Congratulations

Cyd Cowley

Congratulations to Cyd Cowley a York CDT Student who successfully defended his thesis at viva. Cyd's thesis is entitled "Modelling of Alternative Divertor Power Exhaust", his supervisor is Prof Bruce Lipschultz and David Moulton (UKAEA).

Robert Davies

Congratulations to Robert Davies a York CDT Student who successfully defended his thesis at viva. Bob's thesis is entitled "Kinetic ballooning mode studies and the treatment of electromagnetic microinstabilities and turbulence in complex geometry", his supervisor is Dr David Dickinson.

Magnetically confined thermonuclear fusion is promising as a future power source. However, the viability of fusion power plants is strongly influenced by how well the thermal energy can be confined in the plasma fuel. Often, the dominant process governing confinement is microinstability-driven plasma turbulence. This thesis studies microinstabilities and turbulence using gyrokinetic simulations, which may ultimately inform fusion reactor design and operation.

The effect of plasma triangularity on stability in spherical tokamaks (STs) is examined using linear simulations of hypothetical ST equilibria. It is found that the kinetic ballooning mode (KBM), an electromagnetic pressure-driven microinstability, likely prohibits negative triangularity in ST power plants, since negative triangularity closes the “second stability window” for n = ∞ ideal magne-
tohydrodynamic (MHD) ballooning modes. ST equilibria with positive triangularity can access the second stability window, although remain weakly unstable to KBMs.

Secondly, microinstabilities are studied for the optimised stellarator Wendelstein 7-X (W7-X). Electrostatic “stability valley” results are reproduced using stella: a gyrokinetic code which offers flexibility in time-marching schemes by using operator splitting. stella is extended to include Ak and Bk fluctuations linearly using both implicit and explicit schemes. Benchmarking against the code GS2 shows good agreement for electromagnetic tokamak simulations. Using this implementation, the W7-X stability valley at finite β (=plasma pressure/magnetic pressure) is preliminarily explored. The electromagnetic instabilities observed may be relevant to future W7-X experiments.

Finally, a non-interpolating semi-Lagrangian scheme, aiming to efficiently simulate electromag- netic turbulence by eliminating the Courant-Freidrichs-Lewy timestep constraint in nonlinear gyrokinetics, is implemented in stella. A new operator splitting scheme is developed and used to mix single and multi-step numerical methods. Unfortunately, nonlinear tests show low accuracy and cur-
rently unexplained numerical instability. Elucidating the reasons for this would be an interesting area of future research.

In 2023 I'll be starting a postdoc at the Max-Planck Institute for Plasma physics in Greifswald, Germany. Greifswald is home to Wendelstein 7-X, the world's largest stellarator, and I'll be using theory and simulations to find how to optimise stellarators for commercial fusion!

Harry Dudding

Congratulations to Harry Dudding a York CDT Student who successfully defended his thesis at viva. Harry's thesis is entitled "A new quasilinear saturation rule for tokamak turbulence", his supervisors are Dr Frances Casson and David Dickinson

Harry has now moved on to working as a postdoc at CCFE, continuing his research in reduced models of tokamak turbulence. This is to investigate the limitations of their applicability to next-generation tokamaks, and develop improvements.

Michael Mo

Congratulations to Michael Mo a York CDT student who successfully defended his thesis at viva. Michael's thesis is entitled Atomic oxygen densities in pulsed inductively coupled plasmas: Laser spectroscopy & Energy Resolved Actinometry, his supervisors is Timo Gans, Deborah O'Connell and James Dedrick.

Matthew Khan

Congratulations to Matthew Khan a York CDT student who successfully defended his thesis at viva. Matthew's thesis is entitled "Advanced Direct Drive Shock Ignition Studies", his supervisor is Professor Nigel Woolsey.  An abstract from Matthew's thesis is below.

