James Peter Dedrick

Negative ion production is of significant interest for materials processing and neutral beam injection systems for magnetic confinement fusion reactors. The surface production mechanism can be required for high current applications. Dielectric materials, including doped diamond, are of interest for increasing surface production as a potential alternative to low-work function metals and further understanding of the underpinning mechanisms is needed. In this study we use photoemission yield spectroscopy, in conjunction with the Fowler model and mass spectrometry, to measure the negative ion yield and ionization threshold of micro-crystalline diamond (µc-D), micro-crystalline boron doped diamond (µc-BDD) and single crystal phosphorus doped diamond (PDD) under deuterium plasma exposure at sample temperatures between 30 ^∘C and 700 ^∘C. We observe that deuterium plasma exposure of PDD at 400 ^∘C reduces its ionization threshold from approximately 4.0 eV to 2.1 eV, which is similar to the work function of cesium. While the ionization threshold of diamond is observed to be sensitive to sample temperature and the dopant used, this appears to have negligible effect on negative ion production. Developing a better understanding of which material properties are most significant for negative ion production will assist the development of improved negative ion sources.

Atmospheric pressure plasma jets are efficient sources of reactive oxygen and nitrogen species with potential applications in medicine, materials processing, green industry and agriculture. However, selective control over the production of reactive species presents an ongoing challenge and a barrier to the widespread uptake of these devices in applications. This study therefore investigates the production of ozone by a radio-frequency plasma jet driven with tailored voltage waveforms composed of up to five consecutive harmonics, with a fundamental frequency of 13.56 MHz. The plasma is supplied with helium with small admixtures (0.1%–1.0%) of oxygen gas. The ozone density in the far effluent is measured with Fourier transform infrared spectroscopy and the gas temperature in the plasma channel is determined with optical emission spectroscopy. Voltage waveform tailoring is found to enhance the ozone density in the far effluent of the plasma jet in comparison to operation with single-frequency voltage waveforms. Increasing the number of applied harmonics in the driving voltage waveform for a fixed peak-to-peak voltage enhances the ozone density but significantly increases the gas temperature within the plasma channel. Meanwhile, increasing the number of applied harmonics while maintaining a constant RF power deposition allows the density of ozone in the effluent to be increased by up to a factor of 4 relative to single-frequency operation, up to a maximum density of 5.7 × 10 14 cm⁻³, without any significant change to the gas temperature. This work highlights that tailored voltage waveforms can be used to control the density of ozone delivered through the plasma effluent, marking an important step towards realising the potential of these plasmas for applications.

The development of real-time control strategies for key discharge parameters, such as densities, fluxes, and energy distributions, is of fundamental interest to many plasma sources. Over the last decade, multi-harmonic ‘tailored’ voltage waveforms have been successfully employed to achieve enhanced control of key parameters in a wide range of radio-frequency (RF) plasma sources through application of the electrical asymmetry effect (EAE). More recently, the analogous magnetic asymmetry effect (MAE) has been numerically and experimentally demonstrated to achieve a notable degree of control in parallel plate RF plasma sources. The MAE is achieved via selectively magnetising the charged species adjacent to one electrode, altering the charge flux to the surface and enforcing a DC self-bias to maintain quasineutrality. This study addresses the degree of control achieved by the MAE in a non-planar geometry via 2D fluid/kinetic simulations of a magnetised RF capacitively coupled plasma source employing two different magnetic topologies. The simultaneous application of the EAE and MAE is then presented for the same geometry, demonstrating a degree of non-linear behaviour dependant upon the applied magnetic topology. Control of the DC self-bias voltage η dc is demonstrated for a single 600   V pp , 13.56 MHz discharge in both ‘convergent’ (maximum on-axis field strength) and ‘divergent’ (minimum on-axis field strength) magnetic topolgies. MAE induced modulations of η dc   =   0.13   V pp and η dc   =   0.03   V pp are achieved for each magnetic topology, respectively, for magnetic field strengths between 50 and 1000 G. Simultaneous application of an EAE and MAE is achieved through a multi-harmonic ‘peak’-type tailored voltage waveform employing varying harmonic phase offsets between 0 ∘   ⩽   θ   ⩽   360 ∘ . The degree to which the DC self-bias voltage is modulated by the applied EAE is mediated by the orientation and magnitude of the applied magnetic field. The EAE induced DC self-bias modulations exhibit non-linear behaviour in response to a superimposed MAE, such that the resulting DC self-bias differs from an additive combination of the two effects alone Simultaneous application of the electrical and MAEs offers the possibility of further decoupling ion and electron dynamics in RF plasma sources, and represents an improvement over each approach in isolation.

The development of compact, low power, charge–neutral propulsion sources is of significant recent interest due to the rising application of micro-scale satellite platforms. Among such sources, radio frequency (rf) electrothermal microthrusters present an attractive option due to their scalability, reliability, and tunable control of power coupling to the propellant. For micropropulsion applications, where available power is limited, it is of particular importance to understand how electrical power can be transferred to the propellant efficiently, a process that is underpinned by the plasma sheath dynamics. In this work, two-dimensional fluid/Monte Carlo simulations are employed to investigate the effects of applied voltage frequency on the electron, ion, and neutral heating in an rf capacitively coupled plasma microthruster operating in argon. Variations in the electron and argon ion densities and power deposition, and their consequent effect on neutral-gas heating, are investigated with relation to the phase-averaged and phase-resolved sheath dynamics for rf voltage frequencies of 6–108 MHz at 450 V. Driving voltage frequencies above 40.68 MHz exhibit enhanced volumetric ionization from bulk electrons at the expense of the ion heating efficiency. Lower driving voltage frequencies below 13.56 MHz exhibit more efficient ionization due to secondary electrons and an increasing fraction of rf power deposition into ions. Thermal efficiencies are improved by a factor of 2.5 at 6 MHz as compared to the more traditional 13.56 MHz, indicating a favorable operating regime for low power satellite applications.

