Imperial College London


Faculty of Natural SciencesDepartment of Physics

Electronics Workshop Technician



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Publication Type

15 results found

Gusev AI, Lavrinovich I, Bland S, de Ferron AS, Pecastaing L, Parker S, Yan J, Novac BMet al., 2023, New SOS diode pumping circuit based on an all-solid-state spiral generator for high-voltage nanosecond applications, IEEE Transactions on Plasma Science, Pages: 1-8, ISSN: 0093-3813

Semiconductor opening switch (SOS) diodes are capable to switch currents with a density of more than 1 kA/cm 2 and withstand nanosecond pulses with an amplitude of up to 1 MV. SOS diodes, however, require a specific pumping circuit that must simultaneously provide forward and reverse pumping currents with a time of ∼ 500 and ∼ 100 ns, respectively. Such a pumping circuit with energies > 1 J typically requires a gas-discharge switch or a low-efficient solid-state solution. This study proposes a novel approach to pumping SOS diodes based on a spiral generator (SG) (also known as a vector inversion generator). Due to its wave characteristics, the SG produces a bipolar current discharge that meets the time duration and current amplitude required to pump an SOS diode. Moreover, the initial pulse from the spiral typically has a relatively low current amplitude compared to the opposite polarity secondary pulse, so the SOS diode can operate at very high efficiencies. This idea has been tested using an all-solid-state SG coupled with large-area SOS diodes (1 cm 2 ). With this combination, a voltage pulse of 62 kV having a rise time of only 11 ns was obtained on an open circuit load (3 pF, 1 M Ω ). The experiments were highly repeatable, with no damage to the components despite multiple tests. There is significant scope to further improve the results, with simple alterations to the SG.

Journal article

Lavrinovich I, Gusev AI, Bland S, de Ferron AS, Pecastaing L, Parker S, Yan J, Novac BMet al., 2022, 2-kV thyristor triggered in impact-ionization wave mode by a solid-state spiral generator, IEEE Transactions on Plasma Science, Vol: 50, Pages: 3443-3451, ISSN: 0093-3813

Impact-ionization wave triggering of a thyristor enables it to switch significantly higher currents with much faster rise times ( dI/dt ) than through conventional triggering; indeed tests on commercial components demonstrate that both current and dI/dt can be increased an order of magnitude over their specified datasheet values by utilizing impact ionization. However, creating an impact ionization wave places stringent requirements on the generator used to trigger the thyristor—particularly the trigger pulse must have a voltage rise rate ( dV/dt ) of more than 1 kV/ns and an amplitude over twice the thyristors static breakdown voltage. Given the capacitance of a thyristor is relatively large, often hundreds of pF, this is difficult to achieve with many common triggering methods. In this study, we present a bespoke, cost-effective, trigger generator that has been developed based on spiral/vector inversion techniques coupled to an optimized sharpening circuit. Using this generator, both a 2-kV single thyristor and a 4-kV stack of two thyristors in series were triggered in the impact-ionization mode. The thyristors had a wafer diameter of 32 mm and capacitances of 370 pF. With a single thyristor 100 shots were performed with it switching a peak current of 1.25 kA and an associated dI/dt of 12 kA/ μs . With two thyristors, peak currents of 2.6 kA and with dI/dt of 25 kA/ μs were achieved. In all experiments no degradation of the semiconductor structure was observed. The work opens the way for developing very powerful, but still compact, solid-state trigger generators and larger pulsers for a wide range of pulsed power applications.

