Additive Manufacturing of Side-Coupled Cavity Linac Structures from Pure Copper: A First Concept

Abstract

Compared to conventional manufacturing, additive manufacturing (AM) of radio frequency (RF) cavities has the potential to reduce manufacturing costs and complexity and to enable higher performance. This work evaluates whether normal conducting side-coupled linac structures (SCCL), used worldwide for a wide range of applications, can benefit from AM. A unit cell geometry (SC) optimized for 75 MeV protons was developed. Downskins with small downskin angles α were avoided to enable manufacturing by laser powder bed fusion without support structures. SCs with different α were printed and post-processed by Hirtisation (R) (an electrochemical process) to minimize surface roughness. The required accuracy for 3 GHz SCCL (medical linacs) is achieved only for α>45∘. After a material removal of 140 µm due to Hirtisation (R), a quality factor Q0 of 6650 was achieved. This corresponds to 75% of the Q0 simulated by CST®. A 3 GHz SCCL concept consisting of 31 SCs was designed. The effective shunt impedance ZT2 simulated by CST corresponds to 60.13MΩm and is comparable to the ZT2 of SCCL in use. The reduction in ZT2 expected after Hirtisation (R) can be justified in practice by up to 70% lower manufacturing costs. However, future studies will be conducted to further increase Q0.

Improving Fabrication and Performance of Additively Manufactured RF Cavities by Employing Co-Printed Support Structures and Their Subsequent Removal

“…This implies that the gradient model is valid for the single cavities presented here and that the single cavities have the same surface conductivity as a perfect, conventionally manufactured cavity with the same roughness. A comparison with the SCs presented in a previous study [27] reveals an increase in Q λ0 by 10-20%, indicating progress in the manufacturing process. However, it should be noted that the SCs presented in [27] were printed on an L-PBF system based on green laser technology, not red laser technology.…”

“…As in our previous work, we utilize the parameter S λq (adapted from the area rootmean-square average S q , ISO 25178) to describe the surface roughness [27]. The measured surface profile is recorded using a Keyence VK-X3000 laser scanning microscope.…”

“…Additionally, a form correction operator (F-Operator) with a wavelength of λ f = 130 µm is applied. Surface variations characterized by a wavelength λ >> δ appear to have only a minor impact on the surface conductivity [23,27]; therefore, another high-pass filter (λ c filter) is applied. Thereby, λ c is defined as the copper powder size D 50 , and λ c = D 50 = 35.2 µm.…”

“…The simulated Q 0 corresponds to the best possible value for a flat (S qλ = 0) and annealed (σ = 5.8 • 10 7 S m , 100 % IACS) copper surface. As previously described by us in [27], the gradient model developed by Gold et al [36] can be used to simulate the influence of surface roughness on the surface impedance and thus Q 0 with the help of the CST frequency domain solver. For the simulation, the surface roughness is assumed to be uniform over the entire cavity geometry.…”

“…Therefore, a design guide for self-supporting cavity geometries with the condition α > 40 • was developed [26]. However, this condition results in a reduction in Q 0 by up to 18% in the case of TM 010 cavities and thus limits the potential of AM for cavity manufacturing [27]. In this article, we present a manufacturing routine based on L-PBF and an electrochemical post-processing method that allows the realization of arbitrary cavity geometries with the assistance of co-printed support structures.…”