Non-equilibrium inductively coupled plasmas (ICPs) operating in hydrogen are of significant interest for applications including large-area materials processing. Increasing control of spatial gas heating, which drives the formation of neutral species density gradients and the rate of gas-temperature-dependent reactions, is critical. In this study, we use 2D fluid-kinetic simulations with the Hybrid Plasma Equipment Model to investigate the spatially resolved production of atomic hydrogen in a low-pressure planar ICP operating in pure hydrogen (10–20 Pa or 0.075–0.15 Torr, 300 W). The reaction set incorporates self-consistent calculation of the spatially resolved gas temperature and 14 vibrationally excited states. We find that the formation of neutral-gas density gradients, which result from spatially non-uniform electrical power deposition at constant pressure, can drive significant variations in the vibrational distribution function and density of atomic hydrogen when gas heating is spatially resolved. This highlights the significance of spatial gas heating on the production of reactive species in relatively high-power-density plasma processing sources.

High frequency (HF) capacitively coupled plasmas (CCPs) are ubiquitous, having several industrial applications, especially in the semiconductor industry. Inductive heating effects within these plasmas play an important role and therefore understanding them is key to improve industrial applications. For this purpose kinetic research, using particle-in-cell (PIC) codes, offers significant opportunity to study, and improve, industrial plasma processes that operate at the atomic level. However, PIC codes commonly used for CCPs are electrostatic and thus cannot be used to simulate electromagnetically induced currents. Therefore we have developed EPOCH-LTP, a 1D PIC code with a current heating model, that enables the simulation of inductive heating effects in HF CCPs. First simulation results, from an HF CCP (60 MHz) operated at 1 mTorr of argon, show that inductive currents couple most of their power to the electrons at the interface between the bulk plasma and the sheath. Furthermore, the simulation of a dual-frequency CCP, where a HF inductive current and a low-frequency (LF) voltage waveform at 400 kHz are applied, have shown a synergy between the HF and LF waveforms that increase the inductive heating rate.

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. 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, 2 Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. During plasma exposure, samples are biased at −130 V or −60 V and their temperature is varied between 50 ^∘C and 750 ^∘C. 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. The necessity for an additional fitting parameter for the Fowler fits to BDD suggests that its electronic properties are changing under plasma exposure, unlike HOPG. 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. The in-situ measurement of the work function using non-invasive techniques is of interest for the development of negative ion sources.

The production of negative ions is of significant interest for applications including mass spectrometry, materials surface processing, and neutral beam injection for magnetic confined fusion. Neutral beam injection sources maximise negative ion production through the use of surface production processes and low work function metals, which introduce complex engineering. Investigating materials and techniques to avoid the use of low work function metals is of interest to broaden the application of negative ion sources and simplify future devices. In this study, we use pulsed sample biasing to investigate the surface production of negative ions from nitrogen doped diamond. The use of a pulsed bias allows for the study of insulating samples in a preserved surface state at temperatures between 150 ^∘C and 700 ^∘C in a 2 Pa, 130 W, (n_e ∼ 10⁹ cm⁻³, T_e ∼ 0.6 eV) inductively coupled deuterium plasma. The negative ion yield during the application of a pulsed negative bias is measured using a mass spectrometer and found to be approximately 20% higher for nitrogen doped diamond compared to non-doped diamond. It is also shown that the pulsed sample bias has a lower peak negative ion yield compared to a continuous sample bias, which suggests that the formation of an optimum ratio of defects on its surface can be favourable for negative ion production.

The discrete control of ion energy and flux is of increasing importance to industrially relevant plasma sources. The ion energy distribution functions (IEDFs) and net ion flux incident upon material surfaces in intermediate pressure (≈133 Pa, 1 Torr) radio-frequency capacitively coupled plasmas (rf CCPs) are coupled to the spatio-temporal sheath dynamics and resulting phase-averaged sheath potential. For single frequency driven discharges this co-dependence of ion energy and flux on the sheath potential limits the range of accessible operating regimes. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate quasi-independent control of the ion flux and IEDF incident upon plasma facing surfaces in a collisional (≈200 Pa, 1.5 Torr argon) rf hollow cathode discharge driven by multi-harmonic ( n ⩾ 2) tailored voltage waveforms. The application of variable phase offset n = 5 tailored voltage waveforms affords a significant degree of control over the ion flux Γ A r + and mean ion energy ϵ ̂ A r + , modulating each by factors of 2.9 and 1.6, respectively as compared to 1.8 and 1.6, achieved via n = 2 dual-frequency voltage waveforms. The disparate modulations achieved employing n = 5 tailored voltage waveforms demonstrate a significant degree of independent control over the mean ion energy and ion flux for collisional conditions, enabling access to a wider range of operational regimes. Maximising the extent to which ion energy and flux may be independently controlled enables improvements to plasma sources for technological applications such as plasma assisted material manufacture and spacecraft propulsion.