Journal article

Strucka J, Halliday JWD, Gheorghiu T, Horton H, Krawczyk B, Moloney P, Parker S, Rowland G, Schwartz N, Stanislaus S, Theocharous S, Wilson C, Zhao Z, Shelkovenko TA, Pikuz SA, Bland SNet al., 2022, A portable X-pinch design for x-ray diagnostics of warm dense matter, Matter and Radiation at Extremes, Vol: 7, Pages: 1-11, ISSN: 2468-080X

We describe the design and x-ray emission properties (temporal, spatial, and spectral) of Dry Pinch I, a portable X-pinch driver developed at Imperial College London. Dry Pinch I is a direct capacitor discharge device, 300 × 300 × 700 mm3 in size and ∼50 kg in mass, that can be used as an external driver for x-ray diagnostics in high-energy-density physics experiments. Among key findings, the device is shown to reliably produce 1.1 ± 0.3 ns long x-ray bursts that couple ∼50 mJ of energy into photon energies from 1 to 10 keV. The average shot-to-shot jitter of these bursts is found to be 10 ± 4.6 ns using a combination of x-ray and current diagnostics. The spatial extent of the x-ray hot spot from which the radiation emanates agrees with previously published results for X-pinches—suggesting a spot size of 10 ± 6 µm in the soft energy region (1–10 keV) and 190 ± 100 µm in the hard energy region (>10 keV). These characteristics mean that Dry Pinch I is ideally suited for use as a probe in experiments driven in the laboratory or at external facilities when more conventional sources of probing radiation are not available. At the same time, this is also the first detailed investigation of an X-pinch operating reliably at current rise rates of less than 1 kA/ns.

Journal article

Halliday JWD, Bland SN, Hare JD, Parker S, Suttle LG, Russell DR, Lebedev SVet al., 2021, A time-resolved imaging system for the diagnosis of x-ray self-emission in high energy density physics experiments, Review of Scientific Instruments, Vol: 92, Pages: 123507-123507, ISSN: 0034-6748

A diagnostic capable of recording spatially and temporally resolved x-ray self-emission data was developed to characterize experiments on the MAGPIE pulsed-power generator. The diagnostic used two separate imaging systems: a pinhole imaging system with two-dimensional spatial resolution and a slit imaging system with one-dimensional spatial resolution. The two-dimensional imaging system imaged light onto the image plate. The one-dimensional imaging system imaged light onto the same piece of image plate and a linear array of silicon photodiodes. This design allowed the cross-comparison of different images, allowing a picture of the spatial and temporal distribution of x-ray self-emission to be established. The design was tested in a series of pulsed-power-driven magnetic-reconnection experiments.

Journal article

Hutchison CDM, Parker S, Chukhutsina V, van Thor JJet al., 2021, Open hardware microsecond dispersive transient absorption spectrometer for linear optical response, Photochemical and Photobiological Sciences, Vol: 21, Pages: 23-35, ISSN: 1474-905X

An open hardware design and implementation for a transient absorption spectrometer are presented that has microsecond time resolution and measures full difference spectra in the visible spectral region from 380 to 750 nm. The instrument has been designed to allow transient absorption spectroscopy measurements of either low or high quantum yield processes by combining intense sub-microsecond excitation flashes using a xenon lamp together with stroboscopic non-actinic white light probing using LED sources driven under high pulsed current from a capacitor bank. The instrument is sensitive to resolve 0.15 mOD flash-induced differences within 1000 measurements at 20 Hz repetition rate using an inexpensive CCD sensor with 200 μm pixel dimension, 40 K electrons full well capacity and a dynamic range of 1800. The excitation flash has 230 ns pulse duration and the 2 mJ flash energy allows spectral filtering while retaining high power density with focussing to generate mOD signals in the 10–4–10–1 ΔOD range. We present the full electronics design and construction of the flash and probe sources, the optics as well as the timing electronics and CCD spectrometer operation and modification for internal signal referencing. The performance characterisation and example measurements are demonstrated using microsecond TAS of Congo red dye, as an example of a low quantum yield photoreaction at 2% with up to 78% of molecules excited. The instrument is fully open hardware and combines inexpensive selection of commercial components, optics and electronics and allows linear response measurements of photoinduced reactions for the purpose of accurate global analysis of chemical dynamics.

