The ever-growing demand for higher RF gradients has

considerably increased the risk of breakdown in

accelerating structures. Field emission is the most

common form of RF breakdown that generates free

electrons capable of inflicting irreversible damages on the

RF surface. This paper presents a systematic experimental

and simulation programme to understand possible sources

and their influence on RF cavity operation

Kurup, A.

Long, K.

Pozimski, J.K.

Ristic, M.

Seviour, Rebecca

Stables, M.

Zarrebini-Esfahani, A.

English

University of Huddersfield Repository

University of Huddersfield RepositoryZarrebini-Esfahani, A., Seviour, Rebecca, Stables, M., Ristic, M., Kurup, A., Long, K. and Pozimski, J.K.The effects of field emitted electrons on RF surfacesOriginal CitationZarrebini-Esfahani, A., Seviour, Rebecca, Stables, M., Ristic, M., Kurup, A., Long, K. and Pozimski, J.K. (2009) The effects of field emitted electrons on RF surfaces. In: Proceedings of the 23rd Particle Accelerator Conference 4 – 8 May 2009 Vancouver, British Columbia, Canada. JACoW, Vancouver, Canada, pp. 1982-1984.This version is available at http://eprints.hud.ac.uk/15966/The University Repository is a digital collection of the research output of theUniversity, available on Open Access. Copyright and Moral Rights for the itemson this site are retained by the individual author and/or other copyright owners.Users may access full items free of charge; copies of full text items generallycan be reproduced, displayed or performed and given to third parties in anyformat or medium for personal research or study, educational or not-for-profitpurposes without prior permission or charge, provided:• The authors, title and full bibliographic details is credited in any copy;• A hyperlink and/or URL is included for the original metadata page; and• The content is not changed in any way.For more information, including our policy and submission procedure, pleasecontact the Repository Team at: E.mailbox@hud.ac.uk.http://eprints.hud.ac.uk/THE EFFECTS OF FIELD EMITTED ELECTRONS ON RF SURFACE A. Zarrebini-Esfahani#, Mechanical Engineering, Imperial College London, U.K.  R. Seviour, M. Stables, Cockcroft Institute, Lancaster University, Lancaster, U.K.                                 M. Ristic, A. Kurup, K. Long, J. Pozimski, Imperial College London, U.K.  Abstract The ever-growing demand for higher RF gradients has considerably increased the risk of breakdown in accelerating structures. Field emission is the most common form of RF breakdown that generates free electrons capable of inflicting irreversible damages on the RF surface. This paper presents a systematic experimental and simulation programme to understand possible sources and their influence on RF cavity operation.      INTRODUCTION Breakdown which occurs both in RF and DC systems, has been the centre of attention for many year without any general agreement on what triggers this phenomenon [1]. Field emission is the most common problem encountered in superconducting and normal cavities. Grain edges, surface distortions and debris from past breakdown events have been identified as possible emission sites [2, 3]. The presence of defects enhances the local electric field that accelerates emitted electrons. These secondary electrons can cause discontinuities in RF, generate noise, gas burst and damage metallic and insulator surfaces [4]. The heat and stresses exerted by these electrons bombarding the RF surface can lead to surface deformations, creating additional emitting sites [3]. Although breakdown initiates locally, its effects are global.   Currently, the majority of the models focus on surface defects as the only source of emission. In order to develop a better understanding of breakdown, it is vital to study what factors contribute to the formation of such sites, hence lowering the performance of the structure.            PROPOSED RESEARCH PROGRAM This study is in close partnership with the MUCOOL collaboration at Fermilab. Their ultimate goal is to develop muon-cooling systems for Muon Colliders and Neutrino Factory [5].  A series of high power RF tests are performed on button shaped samples using an 805 MHz closed cell pillbox cavity. Further details regarding the MUCOOL Testing Area (MTA) are given in [5-7].   Experimental Study  As demonstrated in figure 3 of [5], tests at the MTA show a striking drop in the supported RF gradient in the 805 MHz cavity when the solenoidal field is applied. This is of high concern for the Neutrino Factory. The quality and condition of the RF surface plays an important role in determining the performance of the accelerating structure. The MTA has been focusing extensively on testing different materials and surface coatings to evaluate their performance when operated within the solenoid field. However, surfaces can be damaged during production and assembly [3]. In order to understand cavity performance and thereby develop better means of production, one needs to look into how fabrication procedure defined the RF surface. This work is a systematic study proposed by the UKRF consortium (Imperial College London, Lancaster University, Cockcroft Institute) aiming to test copper samples fabricated using various techniques.    