Shadowgraph
Exploring a new class of plasma-generated penetrating radiation
More than 130 years after the discovery of X-rays, virtually all radiographic systems still rely on the same fundamental physical principle: bremsstrahlung generated when high-energy electrons strike a metal target.
The Shadowgraph project investigates whether plasma itself can generate penetrating radiation through a mechanism different from conventional X-ray production. Understanding this phenomenon could expand our knowledge of plasma physics and potentially open new directions for imaging technologies.
The inspiration for the Shadowgraph project dates back to the pioneering experiments of Nikola Tesla in the 1890s. Working with high-frequency, single-electrode vacuum tubes, Tesla produced images that he referred to as shadowgraphs. Although these experiments preceded the widespread adoption of X-ray technology, the physical origin of the radiation emitted by Tesla’s tubes has never been fully understood.
The image (Fig. 1) was produced using one of Tesla’s high-frequency, single-electrode vacuum tubes. These experiments demonstrated that Tesla’s apparatus generated radiation capable of penetrating biological tissue, although the physical origin of this radiation has remained a subject of discussion ever since.
More than a century later, advances in plasma diagnostics, radiation detectors, high-speed electronics, and computational modelling make it possible to revisit these historic experiments with a level of precision that was unimaginable in Tesla’s time. The Shadowgraph project at ELLIA Laboratory combines these modern tools to investigate the properties, origin, and potential applications of the radiation generated by Shadowgraph tubes.
The first experimental measurements performed at ELLIA Laboratory have already revealed several unexpected characteristics that distinguish the observed radiation from conventional X-rays. The following sections present the experiments that led to these observations.
Experiment
At the heart of the experimental platform is a simple single-electrode vacuum tube inspired by the historical Shadowgraph concept. When driven by a high-frequency, high-voltage resonant power supply, a stable plasma discharge is formed inside the tube. This discharge is the source region under investigation, and the emitted penetrating radiation is characterized using modern radiation detectors, imaging techniques, and spectroscopic measurements.
Throughout this project, we refer to the experimentally observed penetrating radiation as Coherent Plasma X-ray-Like (CPX). This designation is used as a working research term and does not imply that the underlying physical mechanism responsible for the radiation has been fully established.
Figure 3 illustrates the experimental platform developed at ELLIA Laboratory. A high-voltage AC power supply (HV PS) drives a resonant Tesla transformer through a tertiary coupling coil (TC), generating a high-frequency electric field that excites the single-electrode CPX tube. Under these conditions, a stable plasma discharge is established inside the tube, producing the penetrating radiation investigated in this project.
Depending on the experimental objective, the emitted radiation is characterized using different detection systems. For transmission imaging, the radiation passes through the object under investigation and is converted into visible light by a fluorescent screen, which is recorded using a conventional digital camera. For quantitative measurements, including energy spectroscopy, transmission studies, and spatial characterization of the radiation source, the fluorescent screen is replaced by a pixel detector based on Timepix technology.
Although the radiation source itself is remarkably simple, the characterization of its properties relies on modern experimental techniques, including Timepix pixel detectors, Monte Carlo simulations, precision transmission measurements, and high-speed plasma diagnostics.
The scientific value of the Shadowgraph project lies not in the complexity of the apparatus, but in applying modern experimental methods to investigate a phenomenon that has remained largely unexplored for more than a century.
Using this experimental platform, ELLIA Laboratory has obtained the first quantitative measurements of CPX radiation, revealing several unexpected characteristics that distinguish it from conventional X-rays.
First Experimental Results
The first experiments performed at ELLIA Laboratory confirmed that the radiation emitted by the Shadowgraph tube is capable of producing transmission images of biological tissue. At the same time, modern pixel detectors enabled the first quantitative investigation of the radiation source, providing new insights into its spatial origin and physical characteristics.
The transmission image shown in Figure 4(a) demonstrates that the emitted radiation readily penetrates biological tissue and provides sufficient intensity for practical radiographic imaging. The internal bone structure is clearly resolved, confirming the imaging capability of the experimental source.
