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Student Seminars - December 2023

Friday 8 December 2023, 1.00PM to 2 PM

Speaker(s): Jaka Olenik

Jaka Olenik

“Innovative Control and Prediction of Cold Atmospheric Pressure Plasma Jets”

Non-equilibrium or cold atmospheric-pressure plasma (CAP) has emerged as a crucial tool in modern science and engineering, offering unique advantages for various applications due to its non-equilibrium, ionised gas state at near-room temperatures. Central to the CAP field is the plasma jet, a focused stream of plasma that can be directed at specific targets or materials. This plasma jet technology distinguishes itself by enabling plasma-material interactions at low temperatures, making it ideal for applications ranging from materials processing to biomedical treatments.
The applications of plasma jets are diverse and impactful. In materials science, plasma jets are employed for surface modification, thin-film deposition, and nanomaterial synthesis, enhancing properties like adhesion, wettability, and corrosion resistance. In the biomedical field, they find utility in sterilisation, wound healing, and cancer treatment, where their ability to generate reactive species and electric fields at low temperatures is crucial. These applications underscore the plasma jet's versatility and its capacity to induce desired physical and chemical changes in a controlled manner.
Despite their numerous benefits, the operation of CAP jets encounters significant obstacles that impact their efficiency and effectiveness. These challenges include inconsistencies in performance from one operation to the next (run-to-run variability), gradual changes over extended periods (long timescale drifts), marked variations in temperature and concentrations of reactive species within the plasma field (sharp spatial gradients), and a high degree of sensitivity to external factors such as variations in the distance between the jet tip and the substrate, the electrical properties of the substrate, and ambient humidity. These operational difficulties constitute a major hindrance to realizing the full potential of CAP jet technology.
Addressing these challenges, recent advancements have focused on the live sensing and control of operational parameters. Real-time monitoring and adjustment of voltage, frequency, distance and gas flow rates are critical for maintaining plasma jet stability and ensuring consistent performance. This approach allows for immediate response to any deviations, thereby enhancing the reliability and effectiveness of plasma jet applications.
Moreover, the integration of machine learning techniques, particularly Random Forest algorithms, has shown promise in predicting plasma behaviour. By analysing Optical Emission Spectroscopy (OES) data along with environmental factors such as temperature and humidity, these algorithms can forecast plasma jet characteristics with high accuracy. This predictive capability is instrumental in pre-empting and mitigating instabilities, thus ensuring optimal operation of the plasma jet under varying conditions.
In conclusion, low-temperature plasma jets represent a transformative technology with wide-ranging applications. The ability to control and predict their behaviour through advanced sensing, feedback control systems, and machine learning not only mitigates inherent instabilities but also opens new avenues for application and research. As this field continues to evolve, the intersection of plasma physics, control engineering, and data science is expected to yield further innovations, enhancing the utility and impact of
plasma jet technology in various domains.

Location: Debye Lecture Theatre and Zoom

Post Doc Seminars - December 2023

Tuesday 5 December 2023, 3.00PM to 4 PM

Speaker(s): Matthew Khan

Matthew Khan

The Role of 1D Simulations in STEP Divertor Development

Reactor class tokmaks face a considerable challenge in how to manage exhausted heat from the core. Exceedingly large power fluxes cross the closed field lines of the core, and pass to the scrape-off layer (SOL). This power is rapidly transport down the SOL and towards the of the divertor where, if left unchecked, would readily destroy the plasma facing components.
To overcome this, the divertor is induced into a detached state, where the plasma is radiatively cooled significantly to the point of recombination, via the introduction of high-Z impurities. The plasma detaches from the divertor surface, and a neutral cloud of gas forms behind it, acting as both an energy sink and a momentum buffer.
It is crucial to study how this detached plasma behaves with changes in power, as large spikes in power, from edge-localised mode events (ELMs) for example, have the potential to burn through this neutral cloud and damage the divertor. Additionally, intentionally applied power oscillations are thought to aid in plasma control for double-null configurations.
1D simulations present an opportunity to perform an in depth study key phenomena such as this, typically unobtainable with 2d or 3D simulations, as they require reduced complexity in their design, and far less computing power to run. With this come the unique opportunity to investigate the transient power events described above, as the CVODE numerical solver can be used that has improved temporal accuracy at the cost of slower computation.
However, various techniques must be implemented in order to overcome the higher dimension effects that are lacking in a 1D model. The entirely applicability of 1D simulations must be questioned if key physics cannot be replicated, or if the results differ signifacntly from similar 2D simulations.
The work presented here describes the issues faced by 1D simulation, methods to overcome them, and the final set up for SOL simulations of transient events. This is used to investigate the movement of the detachment front with applied power oscillations, where it is found that the position of the front can differ significantly from the position of the same constant mean power.

