Faculty of Natural SciencesDepartment of Physics

Electronics Workshop Technician

//

### Contact

+44 (0)20 7594 7886s.parker

//

### Location

003Huxley BuildingSouth Kensington Campus

//

## Publications

Publication Type
Year
to

10 results found

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

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

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00577590&limit=30&person=true