Radio Frequency System, Power Converters and Cryomodule installation and tests as a Polish in-kind contribution to the European Spallation Source (ESS)

The European Spallation Source (ESS) project currently enters the final stage of installation. Since 2017, a group of engineers and technicians from The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Science (IFJ PAN) are involved in the project. The contribution to the project can be divided into three main tasks: Radio Frequency Distribution System (RFDS), RF (Radio Frequency) Power Stations and Cryomodules. The RFDS in ESS project is one of the largest installations of this type consisting of 155 RF high power systems. Engineers and technicians from IFJ PAN were responsible for preparation, installation and RF measurements of the above-mentioned system. The team is also involved in preparation and conducting low and high power tests of the RF stations. The IFJ PAN team is also responsible for the preparation as well as vacuum and cryogenic tests for 9 Medium and 21 High Beta Cryomodules, before they are installed in the tunnel. The advanced quality control and quality assurance were mandatory for this work because the costs of failures, as well as potential delays, would have a huge impact in the project realisation. Therefore dedicated methods and approaches have been adapted to this work using experience gained by the IFJ PAN team on previous projects like LHC, XFEL and W7X.


Radio Frequency Power Distribution System
The RFDS system for the ESS accelerator is currently one of the largest installations of this type.The total length of the waveguide and coax lines is approximately 3.5 km.
In addition to the assembly of system components, the IFJ PAN team was also responsible for carrying out RF testing and any necessary fine-tuning to verify that the lines met the acceptance criteria.The preparation phase and the installation were described previously in other articles [2].As of today, the RFDS system is completed, and the team is now focused on the preparation of power converters and their commissioning, as described below.

Radio Frequency Power Station
The main component of the RF system is the power amplifier, which takes the signal from the Low Level RF (LLRF) system and pulse power from the modulator and converts the power into RF waves at 352.21 or 704.42 MHz.Due to the power levels required, most of the power amplifiers at ESS will be klystrons with a peak power in the range of 1 to 3 MW.In total the linac will require 126 klystrons.The Spoke section of the linac requires 400 kW of peak power per resonator at 352 MHz [1].In this case, it was decided to use technology based on tetrodes.

Tetrodes
The Spoke section will have 26 power stations.The main tasks of IFJ PAN are to provide technical support to the ESS experts, in the installation of tetrodes and to connect the power stations (figure 2) to the waveguide systems in the gallery.
In addition, activities such as visual inspection of components, involvement in pressure testing of cooling water systems, electrical and mechanical incoming inspection of the amplifiers, participation in the assembly process of the RF power station (installation of all its key components) and finally, supporting the ESS experts with high power test of each Radio Frequency Power Station (RFPS) in the test stand.Figure 3. Klystrons used at ESS for MB and HB.

Klystrons
When the ESS first becomes operational, it will contain 78 klystrons -6 for a NCL, 36 for Medium Beta (MB) and 36 for High Beta (HB) [3].IFJ PAN participates in the preparation low and high power testing of the klystrons.Three types of klystrons are in use for MB and HB: E37504 (CANON), VKP-8292A (CPI) and TH2180 (THALES) (figure 3).
Prior to a klystron test, many activities must be carried out, such as visual inspection after delivery, electrical testing of integral parts e.g.coils, sensors, as well as the installation of additional components including arc detectors, filament units, junction box, output waveguide etc.After positioning the klys-tron in the final position, the gun tank is filled with oil, the output waveguide is connected to the waveguide system and all electrical and water-cooling connections are made.Only then is the klystron for further operation.
Low Power tests include activities such as checking electrical connections and testing all the electronic devices necessary for system operation.The procedure consists of starting all individual systems, circuits and sensors (e.g.temperature sensors, arc detectors, flow meters, pressure sensors), checking the parameters (vacuum level, current, voltage, pressure, flow, temperature etc.) and checking the reaction of the interlock system.
High power testing can be split into two main parts: conditioning and the actual test.During DC and RF conditioning, various parameter including high voltage (HV) and RF pulse length, repetition rate and drive power are changed, to reach the nominal working levels.Throughout the test, all key parameters (e.g.current, vacuum level, power, etc.) are continuously monitored.During conditioning, the level of ionizing and non-ionizing radiation is monitored in accordance with safety regulations.
When both conditioning procedures are complete, the formal Site Acceptance Test (SAT) can start.All the results obtained during this part of the test, are compared with the Factory Acceptance Test (FAT).During the SAT, tests such as the transfer characteristic, the sensitivity to variation in HV, bandwidth measurements and Filament Roll-off are measured.
In all the above-mentioned elements, starting from assembly of the waveguide line, connecting cables, the incoming inspection and installation of electronic devices in racks, the preparation of klystrons and finally the formal SAT, the IFJ PAN team supports experts from ESS [4].