Location: Debye lecture Theatre & Zoom

Student Seminars - November 2023

Friday 17 November 2023, 3.00PM to 4 PM

Speaker(s): Kittawat Poonsawat and Jidchanok Wangkanai

Kittawat Poonsawat

“Development of actinometry methods for measuring atomic oxygen in plasma jets” 

Atmospheric pressure plasma is of interest. It can be produced in open air without a vacuum system unlike in low pressure. Atmospheric pressure plasma jets (APPJs) are a kind of plasma sources to generate this kind of plasma. APPJs can be easily handled and adjusted. Many of the applications of APPJs, the plasma treatment of pressure- and heat-sensitive materials as well as plasma medicine, rely on providing reactive species like atomic oxygen and/or nitrogen species (RONS) toward a target directly while the gas temperature is still near room temperature.
Reactive oxygen species (ROS) from oxygen-containing plasma play an important role in the plasma treatment. They provide polar functional groups, change the surface morphology, and make the surface become more hydrophilic. Furthermore, oxygen-containing plasma is used for the inactivation of microorganisms and plasma medicine to stimulate tissue regeneration and control the death of cells. The effectiveness of any plasma treatment is related to the mix of reactive species delivered to a substrate. Therefore, the absolute goal is knowing and controlling this mix of species, but complicated because of various interactions and different ROS that exist. Nonetheless, there are many efforts in trying to measure ROS as well as model them, one of which is atomic oxygen (O). Not only does atomic oxygen have its own oxidative properties, but it also contributes as a precursor to the formation and decomposition of a variety of long-lived molecules, including ozone. As a result, the determination of atomic oxygen density is critical.
A standard approach to determine atomic oxygen density is two-photon absorption laser-induced fluorescence (TALIF). However, TALIF requires a specific laser system, making it complex to operate, unsuitable for some industrial plasma setups as well as having the expensive cost. Therefore, this work is focusing on developing the actinometry method which is an easier one than TALIF to become more reliable and widely applicable. The actinometry method has been developed over years from classical actinometry, extended classical actinometry, energy resolved actinometry (ERA) to state enhanced actinometry (SEA). For this work, it adopts the SEA method by considering He (3 3 S) instead of O (3p 5 P) for using optical emission spectroscopy (OES : time and space averaged spectroscopy). Moreover, the effect of cascading emission on atomic oxygen density is investigated in this work by adding the cascading emission term in analytic models. The atomic oxygen density from this work will be compared with that from TALIF.

Jidchanok Wangkanai

Controlling Selective Formation of Free Radicals in Cold Atmospheric Pressure Plasmas

Nonthermal plasmas are a rich source for reactive species including free electrons, positive and negative ions, excited atoms and molecules as well as radical atoms and molecules. Reactive species inside the plasmas are essential not only in inducing the reaction
mechanism but also in having a direct impact on the applications. However, nonthermal plasmas are complex with many reactions from various species including a mixture of radical atoms and molecules and the effects between the plasma and the surface cannot be well-
controlled. Controllable radical fluxes can be useful for enhancing the efficiency of plasma applications. In this work, the plasmas are generated by an atmospheric pressure plasma jet in order to study the plasma effects mainly from radical species. The initial experiment is operating the plasma in an admixture with water vapour (H2O) that specific radicals can ensure to be produced. The radical species namely hydroxyl radicals (OH) will be characterised using spin trapping coupled with analytical techniques by mass spectrometry (MS). The spin trapping can harvest specific short-lived radicals from the plasmas to form more stable species that can be detected. The study will be useful to understand the fundamentals of radical formation as well as radicals and surface interactions.

Location: Debye Lecture Theatre - Zoom

Student Seminars - October 2023

Friday 27 October 2023, 12.00PM to 1PM

Speaker(s): Deborah Selemon, Ethan Attwood and Lloyd Baker

Deborah Selemon

Studying plasma turbulence at MAST-U with SAMI and GS2

Turbulence is the dominant mechanism for heat loss in tokamaks and under-
standing it is critical to the successful operation of a tokamak reactor. Injected microwaves at sufficiently low-frequency bounce off the plasma edge at a density
related to the wave frequency. The underlying theory to be proved by this study is that comparing the polarisation of the back-scattered microwaves to the polarisation of the injected beam can be used to determine the extent to which the scattering turbulence is electromagnetic or electrostatic - thereby discriminating between two different models for edge turbulence. The study involves a mix of modeling with GS2 and experimental methods using a microwave imaging system called the Synthetic Aperture Microwave Imager (SAMI).

