Vacuum arc localization in CLIC prototype radio frequency accelerating structures

A future linear collider capable of reaching TeV collision energies should support accelerating gradients beyond 100 MV/m. At such high fields, the occurrence of vacuum arcs have to be mitigated through conditioning, during which an accelerating structure’s resilience against breakdowns is slowly increased through repeated radio frequency pulsing. Conditioning is very time and resource consuming, which is why developing more efficient procedures is desirable. At CERN, conditioning related research is conducted at the CLIC high-power X-band test stands. Breakdown localization is an important diagnostic tool of accelerating structure tests. Abnormal position distributions highlight issues in structure design, manufacturing or operation and may consequently help improve these processes. Additionally, positioning can provide insight into the physics of vacuum arcs. In this work, two established positioning methods based on the time-difference-ofarrival of radio frequency waves are extended. The first method is based on signal edge detection and the second on cross-correlation. The methods are parametrized and a bias model for the edge method is developed. The localization precision of the methods is also quantified. Under certain conditions, the correlation method is demonstrated to achieve a precision of less than one accelerating cell. The methods are applied to data collected from four CLIC prototype structures: three constant gradient accelerating structures, the T24, T24 open and TD26CC, and one constant impedance deflecting structure, the CLIC Crab Cavity. The TD26CC and Crab Cavity operated as expected, whereas the T24 and T24 open developed hot cells close to the RF input. The T24 open continued conditioning despite the hot cell. Furthermore, evidence of breakdown migration was found when comparing the two positioning methods. It was also discovered that consecutive breakdowns occurring close to each other in time also occur close to each other in space

Sparse Symmetric Linear Arrays with Low Redundancy and a Contiguous Sum Co-Array

Sparse arrays can resolve significantly more scatterers or sources than sensor by utilizing the co-array - a virtual array structure consisting of pairwise differences or sums of sensor positions. Although several sparse array configurations have been developed for passive sensing applications, far fewer active array designs exist. In active sensing, the sum co-array is typically more relevant than the difference co-array, especially when the scatterers are fully coherent. This paper proposes a general symmetric array configuration suitable for both active and passive sensing. We first derive necessary and sufficient conditions for the sum and difference co-array of this array to be contiguous. We then study two specific instances based on the Nested array and the Klove-Mossige basis, respectively. In particular, we establish the relationship between the minimum-redundancy solutions of the two resulting symmetric array configurations, and the previously proposed Concatenated Nested Array (CNA) and Klove Array(KA). Both the CNA and KA have closed-form expressions for the sensor positions, which means that they can be easily generated for any desired array size. The two array structures also achieve low redundancy, and a contiguous sum and difference co-array, which allows resolving vastly more scatterers or sources than sensors.Peer reviewe

Near Field Active Imaging Using Sparse Arrays

Sensor arrays designed for far field operation may experience performance degradation when imaging near field objects. Specifically, sparse active arrays utilizing the additional degrees of freedom provided by the sum co-array are susceptible to these effects, as the co-array depends on both the range and direction of scatterers close to the array. Consequently, a uniform far field sum co-array may become non-uniform in the near field. As a result, co-array processing algorithms, such as image addition, are subject to undesired grating lobes in the presence of near field scatterers. This paper proposes an extension to image addition for mitigating such undesired distortions. The method compensates for near field effects by computing spatially varying transmit and receive element weights. These weights minimize the discrepancy between the desired and achieved near field point spread function, while using as few image addition components as possible. Given a desired point spread function and a set of calibration measurements of the near field array steering vectors, a regularized convex optimization problem is then solved for each pixel of the image.Peer reviewe

Symmetric sparse linear array for active imaging

Sparse sensor arrays can achieve significantly more degrees of freedom than the number of elements by leveraging the co-array, a virtual structure that arises from the far field narrowband signal model. Although several sparse array configurations have been developed for passive sensing tasks, less attention has been paid to arrays suitable for active sensing. This paper presents a novel active sparse linear array, called the Interleaved Wichmann Array (IWA). The IWA only has a few closely spaced elements, which may make it more robust to mutual coupling effects. Closed-form expressions are provided for the key properties of the IWA. The parameters maximizing the array aperture for a given even number of elements are also found. The near field wideband performance of the array is demonstrated numerically in a coherent imaging scenario.Peer reviewe

