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Commissioning, first tests and upgrade of a high-power S-Band Radio Frequency (RF) system for R+D of high-gradient normal-conducting accelerating cavities in breakdown science and RF conditioning

Periodic Reporting for period 1 - HGRF-IFIC (Commissioning, first tests and upgrade of a high-power S-Band Radio Frequency (RF) system for R+D of high-gradient normal-conducting accelerating cavities in breakdown science and RF conditioning)

Reporting period: 2017-05-19 to 2019-05-18

General interest has been shown over the last years for compact and more affordable facilities for hadron-therapy. The High-Gradient (HG) know-how and technology for normal-conducting accelerating RF (Radio-Frequency) electron linac (linear accelerator) structures recently developed for projects such as CLIC (CERN), has raised the achievable accelerating gradient from 20-30 MV/m up to 100-120 MV/m. This gain has come through a better understanding of the high-power RF vacuum arcs or breakdowns (BD) phenomena, the development of quantitative HG RF design methods and refinements in fabrication techniques. This can allow for more compact linacs also for protons, which is potentially important in the new trend in hadron-therapy of using linacs able to provide protons of 70-230 MeV or light ions of 100-400 MeV/u. Linacs are of particular interest for medical applications because they can provide a high degree of flexibility for treatment, such as running at 100-400 Hz pulse rate and pulse-to-pulse beam energy (and intensity) variations. This kind of accelerator is very well suited to treat moving organs with 4D multi-painting spot scanning technique. Project studies like TULIP are taking advantage of these developments and pursuing medical linacs of reduced size.

HG operation, which is carried out under ultra-high vacuum conditions (~10^-9 mbar), is limited by the BD phenomena and is characterized by the BD-Rate (BDR) or number of BD per pulse and meter. New fresh structures initially operate at a reduced performance and must be conditioned through extended high-power RF operation until the maximum operational gradient is reached. This process is a time consuming, and consequently costly task (> 35x10^6 pulses) which is important to understand and reduce. In order to delve into the issues related to this HG phenomena and with the aim of performing high-rate and systematical studies of HG accelerating structures, a HG RF laboratory has been constructed at the IFIC premises (Instituto de Fisica Corpuscular). The IFIC HG-RF laboratory is designed to host a high-power and high-repetition rate facility for testing S-Band (2.9985 GHz) normal-conducting RF structures. The main objective of this project is the commissioning of the laboratory and obtaining the first results of HG accelerating structures designed for proton-therapy applications.
The tasks, in accordance with the project proposal, have been structured into three main workpackages:
System commissioning:
This task has been the most laborious and most time consuming of the project. It has been split in two stages: a first one consisting in setting up the high-power amplifier (klystron) and the power source (modulator), and a second one where the Low Level Radio-Frequency electronics (LLRF) has been designed, constructed and installed and the full system operated up to power limits.
In order to commission the high-power klystron and modulator, proper cooling services and ultra-high vacuum equipment were installed in the laboratory. The klystron has peak RF power output of 7.5 MW, 5 μs pulse width and maximum repetition rate of 400 Hz. The modulator is a novel solid-state modulator from the company Jema Energy, which has been developed and optimized to attain optimal operational conditions for the assembly (modulator+klystron). The optimization process was lengthy and required a lot of permanent feedback joint work with the company in order to build a working product. This experience with IFIC, has brought to the European industry will have a pilot modulator product suitable for medical linac applications, in the forefront of modulators technology.
The LLRF system had to be constructed, installed and fully integrated. This system consists on several components, out of which few had to be developed during the progress of the project. The main work was focused in the following elements: the PXI system, the down-mixer, the log-detector and a safety interlock subsystem. The PXI system, which performs the RF generation and RF signals acquistion, is a highly complex device that behaves as the brain of the laboratory and had to be fully programmed in LabVIEW. A down-mixer with 12 input/output channel, which moves the RF signals down to 62.5 MHz such they can be later sampled, had to be constructed. A logarithmic detector with 8 channels able to extract the envelope of the RF signals had to be re-designed and constructed. And finally, a safety interlock subsystem constructed with PLCs and other custom developed electronics, had to be developed so the different devices and equipment of the laboratory can interplay with each other and with the users in a safe manner.

Structures testing:
Two structures, built at CERN were made to prove the principle of HG acceleration of low energy protons required for hadrontherapy. One of the structures is installed at the HGRF-IFIC test facility, where we will do all the experiments to prove its behavior. In addition, in order to validate the structures design and carry out the scientific work of the project, a more rudimentary similar testing system was built in collaboration with CERN and the other structure has been tested at the CERN premises. The testing has very recently finished, the data has been analyzed and the successful results will be soon published.

RF pulse compressor design:
In order to boost the peak power level from 15 MW to at least 30 MW an RF pulse compressor is needed in one of the test slots of the laboratory. This power upgrade is needed in order to operate the accelerating structures in high gradient regime. Support was granted by CERN experts to guide us on the analytical design and further RF and mechanical design of a compact and spherical pulse compressor. In addition, with the aim of making sure that an operational pulse compressor was available at the lab, a long term loan of and spherical pulse compressor was negotiated with CERN such now it is available at the laboratory premises.

The overall outcome of the project was very successful, with the main objectives accomplished. The laboratory has effectively been operated in “test phase” with dummy loads. Currently, the lab is carrying out tests of a real accelerating structure. Both the technical, engineering and scientific goals of the project have been achieved. This unique test facility has been put into operation and an official inauguration has recently been made in order to expose to the scientific and the local political community.

The progress and results of the project have been periodically reported at different conferences, workshops and publications as listed in the web of the project.
This is a unique test facility that will support the research community and industry in their R&D of HG and high-power technologies and equipment in S-Band. It will help having a better understanding of the Physics of high-power RF in normal-conducting structures together with a phenomenological approach. The extrapolation of successfully implemented design parameters would lead to improved structure performance. The outcome has a crucial impact since it opens the possibility to build smaller facilities, which in the case of medical linacs could allow easier integration in hospitals. As of today, no exhaustive HG studies have been done in S-Band, working frequency of many applications, due to the lack of test facilities with an automatic computer controlled system.