Diagram of the ESS accelerator.
The linear accelerator includes a low-energy section (E≤ 90MeV) with a Radio Frequency Quadrupole (RFQ), a Drift Tube Linac (DTL), and a high-energy section made up of superconducting cavities that are designed to accelerate protons up to 2 GeV. Superconducting cavities can be divided into three categories. The first consists of spoke cavities, operating at 352 MHz and optimized for proton beams at half the speed of light (β= v/c = 0.50), with energies between 90 MeV and 216 MeV. The second and third categories are 704 MHz elliptical cavities known respectively as medium beta (β= 0.67) and high beta (β= 0.86) cavities. The medium beta cavities cover energies ranging from 216 MeV to 571 MeV, while high beta cavities cover energies of 571 MeV to 2.0 GeV.
When the project was launched in 2009, IRFU was one of the first partners in the collaboration. It contributed to the design of a number of major accelerator parts, such as the RFQ and the medium- and high-beta cryomodule sections mentioned above. This work, which began during what is known as the Accelerator Design Update (ADU) phase, was carried out under a French-Swedish collaboration agreement signed by CEA and CNRS.
The ESS construction phase began in January 2013.
The prototyping activities that started during the ADU phase continued between 2013 and 2015 while the French contribution in kind to the ESS accelerator was being prepared. By the end of 2015, IRFU had contributed to some 70% of the total length of the linac, supplying the RFQ and the 30 medium- and high-beta cryomodules, as well as some beam diagnostic features (profile, intensity) and Control System parts.
Some of these new activities concerning the contribution in kind, and considered urgent, were launched under Heads of Agreement (HoA) before the main agreement was signed. This was the case for two diagnostic features for the low-energy beam transport section (Doppler effect species fraction measurements, and emittance measurements), the control system of the low-energy beam transport section, ordering the copper for the RFQ, and a second prototype cryomodule with high beta cavities.
Contribution in kind activities could be started up and carried out with no serious obstacles before the signing of the related agreement, which was postponed until 2016.
The control system of the proton source and the low-energy beam transport (LEBT) is developed by IRFU’s Systems Engineering Division (SIS). It also concerns the beam diagnostic features installed on the LEBT, which are developed at SACM (Doppler Shift Measurements and EMU).
The Emittance Measurement Unit (EMU) is used to measure the angular divergence and intensity of the beam as it leaves the ion source in the two dimensions transverse to the beam direction (x,x’) and (y,y’). SACM is responsible for designing, producing and delivering two Allison scanner systems for taking measurements in each of these directions. One of the challenges that these systems must meet is related to the very high power density deposited at the slits intercepting the proton beam. Delivery of these tungsten parts had to be postponed owing to manufacturing problems, and delivery is now scheduled for November 2016.
The control system of the proton source and the low-energy beam transport (LEBT) is developed by IRFU’s Systems Engineering Division (SIS). It also concerns the beam diagnostic features installed on the LEBT, which are developed at SACM (Doppler Shift Measurements and EMU).
The Emittance Measurement Unit (EMU) is used to measure the angular divergence and intensity of the beam as it leaves the ion source in the two dimensions transverse to the beam direction (x,x’) and (y,y’). SACM is responsible for designing, producing and delivering two Allison scanner systems for taking measurements in each of these directions. One of the challenges that these systems must meet is related to the very high power density deposited at the slits intercepting the proton beam. Delivery of these tungsten parts had to be postponed owing to manufacturing problems, and delivery is now scheduled for November 2016.
The Doppler Shift Measurement Unit is used to measure species fractions in the beam at the ion source exit. It does this by analyzing the interaction radiation of the beam (and therefore of these species) with the gas remaining in the beam vacuum vessel. CEA is responsible for delivering a Doppler unit in July 2016.
Other diagnostic features developed at IRFU, the Non Invasive Profile monitor (NPN) and the neutron-sensitive Beam Loss Monitor (nBLM) are the responsibility of IRFU’s Electronics, Detectors and Computing Division (SEDI).
3D view of the RFQ.