Journal article

Yan J, Parker S, Bland S, 2021, An investigation into high-voltage spiral generators utilizing thyristor input switches, IEEE Transactions on Power Electronics, Vol: 36, Pages: 10005-10019, ISSN: 0885-8993

High-voltage nanosecond pulses are widely used in scientific research, but their wider adoption in industry requires compact, cost-effective, and easy to use generators to be developed. This article presents the modeling and experimental investigations into one method of producing such pulses-a spiral generator with a solid-state-thyristor-based input switch. It includes how the pulses are formed within the spiral, why a high-speed input switch is required, and how the geometry of the spiral dictates its output characteristics and the effects of different loads. Using thyristors, often connected in series to increase the operating voltage of the spiral, enables the spiral generators to have low jitter, high repetition rate, and long lifetime. Modeling of the circuit used a combination of telegraph equations to account for the wave propagation along the spiral and a lumped circuit exchanging charge between the spiral and the input switch and load. The model is verified by the detailed experimental results with the relative error being <; 10% in most cases. The output voltage pulse was often observed to have an initial peak of much lower magnitude than the subsequent peak(s)-which can only be fully explained by considering wave propagation effects. Lower input switch inductance, shorter switching time, larger mean diameter of the spiral, and increasing the width of the copper tape that makes up the spiral can all increase the voltage multiplication efficiency. Although increasing the number of turns that makes up the spiral can increase the output voltage, it can also lower the multiplication efficiency. By understanding the effects of different geometries, the spiral can be optimized to drive different loads-three applications of such spiral generators are then presented-pulses with 10 kV amplitude and 10 kHz repetition rate for driving dielectric barrier discharge plasma, pulses with amplitude of 10 kV and 10 kV/ns rising rate for triggering of advanced solid-sta

Journal article

Yan J, Parker S, Gheorghiu T, Schwartz N, Theocharous S, Bland SNet al., 2021, Miniature solid-state switched spiral generator for the cost effective, programmable triggering of large scale pulsed power accelerators, Physical Review Accelerators and Beams, Vol: 24, Pages: 1-10, ISSN: 2469-9888

This paper presents the design and testing of several different configurations of spiral generator, designed to trigger high current switches in the next generation of pulsed power devices. In particular, it details the development of spiral generators that utilize new ultrafast thyristor technology as an input switch, along with a polarity dependent output gap to improve the efficiency of the spiral generator design. The generator produced 50 kV from a 3.6 kV charging voltage, with a rise time of only 50 ns and a jitter of 1.3 ns—directly comparable, if not better than, a generator employing a triggered spark gap as the input switch. The output gap was constructed in house from commonly available components and a 3D printed case, and showed remarkable repeatability and stability—simple alterations to the output gap could further reduce the rise time. The entire spiral generator, along with control and charging electronics, fitted into a case only 210×145×33  mm.

Journal article

Johnson AS, Wood D, Austin DR, Brahms C, Gregory A, Holzner KB, Jarosch S, Larsen EW, Parker S, Struber C, Ye P, Tisch JWG, Marangos JPet al., 2018, Apparatus for soft x-ray table-top high harmonic generation, Review of Scientific Instruments, Vol: 89, ISSN: 0034-6748

There has been considerable recent interest in tabletop soft X-ray attosecond sources enabled by the new generation of intense, few-cycle laser sources at operating wavelengths longer than 800 nm. In our recent work [Johnson et al., Sci. Adv. 4(5), eaar3761 (2018)], we have demonstrated a new regime for the generation of X-ray attosecond pulses in the water window (284-540 eV) by high-harmonic generation, which resulted in soft X-ray fluxes of ≈109 photons/s and a maximum photon energy of 600 eV, an order of magnitude and 50 eV higher, respectively, than previously attained with few-cycle drivers. Here we present the key elements of our apparatus for the generation and detection of soft X-ray high harmonic radiation in the water window. Of critical importance is a differentially pumped gas target capable of supporting the multi-atmospheric pressures required to phase-match the high energy emission while strongly constraining the gas density, suppressing the effects of ionization and absorption outside the interaction region.