GS2 is a high-performance computing code that solves the gyrokinetic equation by using the δf approach to describe the evolution of turbulence  SAMI is a 2D Doppler back-scattering (DBS) diagnostic designed for multiple high-quality simultaneous measurements of the edge pitch angle on the Mega-Ampere Spherical Tokamak Upgrade (MAST-U). It is a microwave imaging system that uses a phased array of dual-polarized antennas. This dual-polarization feature of SAMI’s antennas is what enables the probing and characterization of the plasma turbulence. The diagnostic operates in 2 modes simultaneously: ’passive’ imaging of plasma emission and ’active’ imaging of the back-scattered signal.
This project will use GS2 to study the feasibility of the underlying theory; the computational model will then be validated experimentally by using SAMI to diagnose and characterize the turbulence in the edge of the UK’s MAST-U tokamak.

Ethan Attwood

Particle-In-Cell for Exascale Computing: Investigations into Ion Cyclotron Emission with Uncertainty Quantification

The edge region of tokamak devices is a crucial part of the plasma, as it includes the exhaust and essentially sets the boundary conditions for the whole device. Unfortunately it is also complex, often requiring multiple models to capture the physics of the bulk plasma, ionised and non-fully ionised impurities, and neutral species, all of which can interact over many spatial and time scales. These different models require different numerical techniques to simulate, for example particle-in-cell (PIC) for the charged impurity species and finite elements (FEM) for the bulk plasma. High fidelity simulations that can be used to inform the design of reactors also need uncertainty quantification (UQ). UQ is often computationally expensive, as it requires many simulations to cover the parameter space, but as we move into the exascale era this is becoming more feasible. The next generation of UK supercomputing is led by the ExCALIBUR ( project, through which a novel code (Neptune Exploratory SOftware – NESO) is being developed at the Culham Centre for Fusion Energy.
The main objective of this project is to investigate Ion Cyclotron Emission (ICE) through state of the art FEM-PIC simulations in a UQ framework that can be run on exascale machines. As an ubiquitous plasma phenomenon, ICE presents a potential passive diagnostic for fast ion characteristics in toroidal fusion plasmas with a high technical readiness level. This requires the development of algorithms for FEM-PIC coupling, efficient off-loading onto GPUs, and integrating the FEM-PIC software into a UQ framework.

Lloyd Baker

Transient modelling and control in scrape off layer plasmas Extremely high heat flux is a major challenge in the exhaust of magnetically confined fusion devices. Computational simulations offer valuable insights, and improve our understanding of physical
processes, such as parallel heat flux and detachment in the ‘scrape-off layer’ (SOL). Fluid models are widely used to efficiently simulate the complex conditions of the SOL, often assuming a highly collisional plasma. However, the divertor region will be subject to transient events and consists of multiple ion and neutral species. These conditions can alter collisionality and introduce high temperature gradients which can cause classical Spitzer-Härm heat flux to become unphysically large. To address this, it is commonplace to employ simplistic global flux limiters to constrain the heat flux to some fraction (alpha) of the free streaming limit. The choice of alpha is critical to the simulated temperature at the divertor target and hence to assessment of the exhaust system.
Hermes-3 is a recent code capable of simulating multiple species. This project first uses Hermes-3 to assess the effect of a range of alpha values on temperature at the divertor targets. The code will then be used in the development and validation of a model to predict the evolution of vertical instabilities within a double null tokamak using divertor power balance for application in plasma control.


Location: Debye Lecture Theatre and Zoom

Post Doc Seminars - July 2023

Friday 28 July 2023, 2.00PM to 3 PM

Speaker(s): Liam Pattinson

Liam Pattinson 

Abstract to follow...

Location: Alfven Room and via Zoom

Student Seminars - July 2023

Friday 21 July 2023, 2.00PM to 3 PM

Speaker(s): Yuyao Wang, Charlie Nicholls, Mark Turner and Lawrence Dior

Yuyao Wan

Abstract to follow...

Charlie Nicholls

Abstract to follow...

Mark Turner

Abstract to follow...

Lawrence Dior

Abstract to follow...