My thesis was focused on inertial confinement fusion (ICF) and the direct drive variant of shock ignition. ICF implosions aim to compress and heat a capsule of fusion fuel to form a central hot spot that ignites a surrounding layer of dense fuel. Shock ignition aims to decouple the assembly and ignition of the fuel to overcome some of the hydrodynamic instabilities that have a detrimental effect on implosions. It achieves this by using a low intensity drive to compress the fuel and prime it for ignition, followed by a high intensity spike that launches a strong shock. This strong shock provides the necessary energy to push the hot spot over the threshold for ignition. My thesis reported on two experiments performed on the Omega-60 laser facility. The first investigated the coupling of the high intensity spike to the target, finding that the convective Raman scattering parametric instability was driven which generated suprathermal electrons with an associated temperature of ~40 keV and a conversion efficiency of ~2%. The second experiment investigated the implosion dynamics of warm deuterium filled capsules using shaped laser pulses that minimised the increase of internal energy within the shell. A laser drive multiplier was tuned with trajectory measurements from a gated self-emission imager, a significant advancement in the ability to more accurately simulate implosion designs of this sort that are relevant for both shock ignition fuel and conventional central hot spot implosions. Significant deviations from a spherical implosion were observed and characterised.

Matthew will be continuing to work within fusion and the YPI, starting a Post Doc studying the energy transport to the divertor of STEP like tokamaks.

Steven Thomas

Congratulations to Steven Thomas a York CDT student who successfully defended his thesis at viva. Steven's thesis is entitled  "The relationship between scrape-off layer filaments and density profiles in ASDEX Upgrade" and his supervisors are  Istvan Cziegler and Ben Dudson.  

Steven has now gone on to working as a post-doctoral research associate for the University of York, based at the Culham centre for fusion energy. He am the responsible officer for the beam emission spectroscopy (BES) diagnostic, where he will be studying ion-scale turbulence in MAST-U." 

Samuel Ward

Eduardo Solis Meza

Congratulations to Eduardo Solis Meza a York CDT student who successfully defended his thesis at viva. Eduardo's thesis is entitled 'Study of extreme ultraviolet laser ablation for fusion applications' and his supervisors are Erik Waganeers and Greg Tallents. An abstract from Eduardo's thesis is below.

Stuart Morris

Congratulations to Stuart Morris a York student who successfully defended his thesis at viva. Stuart's thesis is entitled 'High energy X-ray production in laser-solid interactions' and his supervisors are Chris Ridgers and Gavin Crow. An abstract from Stuart's thesis is below.

The radiation produced in laser-solid interactions has been characterised at intensities relevant to petawatt-class short-pulse lasers (1e20 to 1e22 W/cm2). Particular attention was paid to photons of energy over 1 MeV which emerge from synchrotron radiation in the laser focal spot, and bremsstrahlung radiation as hot electrons traverse the solid. Bremsstrahlung modelling was performed using a novel hybrid-PIC code, where it was found the emission could last on the order of 10-100 ps, and was far more efficient than was thought from previous simulation campaigns (up to 7.4% laser-to-X-ray efficiency). Despite the strong bremsstrahlung emission, specific target geometries were found in which synchrotron radiation could dominate even for petawatt class lasers. These results suggest laser-solid interactions could provide a good source of multi-MeV X-rays, and that synchrotron radiation could provide a useful diagnostic for electron motion in the laser focal spot.

Currently working on a 1-year post-doc in Warwick, where I'm doing EPOCH code development to help model shock-ignition, and to update the ionisation packages. I have offers for more post-doc work when this funding runs out in May.

Joe Allen

Congratulations to Joe Allen, a York Fusion CDT student who successfully defended his thesis at viva on 30 June 2021. Joe's thesis is entitled 'Design of the Synthetic Aperture Microwave Imager-2 for measurement of the edge current density on MAST-U' and his supervisor is Professor Roddy Vann. An abstract from Joe's thesis is below.

