ANL, Lemont, Illinois, USA
The vacuum level in a 2 mm diameter, 0.5 m-long copper corrugated waveguide tube proposed* for a compact high repetition rate wakefield accelerator has been investigated. The analytical calculations have been found to be in good agreement with a result of computer modeling using a finite element method. A representative experiment has been conducted using a smooth copper tube with the same diameter as the corrugated tube and a 1/3 length of the corrugated tube. The vacuum level calculated for this experiment agrees well with the measurement.
*A. Zholentset et al., inProc. 9th International Particle Accelerator Conference (IPAC’18), Vancouver, BC, Canada, 29 April-04 May 2018, ser. IPAC Conference, pp. 1266’1268.
Translume, Inc., Ann Arbor, Michigan, USA
UW-Madison, Madison, Wisconsin, USA
ANL, Lemont, Illinois, USA
A cylindrical, corrugated wakefield accelerating (CWA) structure is being designed to facilitate sub-terahertz Cerenkov radiation produced by an electron bunch propagating in a waveguided structure comprising accelerating sections and transition sections*. The accelerating structure consists of several copper-based 50-cm long sections of internally corrugated tubes with 2-mm inner-diameter. These sections are coupled together using transition sections, which are also copper-based. The transition section has a main body diameter ranging from 2mm to 3.2mm and its length is about 14mm. Two sets of four orthogonal waveguides radiate from the central body. Beside their mechanical coupling function, these transition sections provide for periodic monitoring of the centering of the electron bunch, and for removal of unwanted higher-order EM modes. The fabrication of these transition sections is presented. The fabrication process is based on the use of a sacrificial fused silica glass mandrel, whose body corresponds to the inner volume of the copper element. This fused silica mandrel is subsequently electroplated. The micro-fabrication of a prototype of the transition section is underway. Modelling of various fabrication errors was undertaken to understand their effect and to determine tolerances. Source of machining imperfections are reviewed and their impact compared to the modelling results.
*A. Zholents et al., "A conceptual design of a Compact Wakefield Accelerator for a high repetition rate multi user Xray Free-Electron Laser Facility," in Proc. 9th Int.l Particle Accel. Conf., 2018
ANL, Lemont, Illinois, USA
RadiaBeam, Santa Monica, California, USA
Fabrication of the corrugated structure that generates a field gradient 100 m-1 at 180 GHz is challenging and required an unconventional method of production. The corrugated waveguide with 2 mm inner diameter will be produced by electroplating copper on the aluminum mandrel as proposed in the reference*. A thin seed layer is usually applied to achieve uniform wetting to plate copper on the aluminum mandrel. The copper waveguide is retrieved by removing aluminum and the seed layer. Therefore, uniform copper plating and etching of the seed layer and the Aluminum mandrel is a crucial step to keep the surface free of impurities that are especially necessary for the RF application. Previous studies suggest that electroplated copper has variations in both electrical and mechanical properties compared with those of bulk copper from the batches of production. In this paper, we discuss the copper microstructure produced by the electroforming method and literature study on the variations, which can be attributed to the disparity of the crystallinity of grains structure in plated material.
*A. Zholentset al., "A Conceptual Design of a Compact Wakefield Accelerator for a High Repetition Rate Multi-User X-ray Free-Electron Laser Facility, "in Proc. IPAC 18, 2018, pp. 1266-1268.
ANL, Lemont, Illinois, USA
UW-Madison/PD, Madison, Wisconsin, USA
An effort to build Argonne’s Sub-THz AcceleRator (A-STAR) for a future multiuser x-ray free-electron laser facility proposed in [1] is underway at Argonne National Laboratory. The A-STAR machine will utilize a compact collinear wakefield accelerator (CWA) assembled in modules. To extract the wakefield and monitor beam position downstream of each module, a 45-mm-long transition section (TS) has been proposed and designed. This paper will discuss the design and fabrication chal-lenges for production of the TS.
*A. Zholents et al., "A conceptual design of a Compact Wakefield Accelerator for a high repetition rate multi user Xray Free-Electron Laser Facility," in Proc. IPAC’18, Canada, 2018, pp. 1266-1268.
ANL, Lemont, Illinois, USA
UW-Madison/PD, Madison, Wisconsin, USA
Argonne National Laboratory is developing a Sub-THz AcceleRator (A-STAR) for a future multiuser x-ray free electron laser facility. The A-STAR machine will utilize a compact collinear wakefield accelerator (CWA) based on a miniature copper (Cu) corrugated waveguide as proposed*. The accelerator is designed to operate at a 20-kHz bunch repetition rate and will utilize the 180-GHz wakefield of a 10-nC electron drive bunch with a field gradient of 100 MVm’1 to accelerate a 0.3-nC electron witness bunch to 5 GeV. In this paper, we discuss specific challenges in the mechanical design of the CWA vacuum chamber module. The module consists of series of small quadrupole magnets with a high magnetic field gradient that houses a 2-mm diameter and 0.5-m-long corrugated tubing with brazed water-cooling channels and a transition section. The 45-mm-long transition section is used to extract the wakefield and to house a beam position monitor, a bellows assembly and a port to connect a vacuum pump. The CWA vacuum chamber module requires four to five brazing steps with filler metals of successively lower temperatures to maintain the integrity of previously brazed joints.
*A. Zholents et al., "A conceptual design of a Compact Wakefield Accelerator for a high repetition rate multi user Xray Free-Electron Laser Facility," in Proc. IPAC’18, Canada, 2018, pp. 1266~1268.
ANL, Lemont, Illinois, USA
UW-Madison/PD, Madison, Wisconsin, USA
Thermal stresses generated due to the electromagnetic (EM) heating is a defining phenomenon in the mechanical design of the miniature copper-based corrugated wakefield accelerator (CWA). We investigate the effect of the EM heating due to the high repetition rate electron bunches traveling through a corrugated tube with 1-mm-inner-radius. The steady-state thermal analysis is coupled with computational fluid dynamics, and structural mechanics to determine the thermal effect on the operating conditions of CWA. It could carry a 10 nC drive bunch through the center of corrugated structure that generates a field gradient 100 Mv/m at 180 GHz, accelerating a trailing 0.3 nC witness bunch to 5 GeV. The wakefield produced by the traveling bunches can deposit about 600 W to 3000 W of energy on the inner wall of the device. Also, the instabilities in e-beam trajectories caused by thermal expansion, and the resulting stresses associated high-frequency repetition rate of 10 kHz to 50 kHz are the main concern for the waveguide. Tensile-yield failure due to moderate heating on the surface of the <200 micrometer wide trough regions of the corrugated tube may lead to arcing and loss of the wakefield.
