Niobium superconducting rf cavity fabrication by electrohydraulic forming

Abstract

Superconducting rf (SRF) cavities are traditionally fabricated from superconducting material sheets or made of copper coated with superconducting material, followed by trim machining and electron-beam welding. An alternative technique to traditional shaping methods, such as deep-drawing and spinning, is electrohydraulic forming (EHF). In EHF, half-cells are obtained through ultrahigh-speed deformation of blank sheets, using shockwaves induced in water by a pulsed electrical discharge. With respect to traditional methods, such a highly dynamic process can yield interesting results in terms of effectiveness, repeatability, final shape precision, higher formability, and reduced springback. In this paper, the first results of EHF on high purity niobium are presented and discussed. The simulations performed in order to master the multiphysics phenomena of EHF and to adjust its process parameters are presented. The microstructures of niobium halfcells produced by EHF and by spinning have been compared in terms of damage created in the material during the forming operation. The damage was assessed through hardness measurements, residual resistivity ratio (RRR) measurements, and electron backscattered diffraction analyses. It was found that EHF does not worsen the damage of the material during forming and instead, some areas of the half-cell have shown lower damage compared to spinning. Moreover, EHF is particularly advantageous to reduce the forming time, preserve roughness, and to meet the final required shape accuracy.

Examples of How Increased Formability through High Strain Rates Can Be Used in Electro-Hydraulic Forming and Electromagnetic Forming Industrial Applications

“…EHF has been explored for the production of several Superconducting Radio Frequency half cavities for CERN of 400 MHz, 800 MHz, and 1300 MHz. The main results have been described by Atieh [24] and Cantergiani [25,26]. The 400 MHz frequency cavities made of electronic oxygen-free electronic (OFE) copper are used for the large hadron collider (LHC) and will also be used a priori for the future circular collider (FCC) that was initially foreseen at 700-800 MHz using high purity bulk niobium.…”

“…For such large parts, multiple shots are required to get the final geometry. OFE copper and Niobium constitutive laws have been shown to be very sensitive to strain rates for a few 10 3 s −1 [26]. Thus, proper material models are mandatory as input in simulations to optimize the EHF process parameters, such as the energy level, the number of electrode systems, and the number of successive discharges.…”

“…Figure 15a shows the EHF set-up used to produce the 400 MHz half cells of Figure 15b, and examples of 700 MHz half-cells are given made of OFE copper in Figure 15c and niobium in Figure 15d. The shape accuracy of 400 MHz half-cells formed by EHF was compared at CERN with the shape accuracy of 400 MHz half cells obtained by spinning, including an intermediate annealing step and complete machining of the inner surface [30]. The latter were obtained starting from a blank of 4 mm in order to have enough material for the final machining step to achieve the required inner shape.…”

“…The thicknesses of the formed 400 MHz half-cell were extracted for both EHF and spinning and are shown in Figure 16. Thickness distribution for 400 MHz copper half-cells obtained through EHF or through spinning followed by machining [26].…”

Record high-gradient SRF beam acceleration at Fermilab

“…Over the past three decades, since pioneering realizations at CEBAF at JLab, TRISTAN at KEK and LEP-II at CERN in 1980s, the science and technology of superconducting radio-frequency (SRF) beam acceleration have matured and tremendously advanced. Widespread employment of SRF to improve the performance of existing accelerators and construction of new facilities is mostly due to advantages of the technology such as extremely low RF losses in the cavity walls at cryogenic temperatures, high wall plug to beam power conversion efficiency, possibility of long beam pulse operating modes, reduced interaction of the beam with larger aperture cavities leading to low impedance for high current beams, and high accelerating gradient, E acc [1][2][3][4][5][6]. Table 1 presents key parameters for major linear accelerators based on the pulsed SRF, such as the Spallation Neutron Source at ORNL [7], the European Spallation Source in Sweden [8], the 'Proton Improvement Plan-II' (PIP-II) linac at Fermilab [9], the European X-FEL at DESY [10], MARIE FEL at LANL [11] and the proposed International Linear Collider (ILC) in Japan [12].…”