The Synthetic Aperture Microwave Imager-2 (SAMI-2) is a 2D Doppler backscattering (DBS) diagnostic designed for multiple high quality simultaneous measurements of the edge pitch angle on the Mega-Ampere Spherical Tokamak Upgrade (MAST-U). The specification, design and testing of the SAMI2 microwave front end, predominantly antennas and mixer circuitry, are described in this thesis. A radial profile of the edge pitch angle enables calculation of the edge current density, a difficult quantity to measure, which is valuable for validation of models and understanding of pedestal dynamics and edge plasma instabilities, eg ELMs. In active probing mode, the SAMI-2 diagnostic is designed to make the first measurements of the edge current density by a DBS diagnostic. In passive mode, SAMI-2 will measure Bernstein wave mode conversion, to inform spherical tokamak microwave heating systems  

Bhavin Patel

Congratulations to Bhavin Patel, a York Fusion CDT student who successfully defended his thesis at viva on 25 March 2021. Bhavin's thesis is entitled 'Confinement physics of a steady state net electric burning spherical tokamak' and his supervisor is Professor Howard Wilson & Dr David Dickinson. An abstract from Bhavin's thesis is below.

This work examines the feasibility of a spherical tokamak capable of generating net electricity, which was called BurST for Burning Spherical Tokamak. Different steady state scenarios were examined and areas of operating space were ruled out by different operational limits such that a baseline scenario at 1GW was determined. The feasibility of driving the auxiliary current using neutral beam injection was examined and a suitable configuration was designed such that all the auxiliary power was driven with 94MW. Next the linear micro-stability was examined to understand which types of turbulence will be prevalent in BurST like plasmas. Kinetic ballooning modes (KBMs) and micro-tearing modes (MTMs) were found to the dominant modes. Flow shear was able to wipe out the KBMs and certain MTMs but other MTMs were resilient against it. However, these MTMs were stabilised by the density gradient such that a new equilibrium was designed with a more peaked density profile. This equilibrium was marginally stable to all the modes found, suggesting operating at neoclassical levels of transport may be possible. Finally, the validity of the TGLF quasi-linear model was examined in a BurST regime. It was not possible to obtained converged nonlinear BurST simulations as the MTMs found required multi-scale resolution, but the the TGLF model was tested in progressively more challenging regimes. In low beta ITG regimes, the quasi-linear approximation was found to be valid, but the saturation rule needs development in a high aspect ratio tokamaks. For electromagnetic turbulence like MTMs, it was found the the quasi-linear approximation needs further development.

Philip Bradford

Congratulations to Philip Bradford, a York Fusion CDT student who successfully defended his thesis at viva on 2 March 2021. Philip's thesis is entitled "Laser-driven discharges and electromagnetic fields" and his supervisor is Professor Nigel Woolsey. An abstract from Philip's thesis is below.

"When high intensity lasers interact with solid targets, hot electrons are produced that can exit the material and leave behind a positive electric charge. As this accumulated charge is neutralised by a cold return current, radiation is emitted with characteristics dependent on laser and target properties. This thesis examines how electromagnetic radiation is emitted in experiments with long-pulse and short-pulse lasers.

Radiofrequency electromagnetic pulses emitted during ps-duration laser inter-actions can disrupt scientific measurements and damage electronic equipment close to the target. A study of electromagnetic pulses produced by the Vulcan laser is presented. Strong fields exceeding 100 kV/m and 0.1 mT were measured 1.5 m from the target using conducting probes. Scaling of the EMP field with laser and target parameters shows qualitative agreement with target charging models. A novel EMP mitigation scheme is presented using a dielectric spiral target holder. Experimental results are used to benchmark a frequency-domain dipole antenna model of EMP emission that connects charging physics to EMP fields measured at an arbitrary distance from the target.

