Methods for recording the time profile of single ultrashort ... › publication › fulltext › Methods-f... › publication › fulltext › Methods-f...by VF Tarasenko · 2016 · Cited by 21 · Related articlescoaxial cable, for the first time, was used in [3] to
Methods for recording the time profile of single ultrashort pulses of electron beams and discharge currents in real-time mode
ISSN 2397-7264 Received on 6th January 2016 Revised on 20th February 2016 Accepted on 1st April 2016 doi: 10.1049/hve.2016.0007 www.ietdl.org
Victor F. Tarasenko ✉, Dmitry V. Rybka Institute of High Current Electronics Siberian Branch, Russian Academy of Sciences, Tomsk 634055, Russia ✉ E-mail:
[email protected]
Abstract: The collectors for measuring the time profile of a current pulse of the runaway electron beam that are generated under atmospheric pressure are described. An analysis of changes in pulse shape depending on the bandwidth of the registration path with a temporal resolution of up to 20 ps was performed. It is shown that the electron beam detected behind small-diameter diaphragms has a complex structure, which depends on the parameters of the gas diode. There is a design of the current shunts with a temporal resolution of up to 100 ps in this study. Measurement techniques of voltage pulses with subnanosecond and nanosecond duration are briefly described.
1
Introduction
Subnanosecond-duration electron beams are of great interest for research in various areas [1, 2]. However, in this case, the duration and amplitude of the beam current pulse must be correctly measured. Thus, a small-size collector connected to a coaxial cable, for the first time, was used in [3] to detect electron beam pulses of subnanosecond duration (150–200 ps full width at half-maximum (FWHM)) behind a foil of the vacuum diode. Methods for recording the characteristics of short pulses of electron beams and X-rays are generalised in [1–10]. Note that recently the interest for measuring subnanosecond-duration electron beams has substantially increased due to the studies conducted on gas diodes filled with different gases at pressures of 1 atm and higher [11–16]. The use of gas-filled diodes and nanosecond high-voltage pulsers allows to comparatively simply obtain electron beams with a half-height pulse duration of ∼100 ps [13]. The accumulated experience seems to be insufficient for correct registration of pulses of an electron beam current, discharge current, and voltage across the gap having subnanosecond and picosecond durations [16–28]. The purpose of this review consisted in the design and testing of the collectors for registration of electron beam current pulses with FWHM t = 20–1000 ps. In addition, there are constructions of the current shunts with a temporal resolution of up to 100 ps and a brief description of techniques for the measurement of voltage pulses of subnanosecond and picosecond durations.
2
Development of oscilloscopic methods
The development of oscilloscopic methods in investigations of fast processes in gas discharges is determined by the advent of modern digital oscilloscopes operating in the real-time mode with a bandwidth of ≥30 GHz and a sampling frequency of >80 × 109 points/s [29–31]. These measuring instruments are primarily aimed at the solution of problems in microelectronics, computer engineering, digital communication channels, and so on, and substantially facilitate the work of engineers when developing and checking the correspondence of signals of high-speed serial data transfer to standards. At the same time, the use of real-time digital oscilloscopes with a wide passband Δf makes it possible to register the time profile and amplitude values of single electric signals,
which are formed by current and voltage detectors in studies of pre-breakdown phenomena in the gas-discharge physics. At the frequency band of input analogue paths of oscilloscopes Δf ∼ 30 GHz, it becomes possible to register features of the time profile of signals with a temporal resolution of up to 10 ps. The process of generating a runaway electron (RE) beam in gases at elevated pressures, when a gas diode is excited with high-voltage nanosecond pulses with subnanosecond rise time, is one of such fast processes. In this case, questions arise that are related not only to the correct determination of the RE-beam current shape and the current through a gap, but also related to the determination of voltage across the discharge gap and their mutual positions on the time axis with picosecond accuracy. The first problem in such measurements is associated with both the impossibility of placing a voltage detector directly in the discharge gap and the necessity of conducting indirect measurements on the basis of readings of detectors that are installed in a feeding coaxial line at a certain distance from the discharge gap. The limitations that arise in such a measurement will be discussed in Section 6. Another factor that appears during registering of subnanosecond pulses is the operation of the registration path of a digital oscilloscope operating in real-time mode near the upper edge fh of the passband. The mathematical apparatus for digital signal processing (DSP) that is used in modern oscilloscopes for compensating non-uniformities in the amplitude–frequency characteristics of channels properly operates in the cas