Co-array Music under Angle-Independent Nonidealities

Publisher Copyright: © 2020 IEEE.The difference co-array is crucial in determining the number of resolvable sources in direction-of-arrival (DoA) estimation. This virtual array of pairwise sensor position differences enables sparse arrays to identify vastly more sources than sensors. However, the idealized assumptions giving rise to the co-array, such as isolated omnidirectional gain patterns, may not hold in practice. Consequently, the applicability of the co-array model to real-world arrays needs to be investigated thoroughly. In this work, we consider a general class of angle-independent departures from the ideal model caused by nonideal sensors or compression of the array measurements. We study the impact of these nonidealities on DoA estimation using co-array MUSIC, assuming that the array is calibrated and that an infinite number of snapshots is available. We establish that proper use of the calibration data enables unbiased DoA estimation of more sources than sensors. Nonidealities may nevertheless cause subspace swap at low SNR.Peer reviewe

Breakdown localization in the fixed gap system

Accurate localization of breakdowns in vacuum could help shed light on breakdown related processes that are not yet fully understood. At the DC spark lab at CERN, an instrument called the Fixed Gap System (FGS) has been developed partially for this purpose. Among other things, the FGS has four built-in antennas, which are intended for breakdown localization. The capability of this aspect of the FGS was explored in this report. Specifically, the feasibility of using a method similar to that which is used in cavity Beam Position Monitors (BPMs) was investigated. The usable frequency range of the current experimental setup was also studied. Firstly, a modal analysis of the inner geometry of the FGS was done in HFSS. This showed that the two first modes to be expected in the spark gap quite differ from those of the ideal pillbox – both in field pattern and in frequency ( 4 and 6 GHz vs. 0.2 and 3 GHz). Secondly, S-parameters of the system were measured. These showed that the coupling between antennas is weak below 13 GHz, which is due to the high cut-off frequency of the waveguides in which the antennas are located. The breakdown signal was also measured using an oscilloscope connected to the antennas. However, it was determined that the detected signal was picked up from outside of the system, rendering it useless for localization purposes. It was concluded that either a new approach has to be adopted or the current system must be modified

Results of the Beam-Loading Breakdown Rate Experiment at the CLIC Test Facility CTF3

The RF breakdown rate is crucial for the luminosity performance of the CLIC linear collider. The required breakdown rate at the design gradient of 100 MV/m has been demonstrated, without beam presence, in a number of 12 GHz CLIC prototype structures. Nevertheless, the beam-loading at CLIC significantly changes the field profile inside the structures, and the behaviour with beam needs to be understood. A dedicated experiment in the CLIC Test Facility CTF3 to determine the effect of beam on the breakdown rate has been collecting breakdown data throughout the year 2016. The complete results of the experiment and the effect of the beam-loading on the breakdown rate are presented

Beam-Loading Effect on Breakdown Rate in High-Gradient Accelerating Structures

The Compact Linear Collider (CLIC) study for a future electron-positron collider with a center-of-mass energy up to 3 TeV aims for an accelerating gradient of 100 MV/m. The gradient is limited by RF breakdowns, and the luminosity requirements impose a limit on the admissible RF breakdown rate. RF testing of 12 GHz structure prototypes has shown that gradients in excess of 100 MV/m can be reached with the required breakdown rate. However at CLIC, the structures will be operated with significant beam-loading, modifying the field distribution inside. The effect of the beam-loading must be well understood but has not been previously measured. The commissioning and operation of an experiment to measure the effect of beam-loading on breakdown rate and the measurement results are presented

Fabrication and high-gradient testing of an accelerating structure made from milled halves

Accelerating structures made from parts which follow symmetry planes offer many potential advantages over traditional disk-based structures: more options for joining (from bonding to welding), following this more options for material state (heat treated or not) and potentially lower cost since structures can be made from fewer parts. An X-band structure made from milled halves, and with a standard benchmarked CLIC test structure design has been fabricated and high-gradient tested in the range of 100 MV/m.Accelerating structures made from parts which follow symmetry planes offer many potential advantages over traditional disk-based structures: more options for joining (from bonding to welding), following this more options for material state (heat treated or not) and potentially lower cost since structures can be made from fewer parts. An X-band structure made from milled halves, and with a standard benchmarked CLIC test structure design has been fabricated and high-gradient tested above 95 MV/m