Location: Debye Lecture Theatre and Zoom

Post Doc Seminars - June 2023

Friday 30 June 2023, 2.00PM to 3 PM

Speaker(s): Maurizio Giacomin

Maurizio Giacomin

Theory based scaling law of the L-mode and H-mode tokamak density limits and experimental validation

Among the various limits that restraint the operational space of tokamaks, the L-mode density limit represents the maximum plasma density that can be achieved in magnetic fusion devices before the plasma develops a strong magnetohydrodynamic activity that leads to confinement degradation or even a disruption. Experimental observations have pointed out an important role played by edge physics on setting the maximum achievable density. Motivated by experimental evidence, flux-driven, two-fluid electromagnetic turbulent simulations have been carried out by using the GBS code to identify the main parameters controlling turbulent transport in the tokamak boundary, addressing, in particular, the transition to a regime of catastrophically large turbulent transport, which is associated with a density limit crossing. By leveraging these simulation results, a theory-based scaling law describing the density limit has been derived and successfully compared to simulation results. The density limit scaling has been validated against experimental measurements taken within a multi-machine effort involving the ASDEX Upgrade, JET and TCV tokamaks. While the L-mode density limit scaling shares with the widely used Greenwald density limit scaling the main dependence on the plasma current and tokamak minor radius, it also shows a significant dependence on the power crossing the separatrix. This additional dependence has important implications for the design and operation of future magnetic confinement fusion devices, where the power crossing the separatrix is expected to be significantly larger than in present-day tokamaks.

Location: Debye Lecture Theatre & Zoom

Student Seminars - June 2023

Friday 16 June 2023, 2.00PM to 3PM

Speaker(s): Felicity Maiden, Tobias Schuett, Ben Harris and Anne Marie Norton

Fecility Maiden

Microwave Start-up in Spherical Tokamaks

Microwave start-up uses microwaves to drive the plasma current during the early stages of tokamak operation, from ionisation to the formation of closed flux surfaces. It is a critical problem for tokamak power plants with limited space for a neutron shielded solenoid. However, it has not been widely studied and there are no existing models which include microwave current drive calculations and do not assume closed flux surfaces. The open field lines and low densities and temperatures also lead to different current drive mechanisms and microwave-plasma interactions from flat-top. Therefore, we are developing a new code to simulate microwave start-up.

Tobias Schuett

Radial electric field characterisation across confinement transitions on MAST-U

The radial electric field plays a key role in the L-H transition of tokamak plasmas. While its role in suppressing edge turbulence via shear decorrelation has been established, the composition from neoclassical mean flows and turbulence driven zonal flows is not fully understood, especially in spherical tokamaks like MAST-U. Radial electric field measurements can be obtained both from charge exchange recombination spectroscopy and from turbulent density fluctuations measured by beam emission spectroscopy. This work
aims to perform a comparison between both methods to check zeroth-order consistency as well as to bring out possible discrepancies due to nonlinear zonal flow drive.

Ben Harris

The H2O2 density distribution of He+H2O plasmas in the effluent of a COST-Jet and kINPen

Cold atmospheric-pressure plasma jets (CAPJs) are efficient sources of reactive oxygen species when supplied with a feed gas containing water vapour, making them well-suited to numerous roles in biomedicine. H2O2, the reactive oxygen species of focus in this study, plays a key role in plasma-assisted wound healing by promoting wound contraction and inactivating bacteria. The presence of H2O2 has previously been investigated in the plasma afterglow, plasma-treated liquids, and biological media. However, spatially resolved data is scarce. The distribution of H2O2 dispersing into ambient air from the plasma effluent is largely unknown, making it difficult to quantify the exposure of a substrate. To this end, this work presents the fully spatially resolved density distribution of H2O2 in the effluent of two CAPJs supplied with a helium and water vapour feed gas. These are the COST-Jet, originally designed as a reference standard, and the kINPen, a commercially available plasma source.

The plasma effluent is measured with continuous wave cavity ring-down spectroscopy, with a CAPJ being mounted in an optical cavity to characterise absorption along the line of sight of a mid-infrared laser at various axial and radial positions. The absorption spectra obtained from this are processed to isolate H2O2 absorption, allowing the line-of-sight integrated density of H2O2 to be extracted. The distribution of this parameter allows radial Abel inversions to be performed and the subsequent trends to be characterised in the axial direction, with the result of the Abel inversions giving the fully spatially resolved H2O2 density throughout the plasma

It is found that the H2O2 density profile along the axis of the COST-Jet is highly laminar up to 20 mm from the jet nozzle, at which point the density drops sharply as the H2O2 disperses more widely into the open air. In contrast, the H2O2 density profile of the kINPen is initially less laminar, however the density is more consistent further into the effluent and the same sharp drop is not observed. For both CAPJs, the bulk of H2O2 formation appears to happen either within the plasma channel or the first few millimetres of the effluent.