Experimental Setup The design of the new button consists of a support mandrel and removable cap, allowing the use of a number of forming techniques. The cap sitting on top of the mandrel is the subject for high power testing. Currently, the cap is pressed from a flat oxygen free high thermal conductivity copper (OFHC) sheet.  It is vital to characterise the surface after each step right from material selection up to the end of production. The surface topology and chemical composition of the surface are studied by White light Interferometry and X-ray photo spectrometry (XPS). Scratches introduced during fabrication are removed in steps starting by hand polishing the surface with different grades of sand paper to eliminate larger scratches. A chemical etch followed by an ultrasonic bath would remove grease and larger particles. The surface becomes shiny through electro polishing (EP), using a standard 85% phosphoric acid and 15% butanol mix. Finally, the surface is cleared from any residuals through a high-pressure de-ionised water rinse.   Figure 1: MTA buttons design (left) UKRF button design.  The removal of the damage layer would uncover the virgin copper, ensuring the exposure of the desirable surface to high electric field during testing. A final stage of surface characterisation is conducted once the sample has been tested. By studying the results from each stage of characterisation, it is possible to build a clear picture on how the surface has been changing throughout production. By comparing figures from various ___________________________________________  #a.zarrebini06@ic.ac.uk  WE5PFP001 Proceedings of PAC09, Vancouver, BC, Canada1982Radio Frequency SystemsT06 - Room Temperature RFproduction methods, it would be possible to see which technique causes less damage, hence increasing the performance of the cavity.       Preliminary Results In order to assess how the surface topology is being altered, a series of measurements have been conducted on buttons and flat copper samples taken from the same batch used to form the buttons. These are shown below in table 1 where A, B, C and D refer to modal average of received, mechanical polished, chemical etched and electro polished sample respectively.     Table 1: Surface roughness using Interferometer, modal average taken over 5 samples each case.  Flat Sample Button  Ra (nm) Rq (nm) Ra (nm) Rq (nm) A  106 143 356 459 B 140 194 180 240 C 252 362 220 292 D  93 121 98 120  Ra and Rq are the average surface roughness and the RMS roughness. The further Ra is from Rq, the more pronounced the defects. The pressed cap has a much rougher surface when received compared to flat samples, even when no machining has been performed. This demonstrates the fact that working the metallic surface to create the desired shape alters the surface topology. Figure 2 shows interferometer images of the altering metallic surface in the first and last stages of the surface preparation procedure.         Figure 2: Interferometer images of OFHC copper [8].    The surface is levelled by the phosphoric acid present in the EP mix, while butanol is responsible for providing a greater control over the solution’s conductivity. It is known that electro polishing with an 85% phosphoric acid gives rise to oxygen formation. This leads to creation of sever etched pits on the surface [9]. Oxygen formation can be minimised by careful control over the EP current/voltage, maintaining the anode near the critical cusp point as illustrated in figure 3 [10]. It is essential to maintain this profile as steady as possible to minimise alterations in the final polish.    Figure 3: Characteristic plot for electro polishing.  The XPS investigations of figure 4 show fascinating changes in the chemical composition of Cu surface layers. We see a 96% reduction in carbon and oxygen contaminations after mechanical polishing followed by an additional drop of 50% through chemical etching. However, this process was reversed after EP with a 25 and 100 fold increases in carbon and oxygen respectively, due to the chemistry used. More interestingly is the addition of P impurities from EP, bonding in the surface layer to the Cu via 3S and 2P3/2.    Figure 4: Binding energies of electrons released through XPS of OFHC copper sample [8].  Surfaces, depending on band structure, density of states (DoS) and environmental conditions, are capable of emitting electrons. Secondary Electron yield (SEY) describes the number of electrons emitted from a surface due to the impact of an incident electron. To gain an insight into this behaviour we used the numerical package DMol to calculate variational self consistent solutions to the density functional theory (DFT) equations [11], expressed in a numerical atomic orbital basis, so that we could represent the bonding seen experimental of the P. The solutions to these equations gave the wave functions and electron densities that we used to study the effects of P bonding on the band structure and Dos. The results are shown in figure 5, where we represented the system as an infinite slab of Cu with P introduced into the surface layer. These simulations indicate that the P impurities increase the Dos and band structure, causing overlapping. This indicates a higher ability to stream electrons from the material surface, leading to unpredictable behaviour.    