Cavity Field Control for Linear Particle Accelerators

High-energy linear particle accelerators enable exploration of the microscopic structure of pharmaceuticals, solar cells, fuel cells, high-temperature superconductors, and the universe itself. These accelerators accelerate charged particles using oscillating magnetic fields that are confined in metal cavities. The amplitudes and phases of the electromagnetic fields need to be accurately controlled by fast feedback loops for proper accelerator operation.This thesis is based on the author's work on performance analysis and control design for the field control loops of the linear accelerator at the European Spallation Source (ESS), a neutron microscope that is under construction in Lund, Sweden. The main contribution of the thesis is a comprehensive treatment of the field control problem during flat-top, which gives more insight into the control aspects than previous work. The thesis demonstrates that a key to understand the dynamics of the field control loop is to represent it as a single-input single-output system with complex coefficients. This representation is not new itself but has seen limited use for field control analysis.The thesis starts by developing practical and theoretical tools for analysis and control design for complex-coefficients systems. This is followed by two main parts on cavity field control. The first part introduces parametrizations that enable a better understanding of the cavity dynamics and discusses the most essential aspects of cavity field control. The second part builds on the first one and treats a selection of more advanced topics that all benefit from the complex-coefficient representation: analysis of a polar controller structure, field control design in the presence of parasitic cavity resonances, digital downconversion for low-latency feedback, energy-optimal excitation of accelerating cavities, and an intuitive design method for narrowband disturbance rejection. The results of the investigations in this thesis provide a better understanding of the field control problem and have influenced the design of the field controllers at ESS

Speed control of a peristaltic blood pump

Hemodialysis is today the main treatment used for patients with renal impairment, a serious medical condition. Hemodialysis treatment handles a very delicate system which is why it is of the utmost importance that the dialysis machine has got a reliable safety monitoring system that can detect severe complications during treatment. A new such system is currently under development at Gambro, Lund, which focuses on early detection of venous needle dislodgement(VND). The safety monitoring system determines the heart pulses from the patient on both the venous and arterial side.VND is indicated when no venous heart pulses can be detected. Large pressure disturbances are induced by the rollers on the peristaltic pump which makes it dicult to extract the patient's heart pulses. In order to successfully filter out the large disturbances it is required that the pump has a nearly constant period time. The objective of this thesis was to evaluate the current control system which is a hardware motor speed controller connected in cascade with a software blood flow controller. The project objective was also to find an improved control system for the peristaltic pump which can fulfill the required standard deviation of the relative period time of less than 0.1 %. This was done by first analyzing the pump process in open loop. Results showed that the process had high-frequency disturbances possibly originating from some mechanical unevenness in the motor since the frequency of the disturbances was dependent on the motor speed. It was also found that these disturbances were enhanced by the existing control system. In order to find a new improved control system, a hardware controller similar to the existing controller, was evaluated. With the alternative hardware controller the standard deviations of the relative period times were improved, except at low blood flows (100-150 ml/min). A PI controller was implemented in software using LabView. The PI controller fulfilled the relative period time standard deviation requirement at all blood flows and showed improved performance compared to the original controller. Used together with a feed forward implementation, the performance was further improved

Experimental Study of a Cascade of Low Pressure Turbine Blades with Upstream Periodic Stator Wakes

The objective of this study is to experimentally study film cooling flows. A closed-loop wind tunnel with a four passage linear cascade of US Air Force Research Laboratory (AFRL) ultra-high-lift L1A low pressure turbine (LPT) blades and upstream wake generator is used in conjunction with Particle Image Velocimetry (PIV) flow visualization technique to study turbulent film cooling flows due to the interaction between vanes and blades. Further post-processing in the form of Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) modal analyses is performed to determine the relevant modes that characterize the coherent structures in the flow. An image patching algorithm is also implemented. The results obtained are used to characterize the periodic wake on the cascade flow. The periodic wake has been studied in detail near the leading edge of the suction side. The velocity data led to the mean velocity profile and maximum velocity deficit in the wake. The POD identified the most energetic modes representing the vortex shedding wavelength, and its harmonics, of the wake generator plates. The DMD confirmed the wake passage frequency. Implementation of the image patching algorithm with four domains was presented. The technique was successful in computing the average vector field. Further downstream of the leading edge, the POD modes are shown to become more chaotic and less energetic. The leading order mode pair loses close to half of their energy to lower order modes due to the cascading of turbulent kinetic energy to lower spatial scales and to viscous dissipation losses. When the wake is impinging on the leading edge, the boundary layer separates near the transition point. The boundary layer remains completely attached to the trailing edge when the wake is not impinging on the leading edge

