper is investigated using the time delay method. It has been shown that the recombination of the positive ions at the copper wall, while the atoms at the molybdenum wall, at room temperature, is more intensive. The temperature influence on the recombination of these particles at copper has also been monitored by the same method, showing that the positive ion recombination rate increases and atom recombination rate decreases with temperature.

The influence of additional electrons on memory effect in nitrogen at low pressures

Memory effect in nitrogen based on experimental data of electrical breakdown time delay as a function of afterglow period in the presence of additional electrons has been investigated. The additional electrons were supplied as a result of extraction from the auxiliary electrode pair or nitrogen irradiation with the radioactive source 226 Ra of low activity. The results show that these electrons have an important role in the recombination of positive ions formed in mutual metastable molecules’ collisions and collisions between metastable and highly vibrationally excited molecules in the early afterglow. As a consequence of the ion‐electron recombination N( 4 S) atoms are formed which, as well as N( 4 S) atoms formed in previous discharge, have a significant influence on the memory effect in late afterglow. The presence of N( 4 S) atoms in the late afterglow is tracked by monitoring the secondary emission which they induce via catalytic recombination on the cathode of a nitrogen-filled tube. Also, it has been shown that the contribution of secondary electrons which originate from N( 4 S) atoms and Compton electrons when the radioactive source is and isn’t present can be distinguished.

Digital system for vacuum and gas-filled devices testing

This article describes an improved electrical system aimed at measuring and data acquisition of the breakdown voltage of vacuum and gas-filled devices at low pressures using a discretized dynamic method. The previous system [M. M. Pejovic, C. S. Milosavljevic, and M. M. Pejovic, Rev. Sci. Instrum. 74, 3127 (2002)] included a complex analog circuit for breakdown detection, which did not provide the required reliability. The smallest voltage step which this system could provide was 250 mV. In order to increase the reliability of the breakdown detection, the Keithley model 248 high power supply was added to the system. The breakdown is detected from a monitor output at model 248 rear panel. The disadvantage of this system was the fact that the minimal value of the voltage step in this case is 1 V. An additional Keithley model 2400 Source Meter was introduced as a serial connection with the Keithley model 248 with the aim of decreasing the minimal value of the voltage step, which is in this case 1 mV. PC cont...

Electrical system for measurement of breakdown voltage of vacuum and gas-filled tubes using a dynamic method

This article describes an electrical system aimed at measuring and data acquisition of breakdown voltages of vacuum and gas-filled tubes. The measurements were performed using a nitrogen-filled tube at 4 mbar pressure. Based on the measured breakdown voltage data as a function of the applied voltage increase rate, a static breakdown voltage is estimated for the applied voltage gradient ranging from 0.1 to 1 V s−1 and from 1 to 10 V s−1. The histograms of breakdown voltages versus applied voltage increase rates from 0.1 and 0.5 V s−1 are approximated by the probability density functions using a fitting procedure.

Experimental Investigation of Breakdown Voltage and Electrical Breakdown Time Delay of Commercial Gas Discharge Tubes

This article presents the experimental results of DC dynamic breakdown voltage Ub for small voltage increase rates and electrical breakdown time delay td of commercial gas discharge tubes. It was shown that Ub is a stochastic value with Gauss distribution for voltage increase rates ≥2 V/s. In order to determine the static breakdown voltage Us as a deterministic quantity, the mean values of the dynamic breakdown voltage Ub as a function of voltage increase rate k were extrapolated until the intersection with Ub axis using linear fit. The intersection point (for k = 0) correspond to Us value. Additional experiments were performed in order to verify the temperature stability of these components over the wide temperature range from 25 to 250 °C. The experimental results of electrical breakdown time delay are also presented in the paper. Electrical breakdown time delay if often refereed as delay response and it is also very important parameter of gas filled devices. It was shown when the voltage higher then 310 V is applied to those components, the mean value of electrical breakdown time delay td insignificantly varies to the value of relaxation time τ≈1 s, while the breakdown probability is close to one for the voltages higher then 380 V. These facts show that the commercial gas discharge tubes are very reliable for the protection for voltages higher then 380 V.