Proceedings of PAC09, Vancouver, BC, Canada WE5PFP001Radio Frequency SystemsT06 - Room Temperature RF 1983 Figure 5: DFT simulations of copper with and without P impurity. Dos (left), band structure (right), dotted lines show the Fermi energy, units are in Hartees [8].   Simulation Study  In additional to the above work, it is important to simulate how free electrons can initiate breakdown depending on the way surface defects alter the electric (E) and magnetic (B) field profile of a cavity.  We use Comsol Multiphysics to create a 3D profile of E and B fields of a cavity. A major advantage of working in 3D is the possibility to place various asperities with different shapes and orientations anywhere in the model. Figure 6 demonstrates a cross sectional view of the E filed profile in an 805 MHz cavity. As expected, local field enhancements are observed around the defect.   The electrons behaviour is being studied using a homegrown particle tracking code that uses Matlab to extract the E and B fields from Comsol at any point in space. Being able to communicate directly with Comsol eliminates the need for pre-defined grid points. This allows precise extraction of parameters in order to track electrons from the emission site up to the point of impact.          Figure 6: E field enhancement due to presence of defect on a flat surface in an 805 MHz cavity.  FUTURE PLAN  Although we are just beginning, we have some initial results showing how the manufacturing procedure affects the RF surface. This highlights the importance of the proposed study, which should provide vital information regarding manufacturing and assembly. This can lead to the development of more suitable and efficient techniques at later stages. We intend to expand this effort by looking at additional manufacturing techniques. Eventually, we aim to determine whether these electrons leave the systems or simply are trapped into the RF surface, causing secondary emissions. By examining various parameters at the point of impact, it would be possible to determine the level of heat and stresses generated.     REFERENCES  [1] Norem, J., Z. Insepov, and I. Konkashbaev, Triggers for RF breakdown. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005. 537(3): p. 510-520. [2] Norem, J., A. Hassanein, and I. Konkashbaev. Mechanisms limiting high gradient rf cavities. in Particle Accelerator Conference, 2003. PAC 2003. . 2003. [3] Norem, J. and M. Pellin. Gradient Limits And SCRF Performance.   [cited; Available from: http://web5.pku.edu.cn/srf2007/download/proceedings/TUP08.pdf. [4] Seviour, R., The Role of Elastic and Inelasting Electron Reflection in Multipactor Discharges. IEEE Transactions on Electron Devices, 2005. 52(8): p. 1927 - 1930. [5] Norem, J., et al. Recent RF Results From The MUCOOL Test Area. in Particle Accelerator Conference. 2007. Albuquerque - New Mexico: IEEE. [6] Johnstone, C. MUCOOL Test Area At FermiLab. in Particle Accelerator Conference. 2005. Knoxville, Tennessee: IEEE. [7] Norem, J., et al. The RF Experimental Program In The FermiLab MUCOOL Test Area. in Particle Accelerator Conference 2005. Knoxville, Tennessee: IEEE. [8] Seviour, R., et al. UKNF note-44. [9] Shih-Chieh, C., et al., Superpolishing for Planarizing Copper Damascene Interconnects. Electrochemical and Solid-State Letters, 2003. 6(5): p. G72-G74. [10] Jacquet, P.A., in Metal Finishing 1949. p. 62 - 69. [11] Kieron, B., W. Jan, and E.K.U. Gross, Time-dependent density functional theory: Past, present, and future. The Journal of Chemical Physics, 2005. 123(6): p. 062206. WE5PFP001 Proceedings of PAC09, Vancouver, BC, Canada1984Radio Frequency SystemsT06 - Room Temperature RF

Gradient Limits And SCRF Performance. [cited; Available from: http://web5.pku.edu.cn/srf2007/download/proceeding s/TUP08.pdf.

in Metal Finishing

Mechanisms limiting high gradient rf cavities.

MUCOOL Test Area At FermiLab. in Particle Accelerator Conference.

Recent RF Results From The MUCOOL Test Area.

Superpolishing for Planarizing Copper Damascene Interconnects. Electrochemical and Solid-State Letters,

The effects of field emitted electrons on RF surfaces Original Citation

The RF Experimental Program

The Role of Elastic and Inelasting Electron Reflection in Multipactor Discharges.

Timedependent density functional theory: Past, present, and future.

Triggers for RF breakdown. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,

The effects of field emitted electrons on RF surfaces

The effects of field emitted electrons on RF surfaces

Abstract

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