Ann-Marie Norton

Commissioning the DiPOLE laser for Dynamic Compression of Materials at High Repetition Rates on the High Energy Density instrument at European XFEL

High Energy Density (HED) materials are defined to have energy densities greater than 1011Jm−3; this corresponds to a pressure of approximately 100 GPa. Understanding what happens to materials at HED conditions is vital for many fields. Of particular note is the deformation and failure of critical metallic components used in fusion science, aerospace engineering and laser processing. These components may experience ultra-high pressures, temperatures and strains, causing microstructural changes that eventually lead to macroscopic failures. Examples include ductility in high-performance ceramics and meta-stable high-strength states. Such research is also relevant to astrophysics and planetary science, where understanding the extreme states of materials can shed light on the
formation of planets, interstellar dust clouds, and asteroid impact sites.

The x-ray and laser capabilities of the European XFEL (EuXFEL) High Energy Density (HED) instrument makes it uniquely suited for the research described above. Last year the first in-situ x- ray diffraction and laser shock compression experiment at the HED instrument showed pressures of 30 - 40 GPa and repetition rates of order 0.1 Hz were possible using the pump-probe laser. However, the commissioning of the DiPOLE laser at EuXFEL last month demonstrated High-Repetition-Rate (HRR) laser shock compression at 1 Hz and pressures above 210 GPa. HRR experiments allow more data to be collected, reduce the uncertainties caused by shot-to-shot variations and allow materials with lower scattering/diffraction intensities to be observed (by stacking data from multiple shots). In addition, the higher laser energy (100 J) and arbitrary pulse shaping available with DiPOLE enable new regions of the material phase diagram to be explored.

In this talk, I will use some initial results to demonstrate the success of the DiPOLE commissioning run and the usefulness of the HED platform for researching HED materials.

Location: Debye Lecture Theatre & Zoom

Post Doc Seminars - May 2023

Friday 26 May 2023, 2.00PM to 3PM

Speaker(s): Chris Arran

Chris Arran

Measuring Magnetic Dynamics in High Energy Density Plasmas

The role of magnetic fields in reducing heat flow in plasmas is well known, but less familiar is how heat flow also leads to substantial changes in the magnetic field through the Nernst effect. Steep temperature gradients perpendicular to a magnetic field lead to an induced electric field and advection of the magnetic field with a velocity proportional to the heat flow. In hot plasmas away from equilibrium, the Nernst effect is the dominant process affecting the magnetic field, but due to the complex coupling between heat and the field in laser-plasma interactions this effect is difficult to study.

By heating a gas jet with a nanosecond duration laser pulse within an applied magnetic field, we describe the first direct measurement of Nernst-driven cavitation in the magnetic field inside a plasma. We reconstruct the magnetic field map using proton radiography and show that the heat flow causes rapid expulsion of the magnetic field from the hottest regions of the plasma, before hydrodynamic motion begins to play a role. This allows us to estimate the Nernst velocity as (1.5 ± 0.5) × 106 m/s at early times, with the heat flow reaching a substantial fraction of the free streaming limit. We can therefore explore at plasma densities from 1018 – 1019 cm−3 both how the heat flow advects the magnetic field, and how the magnetic field restricts the heat-flow.

Location: The Debye Lecture Theatre or via Zoom

Student Seminars - May 2023

Friday 19 May 2023, 2.00PM to 3PM

Speaker(s): Celine Schaumans, Ryan Magee and Nicola Lonigro

Celine Schaumans

Exotic Plasma Instabilities in Rotating Tokamak Plasmas.

Toroidal rotation has been shown to stabilize several plasma instabilities, including the internal kink. The internal kink mode is linked to the onset of sawtooth oscillations in tokamaks and must therefore be controlled. In medium-sized spherical tokamaks like MAST-U, neutral beam injection is likely to drive sonic or near-sonic flows. At these flow speeds, a Kelvin-Helmholtz-like global hydromagnetic instability has been found numerically and analytically to grow in the plasma. The existence of this instability introduces an upper limit to the allowable rotation in a tokamak. This work aims to investigate the physics and parametric dependences of both the rotational stabilization of the internal kink and destabilization of the Kelvin-Helmholtz-like instability. In doing so, it seeks to develop the windows of stability which exist between the two.

Ryan Magee

Measuring work function and negative ion yield from boron doped diamond and graphite in deuterium plasmas

Negative-ion sources are of considerable interest for applications such as materials processing and neutral beam injection for magnetic confinement fusion. The efficient production of negative-ions in these sources often relies on surface production. Work function measurements are critical to enable a detailed understanding of the mechanisms that underpin this process for negative-ion production. In this study we used a combination of photoemission yield spectroscopy and the Fowler method to determine the work functions of boron doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) directly after exposure to a low-pressure inductively coupled deuterium plasma (150 W, 2Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. The results show that the increasing work function of the plasma exposed HOPG occurs over the same sample temperature range as the decreasing negative-ion current. In contrast, the work function of BDD does not show a clear relationship with negative-ion current, suggesting that different mechanisms influence the negative-ion production of metal-like HOPG and dielectric-like BDD. For both materials, the maximum photocurrent measured from the samples displays a strong similarity with negative-ion current, suggesting they are driven by a common mechanism.