DESIGN AND IMPLEMENTATION OF AN ALL-COTS DIGITAL BACK-END FOR A PULSE-DOPPLER SYNTHETIC APERTURE RADAR

Radar imaging techniques employing synthetic aperture radar (SAR) are ubiquitous in applications such as defense, remote sensing, space exploration, terrain mapping, and many others. However, to obtain fine image resolution, radar systems must be capable of utilizing large signal bandwidths. By the sampling theorem, a large signal bandwidth equates to a high sampling frequency, resulting in more expensive and complex digital electronics required to digitize and process the waveform. Using linear frequency modulated (LFM) pulses and stretch processing techniques, systems such as frequency-modulated continuous-wave (FMCW) radars reduce the required sampling rate at the expense of longer pulses, higher transmit duty cycle, and decreased pulse repetition frequency. While these tradeoffs are often acceptable, in many situations they are not, and a pulse-Doppler radar system is required. These systems can utilize LFM pulses with nearly any desired pulse length and pulse repetition frequency to perform imaging, but they must have an analog-to-digital converter (ADC) and back-end processing capable of handling the full waveform bandwidth, leading to increased cost, size, or both. At the University of Oklahoma’s Advanced Radar Research Center, a pulse-Doppler radar system for use in a SAR application is designed and built using only commercially available components to decrease the size and cost of the radar, specifically the digital back-end. A minimum size and weight is targeted for this system because it is desired to eventually fly the radar and form images on a lightweight airborne platform, such as a quad- or octo-copter. The challenge with using commercial parts for a custom digital pulse-Doppler radar is that it is difficult to meet the strict timing requirements inherent to pulse-Doppler radar while simultaneously meeting the high-bandwidth requirements imposed by SAR. In this thesis, the design and implementation of the digital back-end for the custom SAR system is presented. The focus is placed on designing a control system and clock distribution scheme in the digital back-end to ensure pulse to pulse coherence while maintaining ideal LFM spectral quality. Additionally, a calibration method is devised to provide accurate range measurements each time the radar is turned on even if the latency between the digital transmitter and receiver changes. At the conclusion of this work, it is shown that the radar system is capable of performing accurate pulse-Doppler radar through the generation of range-Doppler maps from data captured by the radar. The results of these tests indicate that the system is suitable for eventual use in SAR imaging applications

Experimental Investigation of Supersonic Jets Using Optical Diagnostics

The complexity of many fluid flows and phenomena is a well-known characteristic driven primarily by turbulence, which has been a focal point of study for decades. Most engineering applications in fluids will encounter turbulence, and hence the need to understand how turbulence might influence the problem at hand is omnipresent. In many turbulent flows, there are large-scale coherent structures which directly influence macro-scale processes of engineering relevance, such as noise production. Over decades of study, it has been demonstrated that similar structures are often observed across many flowfields, despite differences in characteristic parameters, and this has led to the pursuit of simplified models through the use of these dominant, shared structures. Large-scale, coherent structures are of particular importance in turbulent jets, as they represent efficient sources of sound. Noise reduction of subsonic and supersonic fluid jets represents a large interest in the study of acoustic production in jets, and much of it is viewed in the context of controlling these large-scale structures. Supersonic jets in particular may emit an intense sound known as jet screech as a consequence of these structures. This noise source easily has the potential to be damaging to both structures and humans in close proximity, and is a particular target of noise reduction efforts. Turbulent flowfields from two supersonic, underexpanded, screeching jets are analyzed by means of three non-intrusive, high-speed, optical diagnostics. The first technique is high-speed schlieren. The second technique is pulse-burst particle image velocimetry (PB-PIV). The third technique is known as focused laser differential interferometry (FLDI). Extensive spectral, statistical, and modal decomposition analyses are used in this work to identify, extract, and characterize the most energetic features and coherent structures associated with jet screech. The large field of view of the image-based datasets is fully taken advantage of by creating spatial maps of spectral and statistical quantities, which highlight regions of increased fluctuations or activity. These are shown to agree with, or demonstrate additional features that could not be reproduced by the modal analyses. Modal analyses are used to evaluate the structure of the most energetic components in the flow of both screeching jets

