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

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

The CLIC project is based on normal-conducting high-gradient accelerating structures with an average accelerating gradient of 100 MV/m. The maximum achievable gradient in these structures is limited by the breakdown phenomenon. The physics of breakdowns is not yet fully understood quantitatively. A full knowledge could have strong impact on the design, material choice and construction of rf structures. Therefore, understanding breakdowns has great importance to reaching a gradient of 100MV/m with an acceptable breakdown probability. This thesis addresses the physics underlying the breakdown effect, focusing on a comparison of breakdowns in rf structures and in a dc spark setup. The dc system is simpler, easier to benchmark against simulations, with a faster turnaround time, but the relationship to rf breakdown must be established. To do so, an experimental approach based on optical diagnostics and electrical measurements methods was made. Following an introduction into the CLIC project, a general theoretical introduction into accelerating structures and the presentation of the state of the art breakdown model is given. Afterwards, the results of the breakdown experiments are presented together with the instruments and facilities used. As an initial measurement, spectra of breakdowns in copper rf structures and on copper dc samples were taken in the wavelength range from 300nm to 1000nm. The typical emission time measured is aro und 3 μs. These spectra both show a copper plasma emission spectrum with identified ionization levels up to CuIII and a continuum background which carries about 75% of the total integrated light emission. No significant contribution from other elements was found in rf nor dc. Plasma temperature calculations from these spectra show inconsistent results: depending on the choice of lines, the values spread between 0.3 eV and 1.3 eV. Therefore, the plasma is likely to be a non-LTE plasma. No qualitative differences between rf and dc were found with these experiments. Emerging from the spectroscopy studies, light emission from the spark gap in the dc setup was observed in the case when no breakdown was triggered. This light was identified as an optical transition radiation (OTR), emitted by the field-emitted electrons hitting the copper surface. The measured OTR light intensity was found to be usable to measure the field enhancement factor of the field emitter on the sample surface at high electric field gradients near the breakdown limit where electric measurements could not be applied because of the risk of destruction of the electronics. The same OTR light emission was found also for a 30GHz rf structure. In order to get more insight into the time development of the breakdown plasma, time-resolved spectra were taken in rf and dc. It was found that the light emission of both continuum and lines consistently separate into two part s: an about 10 ns wide first, sharp peak which has to be attributed to the breakdown trigger mechanism, followed by a long intensity build-up and decay with a peak after about 1 μs after the triggering of the breakdown. Directly after the first peak, the intensity drops to practically zero. At the wavelengths of copper emission lines, this intensity of the second long peak is superimposed by peaks of different width, depending on the ion species. It is concluded, that the processes during a breakdown are highly dynamic in plasma temperature and density, not only in time, but also in spatial distribution of the plasma. Time resolved emission line waveforms show the same overall structure in both rf and dc, again indicating strong similarity in the phenomenon. In addition, the electrical characteristics of the dc spark were measured. It was found that the resistance of the spark goes to zero within less than 10 ns, showing that the plasma is a very good conductor. This limits the amount of energy dissipated by the spark. 30GHz breakdowns revealed a flat distribution of energy dissipated by the breakdown at constant input energy, reaching up to full rf energy loss. This different behavior can be explained by energy losses due to acceleration and loss of electrons and ions, which in the dc case still contribute to the current through the plasma. This effect is therefore a secondary effect and does not implicate a difference in the physics of rf and dc breakdowns. Summarizing, it can be concluded that rf and dc breakdowns are similar in their underlying physics in the scope of the experiments presented here.

/pdf/comparative-studies-of-high-gradient-rf-and-dc-breakdowns-vvgm0rjpdw.pdf

Comparative Studies of High-Gradient Rf and Dc Breakdowns

We present single-cell accelerating structures designed for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures are π-mode standing-wave cavities fed with a TM 01 circular waveguide. The structures are fabricated using precision milling out of two metal blocks, and the blocks are joined with diffusion bonding and brazing. The impact of fabrication and joining techniques on the cell geometry and RF performance will be discussed. First prototypes had a measured Q 0 of 2800, approaching the theoretical design value of 3300. The geometry of these accelerating structures are as close as practical to singlecell standing-wave X-band accelerating structures more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. Furthermore, the structures will be powered with short pulses from a MW gyrotron oscillator. RF power of 1 MW may allow an accelerating gradient of 400 MeV/m to be reached.

Prototyping High-Gradient mm-Wave Accelerating Structures

We present the design and parameters of single-cell accelerating structures for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures consist of π-mode standing-wave cavities fed with the TM01 circular waveguide mode. The geometry and field shape of these accelerating structures is as close as practical to single-cell standingwave X-band accelerating structures, more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. The structures will be powered from a pulsed MW gyrotron oscillator. One MW of RF power from the gyrotron may allow us to reach a peak accelerating gradient of 400 MeV/m.

mm-Wave Standing-Wave Accelerating Structures for High-Gradient Tests

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

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30 GHz high-gradient accelerating structure test results