A new local field quantity is presented which gives the high gradient performance limit of accelerating structures due to vacuum rf breakdown. The new field quantity, a modified Poynting vector ${S}_{c}$, is derived from a model of the breakdown trigger in which field emission currents from potential breakdown sites cause local pulsed heating. The field quantity ${S}_{c}$ takes into account both active and reactive power flow on the structure surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both traveling wave and standing wave, and the value of ${S}_{c}$ achieved in the experiments agrees well with analytical estimates.

/pdf/new-local-field-quantity-describing-the-high-gradient-limit-3hyoz4vsgu.pdf

New local field quantity describing the high gradient limit of accelerating structures

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

A novel procedure for the optimization of CLIC main linac parameters including operating frequency and the accelerating gradient is presented. The optimization procedure takes into account both beam dynamics and high power rf constraints. Beam dynamics constraints are given by emittance growth due to short- and long-range transverse wakefields. RF constraints are given by rf breakdown and pulsed surface heating limitations of the accelerating structure. Interpolation of beam and structure parameters in a wide range allows hundreds of millions of accelerating structures to be analyzed to find the structure with the highest ratio of luminosity to main linac input power, which is used as the figure of merit. The frequency and gradient have been varied in the ranges 12-30 GHz and 90-150 MV/m respectively. It is shown that the optimum frequency lies in the range from 16 to 20 GHz depending on the accelerating gradient and that the optimum gradient is below 100 MV/m. Based on our current understanding of the constraints, changing the frequency and gradient from current values of 30 GHz and 150 MV/m to the optimum ones doubles the luminosity for the same main linac input power. Nevertheless, overall extension of the collider and investment cost considerations are not taken into account and impose gradient larger than 100 M/m to 120 MV/m.

/pdf/optimum-frequency-and-gradient-for-the-clic-main-linac-v9zkxki2qd.pdf

Optimum frequency and gradient for the clic main linac

Demonstration of an accelerating gradient of 100 MV/m at a breakdown rate of 10 is one of the key feasibility issues of the CLIC study. A high power RF test program both at X-band (SLAC and KEK) and 30 GHz (CERN) is under way to develop accelerating structures which reach this performance. The centrepiece of the testing program is the nominal CLIC accelerating structure [1], but also includes structures with different RF parameters, with and without wakefield damping waveguides and made with different fabrication technologies. The latter structures address as directly as possible the different aspects of the nominal CLIC structure. The design and objectives of the various Xband and 30 GHz test structures are presented and their fabrication methods and status are reviewed.

/pdf/design-and-fabrication-of-clic-test-structures-4t85hqgt8u.pdf

Design and Fabrication of CLIC Test Structures

The CLIC study is actively investigating a number of different materials in an effort to find ways to increase achievable accelerating gradient. So far a series of rf tests have been made with a set of identical-geometry structures: a W-iris 30 GHz structure, a Mo-iris 30 GHz structure (with pulses as long as 16 ns) and a scaled Moiris X-band structure. A second Mo-iris 30 GHz structure of the same geometry has now been tested in CTF3 with pulse lengths up to 350 ns. The structure was conditioned to a gradient of 140 MV/m with a 70 ns pulse length and a breakdown rate slope of 13 MV/m per decade has been measured.

/pdf/a-high-gradient-test-of-a-30-ghz-molybdenum-iris-structure-5awa1ootcg.pdf

A high-gradient test of a 30 ghz molybdenum-iris structure

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 need for high accelerating gradients for the future 30 GHz multi-TeV e + e - Compact Linear Collider (CLIC) at CERN has triggered a comprehensive study of DC breakdown fields of metals in UHV. The study shows that molybdenum (Mo), tungsten (W), titanium (Ti) and TiVAl reach high breakdown fields, and are thus good candidates for the iris material of CLIC structures. A significant decrease in the saturated breakdown field (Esat) is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of 10 -5 mbar, the decrease in Esat is found to be 50% and 30% for molybdenum and tungsten, respectively. In addition, a 30% decrease is found when molybdenum is conditioned with a CO pressure of ~1·10 -5 mbar. Surface analysis measurements and breakdown conditioning in O2 ambience imply that the origin of the decrease in Esat is closely linked to oxide formation on the cathode surface. ‘Ex-situ’ treatments by ion bombardment of molybdenum effectively reduce the oxide layers, and improve the breakdown characteristics of the metal drastically.

/pdf/investigations-of-dc-breakdown-fields-lhnwjxpz1q.pdf

Investigations of dc breakdown fields

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A high-gradient test of a 30 ghz molybdenum-iris structure

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

Investigations of dc breakdown fields