X-Band LLRF Developments for High Power CLIC Test Stands and Waveguide Interferometry for Phase Stabilisation

This thesis describes the upgrade of the first high power X-band RF test for high gradient accelerating structures at CERN, as required for the e+ e- collider research program; Compact Linear Collider, CLIC. Significant improvements to the control system and operation of the first test stand, Xbox-1, are implemented. The design and commissioning of the new Low Level Radio Frequency, LLRF, system is described in detail. The upgrade also encompasses software, interlock systems, timing, safety and control. The new LLRF requires an up-convertor to convert an input signal at 187.4 MHz to 11.806 GHz. The most common method is a phase locked loop, PLL, an alternative method was envisioned which uses single side-band up-convertor. This necessitated the design and manufacture of a custom cavity filter. The up-convertor and PLL are compared and both are implemented in the new LLRF. The new LLRF system is implemented at Xbox1 and used to RF condition a 50 MW CPI klystron, the final output power was 45 MW for a 50 ns RF pulse length. The phase and amplitude of the LLRF, TWT and klystron are characterised with both the PLL and up-convertor. The klystron phase stability was studied using a sensitivity analysis. The waveguide network between the klystron and the accelerating structures is approximately 30 m. This network is subject to environmental phase changes which affect the phase stability of the RF arriving at the structures. A single path inteferometer was designed which will allow a phase measurement pulse at a secondary frequency to be injected into the waveguide network interleaved with klystron pulses. The interferometer is commissioned in the lab and low power measurements validate its operation. The system is then integrated into the high power network at Xbox1 and used to measure phase shifts in the waveguide network which are correlated with temperature

Range-resolved optical interferometric signal processing

The ability to identify the range of an interferometric signal is very useful in interferometry, allowing the suppression of parasitic signal components or permitting several signal sources to be multiplexed. Two novel range-resolved optical interferometric signal processing techniques, employing very different working principles, are theoretically described and experimentally demonstrated in this thesis. The first technique is based on code-division multiplexing (CDM), which is combined with single-sideband signal processing, resulting in a technique that, unlike prior work, only uses a single, regular electro-optic phase modulator to perform both range-based signal identification and interferometric phase evaluation. The second approach uses sinusoidal optical frequency modulation (SFM), induced by injection current modulation of a diode laser, to introduce range-dependent carriers to determine phase signals in interferometers of non-zero optical path difference. Here, a key innovation is the application of a smooth window function, which, when used together with a time-variant demodulation approach, allows optical path lengths of constituent interferometers to be continuously and independently variable, subject to a minimum separation, greatly increasing the practicality of the approach. Both techniques are applied to fibre segment interferometry, where fibre segments that act as long-gauge length interferometric sensors are formed between pairs of partial in-fibre reflectors. Using a regular single-mode laser diode, six fibre segments of length 12.5 cm are multiplexed with a quadrature bandwidth of 43 kHz and a phase noise floor of 0.19 mrad · Hz -0.5 using the SFM technique. In contrast, the 16.5 m spatial resolution achieved with the CDM technique points towards its applicability in medium-to-long range sensing. The SFM technique also allows high linearity, with cyclic errors as low as 1 mrad demonstrated, and with modelling indicating further room for improvement. Additionally, in an industrial measurement, the SFM technique is applied to single-beam, multi-surface vibrometry, allowing simultaneous differential measurements between two vibrating surfaces