RFQ
The RFQ shown in the 3D view above is the first accelerator cavity of the warm section of the linac (the low-energy part at ambient temperature is not superconducting). The continuous proton beam is injected into the RFQ at an energy of 75 keV and leaves it at 3.6 MeV, after being bunched at a frequency of 352 MHz.
ESS set these beam parameters during the accelerator design update phase in 2013. These changes led to the study of a new RFQ design to meet the new characteristics of the accelerator. The latest design is the third produced by SACM following changes to ESS beam parameters. An initial Critical Design Review (CDR-1) took place on November 18, 2014, after which ESS gave CEA the go-ahead to order the copper required for making the RFQ. The copper was delivered in early 2016. A second review, CDR-2, was held in December 2015 to freeze RFQ detailed design and interfaces; this made it possible to issue the calls for tender for RFQ manufacture at the start of 2016.
ELLIPTICAL CAVITY CRYOMODULES
Downstream of the RFQ, the beam is accelerated by other cavities that are adapted to the increasing speed of particles along the linac. The 30 elliptical cavity cryomodules - nine medium beta and 21 high beta - constitute IRFU’s largest contribution. This covers the following activities: design, prototyping (two cryomodules - one medium and one high beta module will be made and tested at Saclay), and supply of series parts (except for the superconducting cavities), assembly of all 30 cryomodules, and testing at power the first three cryomodules assembled. The cryomodules are 6.6 m long and house four superconducting cavities operating at 2 K. Each of these is powered by an RF power coupler delivering peak power of 1.1 MW. Each medium- and high-beta cavity will be designed to deliver an accelerating voltage of 14.3 MV and 18.2 MV respectively. The mechanical studies for the cryostat were carried out in collaboration with the Institute of Nuclear Physics of Orsay (IPNO). Assembly of the 30 ESS cryomodules will start at the end of 2017. This work will follow on from the cryomodules assembly made for the XFEL project and benefit from the experience acquired and the infrastructure freed up by that project in 2016.
M-ECCTD MEDIUM BETA PROTOTYPE CRYOMODULE
Mechanical studies for M-ECCTD, a prototype cryomodule with medium beta cavities, began in late 2012 in collaboration with IPNO, which is responsible for the design and procurement of the cryostat components. IRFU is responsible for the cavities, couplers, tuning system, magnetic shielding, assembly tools, assembling the cryomodule, installing the RF power test area, and setting up the actual tests in our facilities.
of the medium beta cavities began in September 2013, after ESS had defined a geometric beta value of β= 0.67 for these cavities. work was carried out in collaboration with a student from the University of Lund. The RF design of these cavities had to undergo several optimization iterations to reduce risks relating to certain higher order modes (HOMs). Most of the parts were ordered in 2015, and deliveries began in 2016 with the cavities, the power couplers, cryostat parts for the cryomodule, and the cryogenic lines and RF lines for the test bunker.
H-ECCTD HIGH BETA PROTOTYPE CRYOMODULE
The contribution in kind includes a second prototype cryomodule, called H-ECCTD. It will be very similar to the M-ECCTD, but will have high beta cavities, of which all the design work and mechanical studies were carried out by IRFU. Two high beta prototype cavities were built and tested over the period 2013-2015. The RF tests were a complete success and the cavities met ESS specifications. A call for tender for five cavities for the H-ECCTD cryomodule was prepared in 2015 and issued at the beginning of 2016.
SERIES PRODUCTION CRYOMODULES
During 2013-2015, an IRFU project team was set up, and collaboration with the ESS partners was organized with a view to producing cryomodules in series. The Laboratory for Accelerators and Applied Superconductivity (LASA) of the Italian National Institute of Nuclear Physics (INFN) in Milan will produce the series of 36 medium beta cavities, while STFC in Daresbury, UK, will produce the 84 high beta cavities. LASA has proposed changing the RF and mechanical design of the medium beta cavities from that proposed by CEA. The M-ECCTD cryomodule development plan will therefore have to be modified accordingly to integrate a LASA cavity in this prototype cryomodule for qualification in the final configuration before series production can begin. STFC chose to comply in full with the high beta cavity design proposed by CEA.
Overall 3D view of the M-ECCTD cryomodule
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