Nicola Lonigro

Coherence Imaging Spectroscopy for direct density measurements in the MAST-U divertor. 

The spherical tokamak MAST-U can operate in a variety of magnetic divertor configurations, such as the Super-X divertor, facilitating a comparison of their performance.
A Multi-delay Coherence Imaging Spectroscopy (CIS) camera view of the MAST-U lower divertor has been added to the Multi-Wavelength-Imaging (MWI) diagnostic with the goal of the CIS camera providing direct electron density measurements across the
poloidal cross-section of the divertor. The decrease in the contrast of the fringes making up the CIS interference pattern, due to the Stark broadening of the imaged D$_\gamma$ Balmer line, allows the determination of the emissivity-weighted chordal-averaged density for each pixel of the camera.

A non-linear inversion technique is being developed to determine 2D density profiles under the assumption of toroidal symmetry.

Location: Debye and via Zoom

Post Doc Seminars - April 2023

Friday 14 April 2023, 2.00PM to 3PM

Speaker(s): Arka Bokshi

Arka Bokshi

Abstract to follow...

Location: Debye Lecture Theatre or via Zoom

Student Seminars - April 2023

Friday 21 April 2023, 2.00PM to 3PM

Speaker(s): Luke Thompson, Christina Ingleby and Matt Hill

Luke Thompson

3D effects in tokamak plasma stability

ELMs are powerful transient events that occur in tokamak plasmas after the L-H mode transition. The largest of which ITER is predicted to survive approximately 10 of before requiring decommissioning. As such, controlling ELMs has become imperative to the success of magnetic confinement fusion. One method being trialled is the application of resonant magnetic perturbations, which can be modelled using a Fourier decomposition. In order to test the viability of RMPs as a means of ELM control, the change in energy within a perturbed plasma is investigated.

Christina Ingleby

Characterising x-ray emission from laser-solid interactions and QED plasmas

Laser-matter interactions are well established with current PW lasers, but with the commission of new multi-PW laser facilities, laser intensities beyond 1022 W/cm2 are now achievable. At these laser intensities the EM fields of the laser are so strong that we begin to see QED effects come into play and we enter the QED plasma regime. QED plasmas produce a wealth of particles including hard x-rays through nonlinear Compton scattering (NCS), GeV ions, and electron-positron pairs.
Previous work has shown that there is a bright flash of NCS x-rays produced as we enter the QED regime, and this is expected to provide a key experimental observable. However, at current achievable laser intensities of 1020 -1022 W/cm2, the additional process of inverse bremsstrahlung emission is also present and generates x-rays that are not distinguishable from NCS x-rays in an experimental capacity. For experiments at current facilities, bremsstrahlung emission must be minimised to observe the production of NCS x-rays.
Simulations can be conducted to generate an optimal target design to simultaneously enhance NCS emission and minimise bremsstrahlung emission. The work presented includes results from simulations conducted using EPOCH, and in particular the hybrid-PIC EPOCH extension. The results directly compare the NCS and bremsstrahlung x-ray emission for different laser
intensities within our range of interest (1020 -1022 W/cm2) using an optimised target design.

Matt Hill

Using two-photon absorption laser-induced fluorescence to measure O and N in low-pressure plasmas

Plasma-enhanced pulsed laser deposition (PE-PLD) is a novel method of depositing semiconductor thin films which builds upon the widely-established pulsed laser deposition (PLD) method. It involves ablating a metal target into a plasma plume using a high-powered pulsed laser and a non-metal background plasma such as oxygen, as opposed to a metal compound target and background gas. PE-PLD is still an active area of research with many challenges, including achieving the right amount of non-metal content from the
background plasma to achieve the correct stoichiometry of the thin film. This talk will discuss the plasma diagnostic technique, two-photon absorption laser-induced fluorescence (TALIF), and its use in determining the densities of oxygen and nitrogen used in PE-PLD of metal oxynitride thin films. The talk will also present preliminary results showing how TALIF measures atomic densities of
oxygen and nitrogen plasma species.

Location: Debye Lecture Theatre or via Zoom

Post Doc Seminars - March 2023

Friday 24 March 2023, 2.00PM to 3PM

Speaker(s): Koki Imada

Koki Imada

Abstract to follow ...