Journal article

Johnson AS, Austin DR, Wood DA, Brahms C, Gregory A, Holzner KB, Jarosch S, Larsen EW, Parker S, Struber CS, Ye P, Tisch JWG, Marangos JPet al., 2018, Correction for the Research Article: High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses, Science Advances, Vol: 4, ISSN: 2375-2548

Laser-driven high-harmonic generation provides the only demonstrated route to generating stable, tabletop attosecondx-ray pulses but has low flux compared to other x-ray technologies. We show that high-harmonic generation can producehigher photon energies and flux by using higher laser intensities than are typical, strongly ionizing the medium andcreating plasma that reshapes the driving laser field. We obtain high harmonics capable of supporting attosecondpulses up to photon energies of 600 eV and a photon flux inside the water window (284 to 540 eV) 10 times higherthan previous attosecond sources. We demonstrate that operating in this regime is key for attosecond pulse generation in the x-ray range and will become increasingly important as harmonic generation moves to even longerwavelength driving fields.

Journal article

Johnson A, Austin D, Wood D, Brahms M, Gregory A, Holzner K, Jarosch S, Larsen E, Parker S, Struber C, Ye P, Tisch J, Marangos JPet al., 2018, High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses, Science Advances, Vol: 4, ISSN: 2375-2548

Laser driven high harmonic generation provides the only demonstrated route to generatestable, tabletop attosecond X-ray pulses, but with low flux compared to other X-ray tech-nologies. Here we show that higher photon energies and flux can be obtained from highharmonic generation by using higher laser intensities than are typical, strongly ionizing themedium and creating plasma which reshapes the driving laser field. We obtain high harmon-ics capable of supporting attosecond pulses out to photon energies of 600 eV, and a photonflux inside the water window (284 eV to 540 eV) ten times higher than previous attosecondsources. We demonstrate that operating in this regime is key for attosecond pulse generationin the X-ray range, and will become increasingly important as harmonic generation movesto even longer wavelength driving fields.

Journal article

McGrath F, Johnson AS, Austin DR, Hawkins P, Wood D, Miseikis L, Simpson ER, Castillejo M, Torres R, Parker S, Siegel T, Marangos JPet al., 2017, An apparatus for quantitative high-harmonic generation spectroscopy in molecular vapours, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol: 88, ISSN: 0034-6748

Journal article

Cole JM, Wood JC, Lopes NC, Poder K, Abel RL, Alatabi S, Bryant JSJ, Jin A, Kneip S, Mecseki K, Parker S, Symes DR, Sandholzer MA, Mangles SPD, Najmudin Zet al., 2016, Tomography of human trabecular bone with a laser-wakefield driven x-ray source, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587

A laser-wakefield driven x-ray source is used for the radiography of human bone. The betatron motion of accelerated electrons generates x-rays which are hard (critical energy ${{E}_{\text{crit}}}>30$ keV), have small source size (<3 μm) and high average brightness. The x-rays are generated from a helium gas cell which is near-instantly replenishable, and thus the average photon flux is limited by the repetition rate of the driving laser rather than the breakdown of the x-ray source. A tomograph of a human bone sample was recorded with a resolution down to 50 μm. The photon flux was sufficiently high that a radiograph could be taken with each laser shot, and the fact that x-ray beams were produced on 97% of shots minimised failed shots and facilitated full micro-computed tomography in a reasonable time scale of several hours, limited only by the laser repetition rate. The x-ray imaging beamline length (not including the laser) is shorter than that of a synchrotron source due to the high accelerating fields and small source size. Hence this interesting laboratory-based source may one day bridge the gap between small microfocus x-ray tubes and large synchrotron facilities.

