The compact linear collider (CLIC) is a leading candidate for the next generation high energy linear collider. As any breakdown would result in a partial or full loss of luminosity for the pulse in which it occurs, obtaining a low breakdown rate in CLIC accelerating structures is a critical requirement for the successful operation of the proposed collider. This thesis presents investigations into the breakdown phenomenon primarily in the low breakdown rate regime of interest to CLIC, performed using the CERN DC spark systems between 2011 and 2014.The design, construction and commissioning of several new pieces of hardware, as well as the development of improved techniques to measuring the inter-electrode gap distance are detailed. These hardware improvements were fundamental in enabling the exciting new experiments mentioned below, which in turn have provided significant additional insight into the phenomenon of breakdown.Experiments were performed o measure fundamental parameters of individual breakdowns, including, the turn-on time and the delay before breakdown in order to gain an improved understanding of how breakdowns are triggered and the underlying process behind them. The turn-on time measurements are the highest bandwidth measurements made to date with the CERN DC systems and are closer than ever before to the value which is expected from the present understanding of breakdown simulations.Another key measurement was that of the breakdown rate scaling with electric field. Previous investigations of this relationship in the DC systems were unable to investigate breakdown rates below 10^3 breakdowns per pulse. These new results are able to investigate this relationship down to a breakdown rate of 10^-8 and are hence a considerable improvement. Thanks to these improved results a remarkable similarity to the scaling of the breakdown rate with electric field in RF cavities was discovered. The conditioning, or change in breakdown rate over time was also studied for the first time in the CERN DC spark systems as well as the newly built fixed gap system. The qualitative conditioning behaviour of the Fixed Gap System again showed interesting similarities to that observed in RF structures.Preliminary studies into the effect of pulse length and magnetic field on the breakdown rate were conducted as well. This is the first time the effect of a DC magnetic field was studied in a DC spark system and in contrast to experiments in RF cavities no statistically significant effect was observed. The dependence of the breakdown rate on pulse length, again the first measurement of its kind in a DC system also revealed a similar scaling law to that observed in RF accelerating structures. Both of these preliminary measurements would need to be repeated to confirm the results.

Experimental study of DC vacuum breakdown and application to high-gradient accelerating structures for CLIC

The CLIC study is high power testing accelerating structures in a number of different materials and accelerating structure designs to understand the physics of breakdown, determine the appropriate scaling of performance and in particular to find ways to increase achievable accelerating gradient. The most recent 30 GHz structures which have been tested include damped structures in copper, molybdenum, titanium and aluminum. The results from these new structures are presented in this paper.

/pdf/30-ghz-high-gradient-accelerating-structure-test-results-45t2nnixci.pdf

30 GHz high-gradient accelerating structure test results

The CLIC study group at CERN has built two X-band HDS (hybrid damped structure) accelerating structures for high-power testing in NLCTA at SLAC. These accelerating structures are novel with respect to their rf- design and their fabrication technique. The eleven-cell constant impedance structures, one made out of copper and one out of molybdenum, are assembled from clamped high-speed milled quadrants. They feature the same heavy higher-order-mode damping as nominal CLIC structures achieved by slotted irises and radial damping waveguides for each cell. The X-band accelerators are exactly scaled versions of structures tested at 30 GHz in the CLIC test facility, CTF3. The results of the X-band tests are presented and compared to those at 30 GHz to determine frequency scaling, and are compared to the extensive copper data from the NLC structure development program to determine material dependence and make a basic validation of the HDS design.

/pdf/high-power-test-of-an-x-band-slotted-iris-accelerator-24u2ldk1nw.pdf

High power test of an X-band slotted-iris accelerator structure at NLCTA

In order to build the Compact LInear Collider (CLIC), accelerating structures reaching extremely high accelerating gradients are needed. Such structures have been built and tested using normal-conducting copper, powered by X-band RF power and reaching gradients of 100 MV/m and above. One phenomenon that must be avoided in order to reliably reach such gradients, is vacuum arcs or “breakdowns”. This can be accomplished by carefully designing the structure geometry such that high surface fields and large local power flows are avoided. The research presented in this thesis presents a method for optimizing the geometry of accelerating structures so that these breakdowns are made less likely, allowing the structure to operate reliably at high gradients. This was done primarilly based on a phenomenological scaling model, which predicted the maximum gradient as a function of the break down rate, pulse length, and field distribution in the structure. The model is written in such a way that it allows direct comparison of different criteria, i.e. the peak electric field, the peak local power flow or “modified Poynting vector” Sc, and the global power flow per iris circumference. Using this method, a set of highly optimized accelerating cells were created, as was a C++ library capable of estimating the performance of an RF structure based on these accelerating cells. This library was used in the rebaselinging of the CLIC machine. In addition to this, the thesis also presents a particle in cell simulation of the initial stages of a vacuum arc. This model follows the development of the arc from a small field emitter into a multi-ampere discharge, tracking the evolution of the plasma and the circuit, as well as the interaction between the plasma and the surface. From this model, our understanding of the importance of the ion bombardment under the plasma sheath was improved, especially the ion energy distribution and its effect on sputtering vs. the effective sputtering yields required for the arc to sustain itself and grow.

Avoiding vacuum arcs in high gradient normal conducting RF structures

The CLIC study is investigating a number of different materials at different frequencies in order to find ways to increase achievable accelerating gradient and to understand what are the important parameters for high-gradient operation So far a series of rf tests have been made with a set of identical-geometry 30 GHz and X-band structures in copper, tungsten and molybdenum A new test of a 30 GHz copper accelerating structure has been completed in CTF3 with pulse lengths up to 70 ns The new results are presented and compared to the previous structures to determine dependencies of quantities such accelerating gradient, material, frequency, pulse length, conditioning rate, breakdown rate and surface damage

/pdf/a-high-gradient-test-of-a-30-ghz-copper-accelerating-3vpnpr8ghc.pdf

A high-gradient test of a 30 ghz copper accelerating structure

The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of molybdenum and tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with short pulses (16 ns) in the CLIC Test Facility at CERN. At 30 GHz these structures reached gradients of 150 MV/m for tungsten and 195 MV/m for molybdenum. These experiments were designed to provide data on the dependence of rf breakdown on pulse length and frequency. This paper reports in particular on the high-gradient test of the tungsten-iris structure. At the shortest possible pulse length of 22 ns a gradient of 125 MV/m was reached at X-band, 20 % lower than the 150 MV/m measured at 30 GHz in the CLIC Test Facility. The pulse length dependence and the dependence of the break down rate as a function of gradient were measured in detail. The results are compared to data obtained from the molybdenum-iris experiment at X-band which took place earlier as well as to 30 GHz data.

/pdf/high-gradient-test-of-a-tungsten-iris-x-band-accelerator-3ktrhl54pk.pdf

High-gradient test of a tungsten-iris x-band accelerator structure at nlcta

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High-gradient test of a tungsten-iris x-band accelerator structure at nlcta