P. Kochkin

Analyzing x-ray emissions from meter-scale negative discharges in ambient air

textabstractWhen voltage pulses of 1 MV drive meter long air discharges, short and intense bursts of x-rays are measured. Here we develop a model for electron acceleration and subsequent photon generation within this discharge to understand these bursts. We start from the observation that the encounter of two streamers of opposite polarity launches the electrons, that they are further accelerated in the discharge field and then lose their energy, e.g., by photon emission through Bremsstrahlung. We model electron and photon dynamics in space and energy with a Monte Carlo model. Also the detector response to incoming photons is modelled in detail. The model justifies the approximation that the x-ray bursts are isotropic in space; this assumption is used to conclude that x-ray bursts near the high-voltage electrode with 6.104 photons and characteristic energies of 160 keV closely reproduce the measured spectra and attenuation curves. The nanosecond duration of the bursts as well as their energy spectrum is consistent with model calculations.

Pilot system development in metre-scale laboratory discharge

The pilot system development in metre-scale negative laboratory discharges is studied with ns-fast photography. The systems appear as bipolar structures in the vicinity of the negative high-voltage electrode. They appear as a result of a single negative streamer propagation and determine further discharge development. Such systems possess features like glowing beads, bipolarity, different brightness of the top and bottom parts, and mutual reconnection. A 1D model of the ionization evolution in the spark gap is proposed. In the process of the nonlinear development of ionization growth, the model shows features similar to those observed. The visual similarities between high-altitude sprites and laboratory pilots are striking and may indicate that they are two manifestations of the same natural phenomenon.

Relativistic electrons from sparks in the laboratory

Abstract Discharge experiments were carried out at the Eindhoven University of Technology in 2013. The experimental setup was designed to search for electrons produced in meter‐scale sparks using a 1 MV Marx generator. Negative voltage was applied to the high voltage (HV) electrode. Five thin (1 mm) plastic detectors (5 cm2 each) were distributed in various configurations close to the spark gap. Earlier studies have shown (for HV negative) that X‐rays are produced when a cloud of streamers is developed 30–60 cm from the negative electrode. This indicates that the electrons producing the X‐rays are also accelerated at this location, that could be in the strong electric field from counterstreamers of opposite polarity. Comparing our measurements with modeling results, we find that ∼300 keV electrons produced about 30–60 cm from the negative electrode are the most likely source of our measurements. A statistical analysis of expected detection of photon bursts by these fiber detectors indicates that only 20%–45% of the detected bursts could be from soft (∼10 keV) photons, which further supports that the majority of detected bursts are produced by relativistic electrons.

Meter-scale spark X-ray spectrum statistics

Abstract X‐ray emission by sparks implies bremsstrahlung from a population of energetic electrons, but the details of this process remain a mystery. We present detailed statistical analysis of X‐ray spectra detected by multiple detectors during sparks produced by 1 MV negative high‐voltage pulses with 1 μs risetime. With over 900 shots, we statistically analyze the signals, assuming that the distribution of spark X‐ray fluence behaves as a power law and that the energy spectrum of X‐rays detectable after traversing ∼2 m of air and a thin aluminum shield is exponential. We then determine the parameters of those distributions by fitting cumulative distribution functions to the observations. The fit results match the observations very well if the mean of the exponential X‐ray energy distribution is 86 ± 7 keV and the spark X‐ray fluence power law distribution has index −1.29 ± 0.04 and spans at least 3 orders of magnitude in fluence.

In‐Flight Observation of Positron Annihilation by ILDAS

Abstract We report a 511‐keV photon flux enhancement that was observed inside a thundercloud and is a result of positron annihilation. The observation was made with the In‐flight Lightning Damage Assessment System (ILDAS) on board of an A340 test aircraft. The aircraft was intentionally flying through a thunderstorm at 12‐km altitude over Northern Australia in January 2016. Two gamma ray detectors showed a significant count rate increase synchronously with fast electromagnetic field variations registered by an on‐board antenna. A sequence of 10 gamma ray enhancements was detected, each lasted for about 1 s. Their spectrum mainly consists of 511‐keV photons and their Compton component. The local electric activity during the emission was identified as a series of static discharges of the aircraft. A full‐scale Geant4 model of the aircraft was created to estimate the emission area. Monte Carlo simulation indicated that the positrons annihilated in direct vicinity or in the aircraft body.

In‐Flight Observation of Gamma Ray Glows by ILDAS

Abstract An Airbus A340 aircraft flew over Northern Australia with the In‐Flight Lightning Damage Assessment System (ILDAS) installed onboard. A long‐duration gamma ray emission was detected. The most intense emission was observed at 12 km altitude and lasted for 20 s. Its intensity was 20 times the background counts, and it was abruptly terminated by a distant lightning flash. In this work we reconstruct the aircraft path and event timeline. The glow‐terminating flash triggered a discharge from the aircraft wing that was recorded by a video camera operating onboard. Another count rate increase was observed 6 min later and lasted for 30 s. The lightning activity as reported by ground networks in this region was analyzed. The measured spectra characteristics of the emission were estimated.

Some EMC aspects of a 2 MV marx generator with sensitive diagnostic equipment in the immedeate vicinity

A 2 MV lightning surge generator is a powerful source of interference, in particular when the voltage waveform is chopped by a shorting spark gap. We not only do need to protect the sensitive diagnostic electronic equipment that operates on small signals, but we also should guarantee its flawless operation during the discharge. The paper discusses the setup, the EMC measures taken, and the results of verification measurements. Some typical results obtained in the research with this instrument will be shown as well.