Journal article

Yu X, Zhiwen Y, Houston CM, Zecharia AY, Ma Y, Zhang Z, Uygun DS, Parker S, Vyssotski AL, Yustos Y, Franks NP, Brickley SG, Wisden Wet al., 2015, Wakefulness is governed by GABA and histamine co-transmission, Neuron, Vol: 87, Pages: 164-178, ISSN: 0896-6273

Histaminergic neurons in the tuberomammilary nucleus (TMN) of the hypothalamus form a widely projecting, wake-active network that sustains arousal. Yet most histaminergic neurons contain GABA. Selective siRNA knockdown of the vesicular GABA transporter (vgat, SLC32A1) in histaminergic neurons produced hyperactive mice with an exceptional amount of sustained wakefulness. Ablation of the vgat gene throughout the TMN further sharpened this phenotype. Optogenetic stimulation in the caudate-putamen and neocortex of “histaminergic” axonal projections from the TMN evoked tonic (extrasynaptic) GABAA receptor Cl− currents onto medium spiny neurons and pyramidal neurons. These currents were abolished following vgat gene removal from the TMN area. Thus wake-active histaminergic neurons generate a paracrine GABAergic signal that serves to provide a brake on overactivation from histamine, but could also increase the precision of neocortical processing. The long range of histamine-GABA axonal projections suggests that extrasynaptic inhibition will be coordinated over large neocortical and striatal areas.

Journal article

Price CJ, Donnelly TD, Giltrap S, Stuart NH, Parker S, Patankar S, Lowe HF, Drew D, Gumbrell ET, Smith RAet al., 2015, An in-vacuo optical levitation trap for high-intensity laser interaction experiments with isolated microtargets, Review of Scientific Instruments, Vol: 86, ISSN: 0034-6748

We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments. The system uses a focused, vertically propagating continuous wave laser beam to capture and manipulate micro-targets by photon momentum transfer at much longer working distances than commonly used by optical tweezer systems. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution. Optical trapping of 10 ± 4 μm oil droplets in vacuum was demonstrated, over timescales of >1 h at extended distances of ∼40 mm from the final focusing optic. The stability of the levitated droplet was such that it would stay in alignment with a ∼7 μm irradiating beam focal spot for up to 5 min without the need for re-adjustment. The performance of the trap was assessed in a series of high-intensity (1017 W cm−2) laser experiments that measured the X-ray source size and inferred free-electron temperature of a single isolated droplet target, along with a measurement of the emitted radio-frequency pulse. These initial tests demonstrated the use of optically levitated microdroplets as a robust target platform for further high-intensity laser interaction and point source studies.

Journal article

Baker R, Gent TC, Yang Q, Parker S, Vyssotski AL, Wisden W, Brickley SG, Franks NPet al., 2014, Altered activity in the central medial thalamus precedes changes in the neocortex during transitions into both sleep and propofol anesthesia, The Journal of Neuroscience, Vol: 34, Pages: 13326-13335, ISSN: 0270-6474

How general anesthetics cause loss of consciousness is unknown. Some evidence points toward effects on the neocortex causing “top-down” inhibition, whereas other findings suggest that these drugs act via subcortical mechanisms, possibly selectively stimulating networks promoting natural sleep. To determine whether some neuronal circuits are affected before others, we used Morlet wavelet analysis to obtain high temporal resolution in the time-varying power spectra of local field potentials recorded simultaneously in discrete brain regions at natural sleep onset and during anesthetic-induced loss of righting reflex in rats. Although we observed changes in the local field potentials that were anesthetic-specific, there were some common changes in high-frequency (20–40 Hz) oscillations (reductions in frequency and increases in power) that could be detected at, or before, sleep onset and anesthetic-induced loss of righting reflex. For propofol and natural sleep, these changes occur first in the thalamus before changes could be detected in the neocortex. With dexmedetomidine, the changes occurred simultaneously in the thalamus and neocortex. In addition, the phase relationships between the low-frequency (1–4 Hz) oscillations in thalamic nuclei and neocortical areas are essentially the same for natural sleep and following dexmedetomidine administration, but a sudden change in phase, attributable to an effect in the central medial thalamus, occurs at the point of dexmedetomidine loss of righting reflex. Our data are consistent with the central medial thalamus acting as a key hub through which general anesthesia and natural sleep are initiated.

Journal article

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