In a separate experiment, coil targets were driven with three ns laser beams from the Vulcan laser, generating multi-tesla quasi-static magnetic fields. Dual-axis proton deflectometry was used to measure electric and magnetic fields around the coils. Results suggest that wire electric fields of order 0.1 GV/m develop on a 100 ps timescale. Maximum currents of 10 kA were observed towards the end of the laser drive for 1 mm- and 2 mm-diameter loop targets, corresponding to an axial magnetic field of B ≈ 12 T in the 1 mm loops. Deflectometry results agree well with a plasma diode model, whereas B-dot probe measurements of the magnetic field were approximately 10× larger. Analytic and computational modelling of charged particle motion in electric and magnetic fields is presented. Prospects for an all-optical platform for magnetized high energy density physics experiments are discussed."

Chris Underwood

Congratulations to Chris Underwood, a York Fusion CDT student who successfully defended his thesis at viva on 18 December 2020. Chris's thesis is entitled "Optimising Production of High Energy Radiation using Laser Wakefield Acceleration" and his supervisor is Dr Chris Murphy. An abstract from Chris's thesis is below.

This thesis presents work from experiments and simulations on characterising and optimising electron and X-ray sources created using laser wakefield accelerators.

The flux, critical energy of the spectrum, source size and divergence of a laser wakefield accelerator driven bremsstrahlung X-ray source were characterised experimentally and through simulation. The source had the highest energy photon spectrum measured from a laser wakefield bremsstrahlung source, with critical energies over 100 MeV. The source is shown to be tunable over the measured characteristics with changes in the plasma density and converter parameters.

A 5% energy spread 1.2 GeV electron beam was experimentally created using a density profile injection mechanism, the highest energy recorded using this injection technique. The density profile formation was investigated using fluid simulations, and the effect of the profile on electron generation was explored using particle-in-cell simulations.

Optimisation of electron beams using a machine learning technique was deployed experimentally in the form of a Bayesian optimisation algorithm. The algorithm was shown to be an effective method of finding a global optimum, and for creating electron beams with different characteristics. Comparisons between optimising the total beam energy and optimising for the energy in a narrow divergence were made. This difference in optimal position in parameter space was shown to be based on pulse evolution. This approach found the global optimum, in a four input parameter space for accelerated charge, in 􏰁< 20 data points. This efficient optimisation of a laser wakefield accelerator will increase the usable time of future devices using this approach.

A new design for a high repetition rate plasma mirror was characterised. The mirror was created from a flowing liquid which refreshes the surface at a rate suitable for operation at ∼1 kHz, compared with other liquid based plasma mirrors operating at ∼1 Hz. The injection of subsequent laser pulses into a staged wakefield accelerator operating at high repetition rate could be achieved with this plasma mirror

Joshua Boothroyd

Congratulations to Joshua Boothroyd, a York Fusion PhD student who successfully defended his thesis at viva on 14 December 2020. Josh's thesis is entitled " Efficient Generation of Atomic Chlorine by a Low-temperature Plasma and Application to Atmospheric Chemistry" and his supervisor is Professor Timo Gans. An abstract from Josh's thesis is below.

Well done Josh!

Short-lived reactive species, such as hydroxyl (OH) and atomic chlorine (Cl) radicals, play a crucial role in atmospheric self-regulation and low temperature plasma applications. Direct measurements of radical concentration and reactivity (loss rate) are challenging in the atmosphere. Indirect techniques have

been shown to be of value, notably for OH reactivity measurements. Low-temperature plasmas have potential as efficient sources of radicals at atmospheric pressure for use in these indirect techniques.

In this work, atomic chlorine generated by a capacitively coupled, radio-frequency driven plasma was applied to a competitive reactivity method for measuring the reactivity of atomic chlorine in a gas sample. Argon with a small admixture (0.04{0.096%) of molecular chlorine was used as the plasma feed gas.

Proton transfer reaction mass spectrometry (PTR-MS) was used to indirectly quantify the reactive species downstream of the plasma through adding volatile organic compounds to the plasma effluent and monitoring the resulting mixture. Optical emission spectroscopy of the plasma identified humid air impurities (through OH and N2 rotational band emission). The loss rate of atomic chlorine in a mixture of toluene/isoprene, using diethyl-ether as the reference was also attempted for the first time.