Modelling, simulation and multivariable control of plasma etching of silicon and silicon dioxide

Plasma etching has been used extensively in the microelectronics industry for integrated circuit fabrication. However, the optimisation of this process is quite challenging because the plasma etching process is complex and not fully understood. An experimental and theoretical study of the etching characteristics of silicon (Si) and silicon dioxide (SiC^) in a sulphur hexafluoride (SFg) with Argon is reported. The selected manipulated variables or inputs are radio-frequency (RF) discharge power density, chamber pressure and gas component ratios. The semi-outputs or process variables are the relative percent concentration of plasma species: fluorine [F], [SFX] (x=3-»6) and the electric field to pressure ratio E/p. The outputs or performance variables are Si and Si02 etch rates and (centre-to-edge ) etching uniformity. The etch rates of silicon and silicon dioxide in SFg/Ar plasma are statistically investigated based on the effects of different settings of the manipulated variables. Optical emission spectrometry and laser interferometry have been employed to monitor spectral emission and etch rate in real time for plasma diagnostics, endpoint detection, and process control. The results obtained from this study have shown that etching Si and Si02 in SFg/Ar leads to higher etch rates in comparison to other reported systems. Variations in optical parameters associated with manipulated conditions, such as RF power density etc., have been studied over a limited parameter space to obtain their effects on the etch rates of Si and Si02. A dynamic mass balance has been employed to construct a comprehensive reactor model for a basic study of plasma etching o f Si and Si02 with SF6/Ar. The model includes diffusion and convection of molecular fragments in a duct geometry, which is estimated by using an effective diffusion length which takes surface reflection into account. Electron impact dissociation and ionisation reactions which depend on the electric field and gas density are the dominant sources of active species generation. Fluorine atom generation is also described by dissociative chemisorption. Fundamental plasma parameters such as electron density and electric field are estimated from impedance measurements in designed experiments under the various operating conditions. Results presented show relatively good agreement between the model predictions and the experimental data. Using regression analysis a steady-state model which relates the manipulated conditions to both the process and performance quantities has been developed. Optical emission spectroscopy and laser interferometry (both non-intrusive technologies) are again employed in order to maintain the integrity of the etching environment. This information can be used to find correlations and also feed into the model to track proper operating conditions. It is found that a fast, uniform Si and Si02 etch rate could be achieved in the SFg/Argon process by using high RF power density, low pressure, high SFg/Ar ratio. Correlations are developed to directly relate inputs, semi-outputs and outputs in the SFg/Ar system. Response surface methodology (RSM) is used as a basis for further modelling of the non-linear plasma etching process. Results presented in this study compare favourably with the known discharge characteristics, some interpretations of the etching and discharge mechanism and also the comprehensive reactor model. The singular value decomposition (SVD) technique has been applied to determine the pairings between performance quantities, process and manipulated variables. The non-intrusive techniques are also used for dynamic measures of interaction and are found to very rarely change the variable pairings. Step tests are run to determine process variable time constants for use in dynamic process simulation. The SVD pairings with input and output structural compensators designed by using the SVD technique form a multi input-multi output (MIMO) decoupled control system. The robust multivariable control system analysis based on structured uncertainties of inputs and outputs has been formulated as a "block diagonal bounded perturbation" problem (BDBP). The solution to this problem involves the structured singular value (SSV), a generalisation of the singular value decomposition, which is useful for robust multivariable control analysis since the model uncertainty due to the non-linear behaviour of the plasma etching is highly structured. The robust stability and performance properties of the system subject to disturbances and structured perturbations are developed. Results presented show that the closed loop transient responses for SVD pairings with the structurally compensated MIMO control strategies are typically much faster than the conventional scheme. Both of the control strategies satisfy the robustness requirements but the robust stability of conventional control is worse for multiplicative input uncertainties and the structurally compensated scheme is less sensitive to input perturbations