Location: Debye Lecture Theatre or via Zoom

Student Seminars - March 2023

Friday 17 March 2023, 2.00PM to 3PM

Speaker(s): Theo Gheorghiu. Ben Pritchard and Daniel Greenhouse

Theo Gheorghiu

Applying Random Walks to Zonal Flow Models; How can we describe transport influenced by coherent structures?

Zonal flows have been recognised as being important to transport in the edge and core of MCF devices. The predominantly radial electric field driven phenomena results in distinctive shear layers forming internal transport barriers now known as the ExB staircase, and with shear layers also capable of transiently forming in the SOL – in both cases, ExB shear is associated with suppression of turbulence. In geophysics and 2D fluid dynamics we have a good understanding of how shear structures act as transport barriers by both analysis and the study of the dynamics of passive particles.
The Hasegawa-Wakatani (HW) equation is studied due to its suppression of radial turbulence by ExB shearing effects. We apply an observational random walk model to the dynamics of passive particles undergoing ExB motions in the classical and modified HW system. We can identify diffusion-like equations based on the data, and find that only in the simulations where distinctive zonal flows 

Ben Pritchard

SAMI-2 commissioning: A synthetic aperture microwave imaging diagnostic.

SAMI-2 is a novel diagnostic designed without optical components for focussing and to operate in the 20 – 40 GHz microwave range. SAMI-2 uses a phased array approach for post-processing image focusing, removing the need for any optical components to perform in-situ focusing during data collection, and removing the need for additional maintenance overheads. The frequency range has been designed to be reflected at density cut offs near the edge of the MAST-U plasma, which SAMI-2 is installed on. This talk will outline how SAMI-2 works and the current calibration work needed to make the phased array concept work. are formed, the diffusion-like equation features a non-local and non-linear fractional derivative term – seemingly the first time this has been identified from data.
If these coherent structures – the zonal flows – are responsible for the fractional terms as we may infer, this suggests that we can provide an intuitive and kinetic explanation for fractional transport and its exact relation to transport barriers of various kinds. If we can do this, it is feasible to describe transport through complicated turbulent phenomena with a simple(ish) stochastic evolution equation.


Daniel Greenhouse

Integrated Data Analysis Technique for Investigating Divertor Physics

The control of heat and particle flux from the core plasma to chamber walls remains a key challenge for nuclear fusion tokamak reactors. Divertors are designed to manage this flux and MAST-U is exploring the effects of novel magnetic configurations in the divertor region. To aid the understanding of processes occurring in the divertor, an integrated data analysis (IDA) system based on Bayesian inference is being developed toward experimental data at MAST-U.

The IDA offers inference of electron temperature (Te), electron density (ne) and neutral density (n0) plasma characteristics (fields) over a two-dimensional, poloidal cross section. Tests on synthetic data have shown that this approach can circumvent limited diagnostic coverage in the divertor and achieve 4.7%, 2.7% and 8.7% mean absolute percentage error for Te, ne and n0 respectively across the relevant divertor region. These parameters can be used to derive further quantities which give insight into plasma processes in the divertor.
This talk will outline the statistics that underpins the IDA as well as the challenges faced by the IDA and our strategies for over-coming them.


Abstract to follow...

Location: Debye Lecture Theatre or via Zoom

Post Doc Seminars - February 2023

Friday 24 February 2023, 2.00PM to 3PM

Speaker(s): Alexandra Dudkovskaia

Alexandra Dudkovskaia

Neoclassical tearing modes (NTMs) are resistive magneto-hydrodynamic plasma instabilities. They change the equilibrium tokamak magnetic geometry, creating a chain of magnetic islands and destroying the toroidal symmetry of the tokamak plasma. The NTM island width increases rapidly, reaching values of 10−30% of the tokamak minor radius and limiting plasma beta (fusion output). One of the NTM control techniques is to generate microwaves at the electron cyclotron frequency to decrease the island width.
According to experimental observations, there is some threshold island width, wc, of a few ρbi, i.e. (1 − 2)cm, where ρbi is the trapped ion banana orbit width, below which NTMs are fully stabilised. The aim is to predict and quantify this wc in theory. To resolve wc ∼ ρbi, kinetic theory needs to be employed. In [Plasma Phys. Control. Fusion 63 (2021) 054001] a drift kinetic theory is developed to calculate how plasma responds to small magnetic islands of width ρbi. The theory is valid in the limit of rare collisions (i.e. relevant to the banana collisionality plasma) and low beta large aspect ratio tokamaks. In [Nucl. Fusion 63 (2023) 016020] this theory is extended to a finite beta arbitrary geometry (i.e. D-shaped) tokamak plasma. The plasma shaping effects (such as plasma elongation, triangularity and finite aspect ratio) on the NTM threshold are investigated. In particular, it is found that a higher triangularity plasma is more susceptible to NTMs, which is in accordance with the recent tearing mode onset relative frequency measurements in DIII-D. Second, the NTM threshold dependence on the tokamak inverse aspect ratio is extended to a finite aspect ratio limit. Third, the NTM threshold dependence on poloidal beta is obtained and successfully benchmarked against the EAST threshold island width measurements.