Chen Geng

Congratulations to Chen Geng, a York Fusion PhD student who successfully defending his thesis at viva in November 2020. Chen's thesis is entitled "the Physics of a Collisionless Micro-scale Tearing Mode" and his supervisors are Professor Howard Wilson and Dr David Dickinson. An Abstract from Chen's thesis is below.

Microtearing modes and electron temperature gradient modes are two types of microinstability that are driven by the electron temperature gradient in magnetised plasmas. Both have been widely studied and are well known respectively as an electromagnetic tearing parity mode and an electrostatic twisting parity mode. Microtearing modes, as the tearing parity one, can cause fine scale reconnection of the magnetic field lines in the vicinity of rational flux surfaces. This leads to formation of magnetic islands, which increases the heat and particle flux across the magnetic confinement devices. Microtearing modes therefore are considered as a candidate for anomalous electron heat transport in tokamak plasmas.

Gyrokinetic theories are used in modelling the physics drive mechanism and the stability of micro-scale modes. Early theories for microtearing modes in slab geometry concludes that the drive mechanism of this mode requires a finite collision frequency; thus it is stabilised at low collision frequencies. However, we find in linear gyrokinetic simulations that a fine scale tearing parity instability, driven also by the electron temperature gradients, persists even in the collisionless or electrostatic limit. We demonstrate that this mode has a much larger radial wavenumber than the binormal one and poses numerical challenges to resolve in simulations. The mode growth rate is sensitive to electron finite Larmor radius effects, which are often neglected in previous studies. We develop two linear analytic gyrokinetic models to identify that this collisionless tearing parity mode is consistent with a higher harmonic of the electron temperature gradient mode, which becomes more unstable than the conventional twisting eigenmode under the parameter range in this thesis.

When including electromagnetic fields, this mode is capable of forming magnetic islands even in the absence of collisions. Our study provides an example that tearing parity micro-instabilities can arise from various physics drives. This result brings up thoughts for further studies on turbulent transport in magnetised plasmas.

Will Trickey

Congratulations to Will Trickey, a York Fusion CDT student who successfully defended his thesis at viva in November 2020. Will’s thesis is entitled “Novel Approaches to Indirect Inertial Confinement Fusion” and his supervisor is Dr John Pasley. An abstract from Will’s thesis is below.

Well done Will!

This thesis describes work that developed new techniques towards indirect drive inertial confinement fusion. The work predominantly used the 1-dimensional (1D) and 2-dimensional (2D) versions of the radiation hydrodynamics HYADES. 

The scaling of ablation pressures produced by irradiation of soft X-rays was investigated. Materials with atomic numbers between 3.5 and 22 were irradiated by X-ray sources with radiation temperatures ranging from 100 eV to 400 eV. For each material, pressure scaling laws were determined as a function of temperature and time. Additionally, the maximum drive temperature for subsonic ablation was found for all the materials. Materials with high atomic number tend to have weaker pressure scaling but higher maximum subsonic drive temperatures.

The next study found the laser drive parameters required to produce shock-ignition-like pressures through indirect drive. First, 1D simulations found an X-ray drive profile that is capable of producing shock-ignition-like pressures in a beryllium target. From there, 2D simulations were carried out to simulate the laser to X-ray conversion in a hohlraum. A laser drive profile was found that was capable of producing the required X-ray intensity profile. 

The final piece of work developed a new technique for controlling the X-ray flux inside hohlraums using burn-through barriers. Hohlraum designs that use multiple chambers separated by burn-through barriers were proposed. The burn-through barriers are used to modulate the spatial and temporal properties of the X-rays as they flow between the chambers. It is shown how a number of different barrier designs can be used to manipulate the properties of the X-rays in both time and space.