IFJ PAN contribution for cryomodules testing
The IFJ PAN team is actively participating in testing and preparation of installation of the cryomodules belonging to the Superconducting Cold Linac (SCL) section of the accelerator.The main responsibility of IFJ PAN is to perform the on-site SAT for 30 elliptical cryomodules.In addition, reception check of the 13 Spoke cryomodules is done after transportation from FREIA laboratory, where their SAT took place.
IFJ PAN members are involved in reception and checking the spoke cryomodules by doing incoming inspections at the ESS site.After inspection, the cryomodules with the cooperation of ESS personnel, are prepared for installation in the tunnel [5].
The main task of the IFJ PAN specialists is the SAT of the elliptical cryomodules as mentioned before.IFJ PAN is responsible for preparing for the RF test at 2 K, of 9 MB (figure 4) and 21 HB elliptical cryomodules [6].During preparation of the cryomodules for tests at an early stage, it is necessary to check if there were any irregularities following transportation.Due to that, the first step after arrival of the cryomodule to ESS is to perform a mechanical and electrical incoming inspection at the preparation area.The instrumentation (e.g.compressed air and helium guard system, vacuum equipment, doorknobs, cabling etc.) can be then installed on the cryomodule and the cryomodule is leak tested.When the inspection and vacuum tests have passed, the cryomodule can be installed inside the test-stand bunker.
After placing the cryomodule into the test bunker, numerous mechanical and electrical connections must be made, such as: jumper/cryogenic lines connection with installation of the thermal shield and multi-layer insulation (MLI), waveguides and auxiliary lines connection, connection of the cryogenic and RF cables etc.Then, vacuum tests of the beamline and cryogenic lines are carried out.For that purpose, first the particle-free pump station is connected to the cavities string inside the clean room and residual gas analysis (RGA) and leak check is performed for the beam line.Before the cooldown starts, the cryogenic lines are pressurized and a leak test of the insulation vacuum/beam volume is done (figure 5).Also prior to cooling down, the pump and purge procedure of the process pipes is performed, cryogenic valves initialization, pressure sensors calibration and many other checks are carried out.In each cryomodule two cryogenic circuits: 4.5 K (cavities) and 40 K (thermal shield) can be distinguished.At nominal gradient the Superconducting Radio Frequency (SRF) cavities are operated at 2 K, 31 mbar He bath pressure while the thermal shield is maintained at 40 K and a pressure of 14 bar.The power couplers are at 4.5 K.The cool down process usually starts from parallel helium flow through the thermal shield and 4.5 K circuits.Helium gas is provided by dedicated cryoplant.Each cryomodule is equipped with two main cryogenic valves.In the beginning, SRF cavity tanks are filled in parallel with liquid helium from the bottom and later there is a transition to a Joule-Thomson valve and then cavity tanks are filled from the top.Once a stable helium level has been achieved, the pumping down process to 31 mbar is performed (figure 6).For better 2 K operation, a heat exchanger is installed in the cryomodule which cools down the supplied helium gas before it is passed through the Joule-Thomson valve.
Once the temperatures and cryogenic condition have stabilised at 2 K, all required RF tests are performed under the responsibility of the ESS SRF Section.When the RF tests at 2 K, as well as the static and dynamic heat loads measurements for the cryomodule are complete, the cryomodule is warmed up.This takes a couple of days [7].When the cryomodule is warm additional vacuum checks are performed before the cryomodule is disconnected from the test stand.Outside the bunker an outgoing inspection is performed and the cryomodule is secured and ready for tunnel installation [8,9].

Conclusion
IFJ PAN is a team that has been supporting ESS experts for many years in various fields, from installation to testing.The IFJ PAN team works according to the quality system created by themselves based on experience gained in previous projects.One of the few key aspect of this quality system is a dedicated database called Quality Database (QDB).The QDB is regularly developed based on experience and needs.The stored data is used for numerous analyses and preparation of reports which are shared with ESS experts.
It should be acknowledged that the participation of IFJ PAN employees in this project, as the Polish In-Kind contribution to the ESS, it is very valuable both for the implementation of the project as well as for the exchange of experience and the acquisition of new skills and science.

Figure 1 .
Figure 1.Part of the waveguide system in Normal Conducting Linac (NCL) area.

Figure 2 .
Figure 2. RFPS unit.Figure 3. Klystrons used at ESS for MB and HB.

Figure 5 .
Figure 5. Leak test of the 2 K cryogenic volume (leak signal vs pressure).

Figure 6 .
Figure 6.Plot of cavity tank helium bath pressure and liquid helium level during pumping down to 31 mbar.