Location: Debye Lecture Theatre or via Zoom

Student Seminars - February 2023

Friday 17 February 2023, 2.00PM to 3:00 PM

Speaker(s): Sid Leigh & Michal Kryjak

Sid Leigh

Modelling Neoclassical Tearing Mode Instabilities at the Threshold Scale

High-pressure tokamak plasmas are vulnerable to magnetohydrodynamic instabilities that modify the equilibrium magnetic geometry, negatively impacting confinement. One instability is the neoclassical tearing mode (NTM), where perturbations called ‘magnetic islands’ form on toroidal flux surfaces. Islands short-cut radial transport, removing the pressure gradient-dependent bootstrap current inside them and amplifying the original perturbation and removing more pressure from the tokamak. Islands grow only when the normalised plasma pressure and the island width both exceed thresholds, otherwise they heal away. We are developing a predictive theory of the threshold island width, and a rebuilt simulation code based on the algorithm of Refs. [1] and [2]. This talk outlines the physics challenge of modelling NTMs at the threshold island width scale, the redevelopment of the code, the practical challenges encountered, and the plans ahead.
Our work is supported by the Fusion Centre for Doctoral Training (grant EP/S022430/1), and by UKRI EPSRC High End Computing grant EP/R029148/1. This work used the ARCHER2 UK National Supercomputing Service (
[1] Imada et al, Phys. Rev. Lett. 121 175001 (2018) DOI:10.1103/PhysRevLett.121.175001
[2] Imada et al, Nucl. Fusion 59 046016 (2019) DOI:10.1088/1741-4326/ab00ba

Michal Kryjak

Comparison of new edge code Hermes-3 to leading 2D scrape-off layer codes

Hermes-3 is a new plasma edge code under the BOUT++ framework. Unlike other codes, it is able to perform self-consistent simulations of 1D/2D plasmas as well as of 3D plasma turbulence. In this work, we perform a code comparison between Hermes-3 and the leading edge codes SOLPS-ITER and SOLEDGE2D through 2D simulations of the ST-40 spherical tokamak.
The work is part of a collaboration with Tokamak Energy and the University of Tuscia.

Location: Debye Lecture Theatre or via Zoom

Student Seminars - January 2023

Friday 27 January 2023, 2.00PM to 3:00 PM

Speaker(s): Cyd Cowley, Max Kellermann-Stunt & Arun Nutter

Max Kellermann-Stunt

Abstract to follow...

Arun Nutter

The exciting, recent developments in inertial confinement fusion (ICF) after results at the National Ignition Facility (NIF) have demonstrated laser driven fusion is the best way to pursue fusion energy and have ignited increased interest and motivation for ICF based projects in many countries. To maximise progress in ICF, we need to be able to accurately model an implosion, this will enable better designs and improve the analysis of experiments. Driving an implosion efficiently requires laser energy to be absorbed via inverse bremsstrahlung, which heats the coronal plasma that ablates a shell, compressing the fuel while keeping it cold. The coupling of laser energy into the corona and resulting implosion velocity is reduced by the presence of laser-plasma instabilities (LPI). These instabilities couple laser energy into electromagnetic and electrostatic waves in the plasma that can also accelerate hot electrons, which may preheat the cold fuel and reduce compression. 
Implosion dynamics are typically modelled using radiation hydrodynamics codes, but these codes do not include the wave physics needed to describe LPI and instead use multipliers to account for these processes. To address this we are creating a fast, computational model to ensure the effects of LPI are included in simulations.

Cyd Cowley

Title: Comparisons of detachment control across multiple divertor configurations in MAST-U

Abstract: One of the primary aims of the MAST-U tokamak is to study how advanced divertors such as the Super-X can help mitigate plasma exhaust. A key way in which advanced divertors are predicted to aid plasma exhaust is through better access to, and control over the process of detachment. In this work a Multi-Wavelength Imaging (MWI) diagnostic is used to track the effective location of a detached divertor plasma using hydrogenic Fulcher-band emission. The control of this detachment front with respect to changes in upstream density is analysed, and in comparable regions detachment is controlled measurably easier in a Super-X compared to a standard configuration.


Location: Debye Lecture Theatre or via Zoom