Analysis of low-pressure dc breakdown in nitrogen between two spherical iron electrodes

The influence of afterglow period τ, voltage increase rate k, and electrode gap d on breakdown voltage Ub for a nitrogen-filled tube with spherical electrodes of diameter D⪢d and p=6.5mbar has been investigated. The data for the breakdown voltage were obtained for the case when there is a presence of N(S4) atoms, which release secondary electrons via recombination on the cathode. By fitting the experimental data of breakdown voltage mean values as a function of the voltage increase rate, the static breakdown voltages for afterglow periods of 15 and 100 s were estimated. The electrical field as a function of the electrode gap using breakdown voltage mean values was also determined. It is shown that experimental results of the breakdown voltage mean value as a function of pd in the interval of d from 0.82 to 1.62 mm can be very well described with Paschen’s law, valid for the case of parallel-plate electrodes.

Response of pMOS dosemeters on gamma-ray irradiation during its re-use

Response of pMOS dosemeters during two successive irradiations with gamma-ray irradiation to a dose of 35 Gy and annealing at room and elevated temperature has been studied. The response was followed on the basis of threshold voltage shift, determined from transfer characteristics, as a function of absorbed dose or annealing time. It was shown that the threshold voltage shifts during first and second irradiation for the gate bias during irradiation of 5 and 2.5 V insignificantly differ although complete fading was not achieved after the first cycle of annealing. In order to analyse the defects formed in oxide and at the interface during irradiation and annealing, which are responsible for threshold voltage shift, midgap and charge-pumping techniques were used. It was shown that during first irradiation and annealing a dominant influence to threshold voltage shift is made by fixed oxide traps, while at the beginning of the second annealing cycle, threshold voltage shift is a consequence of both fixed oxide traps and slow switching traps.

Dose response, radiation sensitivity and signal fading of p-channel MOSFETs (RADFETs) irradiated up to 50 Gy with 60Co

This paper reports response of p-channel MOSFETs (RADFETs) to (60)Co gamma radiation in the 10-50 Gy dose range and signal fading (room temperature annealing) for 100 days after irradiation. RADFETs with three different thicknesses of the gate oxide layer were used. Irradiations were performed at gate biases ranging from 0 to 5 V. Threshold voltage shift was monitored during the irradiations and the subsequent fading. The dependence of the threshold voltage shift on the radiation dose is linear for the RADFETs with 100 nm- and 400 nm-thick gate oxide layers irradiated under the gate biases ranging from 1.25 to 5 V. Also, an exponential dependence of the radiation sensitivity on the gate bias during irradiation was found. The signal fades at room temperature without a gate bias. The results demonstrate that these RADFETs are suitable as sensors of gamma radiation. The threshold voltage shift of the RADFETs with 400 nm- and 1 μm-thick gate oxide layers decreases significantly during the first day after irradiation, which, unfortunately, makes these devices incapable of holding dosimetric information for long periods of time.

Investigation of post-discharge processes in nitrogen at low pressure

The processes which are a consequence of neutral active particles presence in post-discharge nitrogen-filled tube at 13.3 mbar pressure have been analyzed. The analysis has been performed based on the experimental data of electrical breakdown time delay as a function of afterglow period τ. The most significant parameters such as applied voltage, discharge current, time, and exposure to radiation have been varied. It has been shown that the increase in applied voltage and discharge time, as well as exposure to UV radiation, leads to the decrease of the mean value of electrical breakdown time delay t¯d. This decrease occurs for τ>70 ms, when N(S4) atoms play a dominant role in breakdown initiation. The increase in discharge current leads to the decrease of t¯d values for τ≤70 ms, when positive ions dominantly induce breakdown. The most important reactions which lead to formation of positive ions and neutral active particles are also presented in the paper.