3D Printing of RF Cavity Resonators

In modern accelerator facilities, charged particles (e.g. electrons, protons, etc.) are accelerated to relativistic velocities, in some cases close to the speed of light, with the help of high-frequency cavity resonators (RF cavity resonators). These RF cavity resonators are structures made of highly conductive materials (e.g., aluminum, copper, their alloys, or superconductors) that enclose an open volume - the cavity. A radio frequency (RF) electromagnetic field is injected into the cavity and interacts with the particles as they pass through the cavity. If the frequency of the RF field corresponds to the resonant frequency defined by the cavity geometry, the energy of the RF field increases with each oscillation. This results in an exaggeration of the field amplitudes, with the electric field component being used to accelerate particles.

The goal of cavity design is to maximize the effective accelerator voltage over the shortest possible accelerator distance in order to realize efficient and compact accelerator systems.

For the following reasons, this requires some complexity in the internal cavity resonator geometry:

  1. The particles should undergo multiple RF cycles (experience many acceleration bursts) as they pass through the cavity in order to make the best use of the power coupled into the cavity. However, to ensure that the particles interact only with the accelerating E-field phases of the RF cycles, they must be shielded at certain intervals by complex geometries in the cavity.
  2. Due to ohmic losses, the RF power coupled into the cavity leads to thermal stress in normally conducting cavity resonators and thus to a change in the cavity geometry due to material expansion. The resulting change in resonant frequency prevents resonant power coupling and thus maximum E-field amplitude. To compensate for the thermal load, cavity resonators must be equipped with complex cooling channel structures.

The complexity of the internal geometries and the cooling channels of cavity resonators do not allow traditional (machining) manufacturing in one piece. Instead, cavity resonators are traditionally manufactured from several individual subcomponents, which are then joined by joining processes such as brazing, electron beam welding or similar to guarantee high electrical and thermal conductivity as well as vacuum tightness. This traditional manufacturing method is time-consuming, error-prone and very expensive.

3D printing processes (or additive manufacturing techniques) offer the possibility of producing complex geometries by applying material layer by layer and joining it locally. Compared to traditional processes, this results in significantly greater design freedom on the one hand, and on the other, the possibility of producing one-offs and small batches much faster and also at lower cost. Nevertheless, this technology has hardly played a role so far for the production of normal-conducting cavity resonators, as they are predominantly used in accelerator systems. This is mainly due to the fact that normal-conducting cavity resonators are usually made of high-purity copper, and this material has so far been difficult or impossible to process using 3D printing methods due to its high thermal conductivity and reflectivity (in the infrared range).

However, the excellent electrical and thermal properties of high-purity copper are crucial for many industries. Therefore, considerable efforts have been made in recent years and ultimately decisive progress has been achieved, making 3D printing of high-purity copper possible. In this context, the 3D printing processes of selective laser melting (SLM) and selective electron beam melting (EBM) are particularly suitable for processing high-purity copper

The Institute for Applied Physics and Metrology (LRT2) has set itself the task of developing cavity resonators that can be manufactured using the SLM or EBM processes.

The primary goal is to demonstrate that 3D printing offers the fundamental potential to produce cavity resonators with comparable high RF performance at significantly reduced cost compared to traditional manufacturing methods. In the meantime, we were able to develop a prototype drift tube linear accelerator (DTL) whose cavity resonator could be fabricated from high-purity copper using SLM (publication). This prototype already shows RF performance comparable to traditionally fabricated LINAC structures at low to medium RF powers. An evaluation and optimization of the RF performance at high RF power shall follow. For this purpose, a high power test bench is to be developed and realized within the scope of our project. It is also planned to develop methods to reduce the surface roughness of the cavity walls after printing and thus increase the RF performance.

A DTL was chosen because this type of linear accelerator is used in many accelerator facilities around the world, further highlighting the potential of 3D printing. At the same time, we also want to research how the geometric design freedom gained by 3D printing can be used to realize completely new cavity geometries and to optimize already known cavity resonator concepts.

Worldwide, more than 30000 ion and electron accelerators are used for a variety of applications such as basic physics research, radiation therapy for the treatment of tumors or materials research. In the realization of accelerator systems, traditional manufacturing methods cause enormous costs and at the same time limit the geometrical design freedom. We are therefore convinced that 3D printing processes will revolutionize the design and manufacture of RF cavity resonators (patent).