ESS Accelerator Research Articles

Future ESS upgrade to Medium Pulse Length – What are the technical challenges for the accelerator and target?

A compression of the ESS proton pulse from the present 2.86 milliseconds to a medium pulse length of a few tens of microseconds which is better matched to the moderator time-constant of thermal neutrons would considerably boost the performance for many instruments at ESS. Generating such a proton pulse with preserved instantaneous beam power requires a storage ring to be added to the ESS accelerator. Such a ring has been studied within the ESSnuSB neutrino super-beam study. The proton pulse length extracted in single turn extraction from this ring would be approximately 1 microsecond long which could be destructive for the present ESS target and is very short compared to the moderator time constant. The more desirable medium length pulse could possibly be generated by multi-turn extraction and a new concept using a cyclotron like extraction scheme in a synchrotron or a FFA. Another way to generate the longer pulses is to extract a bunch train using fast strip line kickers but this would require a larger storage ring. Using a “bunch train” has been successfully applied at the CERN ISOLDE facility to avoid destruction of sensitive liquid metal targets used for Nuclear Physics experiments. Other challenges are linked to the injection into the storage rings and the understanding of the target, moderator and neutron extraction systems with short and medium pulse length.

Design and fabrication of stripline BPM for the ESS MEBT

Beam Position Monitors (BPM) are non-destructive diagnostics devices used in particle accelerators to measure the transverse beam position (beam center of mass) and phase of the accelerated beam. There are various ways of realization of the BPMs in accelerators having different measurement requirements, particle type or energy. Several types of BPM pickups have been studied including the button type, grounded striplines and matched striplines. The choice and final design is based on a matched stripline to accommodate the signal transmission to electronics and provide relatively higher signal level for low velocity (β=0.088) proton beam within MEBT. Due to mechanical space limits, all the BPMs are embedded inside quadrupoles; hence it is required to use non-magnetic materials in the BPM sensor, in particular the feedthroughs. The BPM sensor is composed of several components, which are separately machined with tolerances of 10μm, and then welded together by the use of electron beam welding. The analysis show for long bunches, the multi bunch transfer impedance is different than of single bunch due to bunch signals overlapping. The design, fabrication and tests of the BPM sensors were performed at ESS-Bilbao and their installation and the electrical checks at ESS-Lund. Prior to installation in the ESS accelerator tunnel, one BPM stripline was installed and the BPM system tested with the CEA-IPHI beam. This paper gives an overview of the electromagnetic and mechanical design and the fabrication of BPM striplines. Furthermore, the results of the RF measurements are compared with the simulations.

Engineering Design Integration & Non-Destructive Testing for the ESS accelerator

The ESS linac is under construction by the ESS partner institutes (so- called In-Kind Contributors - IKC) and will operate the most powerful proton beam ever for neutron spallation source. The linac delivers 5 MW via 2 GeV protons at a repetition rate of 14 Hz at the He-cooled solid tungsten target. The pulsed neutrons, result of the spallation, will reach the science instruments after having been moderated. The engineering effort needed to assemble the linac and its RF sources (klystrons), commenced with the design integration and currently is undergoing installation planning. In addition, the first dedicated engineering properties experiments, so-called Non-Destructive Testing for Accelerators (NDTA), take place to map and pilot the innovative testing strategy for the ESS linac structural materials. The Engineering Resources Group (ERG) of the Accelerator Division (AD) has been created to provide services of design integration, mechanical engineering and system engineering to ESS Accelerator Systems (ACCSYS) that are applicable across work package boundaries and principles of the linac systems. In parallel, part of the machine integration is the physical plant coordination and supervision. At last, in order to fulfill the missions of feasibility and planning, the ERG designs and leads the development of the technical laboratories for the accelerator systems. The current citation describes the engineering proposal for the mechanical design study and integration of the linac machine, its non- destructive testing and the essential development of the technical areas to service the long-term operational needs.

Developing the Particle Free Vacuum System at ESS

This paper discusses the acceptance criteria for particle free, the development of particle free pumping carts and the results of particle testing performed in the ESS accelerator.

Activation assessment of the soil around the ESS accelerator tunnel

Activation of the soil surrounding the ESS accelerator tunnel calculated by the MARS15 code is presented. A detailed composition of the soil, that comprises about 30 chemical elements, is considered. Spatial distributions of the produced activity are provided in both transverse and longitudinal directions. A realistic irradiation profile for the entire planned lifetime of the facility is used. The nuclear transmutation and decay of the produced radionuclides is calculated with the DeTra code which is a built-in tool for the MARS15 code. Radionuclide production by low-energy neutrons is calculated using the ENDF/B-VII evaluated nuclear data library. In order to estimate quality of this activation assessment, a comparison between calculated and measured activation of various foils in a similar radiation environment is presented.

Beam commission of the high intensity proton source developed at INFN-LNS for the European Spallation Source

At the Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud (INFN-LNS) the beam commissioning of the high intensity Proton Source for the European Spallation Source (PS-ESS) started in November 2016. Beam stability at high current intensity is one of the most important parameter for the first steps of the ongoing commissioning. Promising results were obtained since the first source start with a 6 mm diameter extraction hole. The increase of the extraction hole to 8 mm allowed improving PS-ESS performances and obtaining the values required by the ESS accelerator. In this work, extracted beam current characteristics together with Doppler shift and emittance measurements are presented, as well as the description of the next phases before the installation at ESS in Lund.

Cryogenic Distribution System for the ESS Superconducting Proton Linac

The European Spallation Source is an accelerator-driven neutron scattering facility, currently under construction in Lund, Sweden. Its proton linac will include 43 superconducting cavity cryomodules. The nominal operating temperature for the cavities is 2K, with 40-50K thermal shielding. Required cooling powers will be provided by the accelerator cryoplant and delivered in 4.5 K and 40K temperature levels. The 2K temperature level will be produced at each cryomodule. The cryomodules will be connected with the cryoplant by a dedicated cryogenic distribution system. The system will be composed of a cryogenic transfer line with a splitting box and a cryogenic distribution line with 43 valve boxes and an end box. A number of high-level requirements for the ESS linac impose a highly branched structure for the cryogenic distribution system and strongly influence specific choices related to the detailed designs of valve boxes and cryogenic lines. The design of the valve boxes must allow for warming up and cooling down of one or more cryomodules without affecting the others. This feature will help to meet a high availability requirement for the ESS accelerator by shortening the duration of some potential and unavoidable cryomodule repairs. This paper describes main functional and technical requirements for the cryogenic distribution system and presents the ESS in-house preliminary design. The paper also gives a few main points of the project execution plan, which points towards in- kind contribution from two different European institutions.