To investigate the spatial origin of the emitted radiation, pinhole imaging was performed using an ADVACAM Timepix pixel detector. As shown in Figure 4(b), the measurements indicate that the dominant source of the radiation is located within the plasma discharge rather than at the metallic electrodes. This observation provides important experimental evidence regarding the origin of the emitted radiation.
These initial observations established the foundation for a more comprehensive quantitative investigation, including energy spectroscopy, transmission measurements, and comparisons with conventional X-ray sources.
Energy Spectrum and Transmission Characteristics
To investigate the physical properties of the emitted radiation, a series of quantitative transmission measurements was performed using a Timepix pixel detector together with absorbers of different materials and thicknesses. The same experimental platform was subsequently used to measure the energy spectrum of the emitted radiation.
Figure 5 shows the experimental arrangement used for the transmission measurements. Aluminum absorbers of different thicknesses were positioned directly in front of the detector, allowing the transmitted intensity to be measured simultaneously for multiple absorber thicknesses within a single exposure. This approach provides highly reproducible transmission data under identical experimental conditions.
The resulting cluster map is shown in Figure 6(a). Each horizontal band corresponds to a different aluminum thickness, producing a distinct attenuation level. By projecting the detected intensity along the vertical direction, the relative transmission through each absorber can be extracted quantitatively, as shown in Figure 6(b). These measurements form the basis for direct comparisons between the attenuation characteristics of CPX radiation and those expected for conventional bremsstrahlung X-rays.
The measured transmission ratios revealed attenuation behavior that differs significantly from that expected for conventional bremsstrahlung X-rays. This unexpected observation motivated a detailed comparison with Monte Carlo simulations and energy spectrum measurements presented in the following section.
Energy Spectra and Novel Transmission Characteristics
To further characterize the emitted radiation, the energy spectrum and transmission through aluminum absorbers were measured using the Timepix pixel detector. The measured transmission characteristics were then compared with Monte Carlo simulations of conventional bremsstrahlung X-rays performed using the Geant4 simulation toolkit.
Figure 7(a) shows the measured energy spectrum of CPX radiation together with spectra recorded after transmission through aluminum absorbers of different thicknesses. As expected, increasing absorber thickness reduces the detected intensity and shifts the average photon energy toward higher values due to preferential absorption of lower-energy photons.
A more remarkable observation is presented in Figure 7(b), where the measured transmission ratios are compared with Geant4 simulations of conventional X-ray attenuation. Conventional bremsstrahlung has a spectrum that hardens with increasing absorber thickness. Furthermore, Absorbers tend to become more transparent to higher energy X-rays, leading to transmission approaching unity. However, the measured CPX transmission follows substantially different behaviour. This discrepancy suggests that the interaction of CPX radiation with matter cannot be fully described by the attenuation characteristics expected for conventional bremsstrahlung X-rays.
These observations represent one of the principal experimental findings of the Shadowgraph project and motivate further investigation into the physical nature of the emitted radiation.
Summary and Outlook
The experimental results obtained so far indicate that the radiation generated by the Shadowgraph tube exhibits several characteristics that differ from those expected for conventional bremsstrahlung X-rays. These include its measured transmission behaviour, spectral properties, and the apparent spatial origin of the radiation within the plasma discharge. Together, these observations motivate further systematic investigation into the physical mechanism responsible for the emitted radiation.
If these findings are confirmed through independent experiments and a consistent theoretical framework, they could point toward physical processes that are not adequately described by current models of radiation generation in plasma. Such an outcome would have important implications for our understanding of plasma physics and high-field electrodynamics.
From a technological perspective, the Shadowgraph concept offers several attractive features. The radiation source is compact, mechanically simple, and operates without a conventional X-ray target. Its relatively low power consumption makes operation from portable battery systems feasible, opening the possibility of lightweight and transportable imaging devices. Potential future applications include medical imaging, non-destructive testing, industrial inspection, mining and ore sorting, security screening, and scientific instrumentation.
The primary objective of the Shadowgraph project is to establish the physical origin of the observed radiation through rigorous experimental investigation. If the phenomenon proves to represent a genuinely new radiation-generation mechanism, it could provide the foundation for both new scientific understanding and a new generation of compact imaging technologies.