Within the straight continuity of the undertaken work on GUINEVERE – a demonstrator dedicated to feasibility studies on hybrid reactors technologies (deuteron beam: 250 W, reactor: 150W) - LPSC is involved in the MYRRHA (Multipurpose hYbrid Research Reactor for Hightech Applications) project.
Supported by the SCK-CEN, the MYRRHA project aims at the construction of an ADS (Accelerator Driven System) demonstrator (50 to 100 MWth) in order to study the feasibility of high radiotoxicity nuclear wastes. The nuclear reaction, in the subcritical core of this fast spectrum reactor, will be sustained thanks to spallation neutrons. These neutrons will be produced by means of a proton beam impacting a target: a Lead-Bismuth eutectic.
A high power accelerator will produce a continuous wave (CW) proton beam (4 mA – 600 MeV). In addition to the high beam power, one has to consider that frequently-repeated beam interruptions can induce high thermal stresses and fatigue on the reactor structures, the target or the fuel elements, with possible significant damages especially on the fuel claddings. Therefore the accelerator will have to be extremely reliable. The present tentative limit for the number of allowable beam trips is: 10 unexpected interruptions longer than 3 seconds per 3-months operation cycle. The reference solution for the accelerator (studied in the MAX project) is a superconducting accelerator with a fault tolerant scheme for rapid RF failure mitigation. To maximize the reliability this accelerator design is based on a redundant injection line. The superconducting line will be composed of one family of spoke cavities and two families of 5-cells elliptical cavities.In the 1st step of the R&D programme dedicated to this multi-megawatt demonstrator, the LPSC construct, implement and test a prototype of the Low Energy Beam Transfer line (LEBT) of the proton injector (10 mA, 30 keV). The LEBT main goal is to produce and shape the proton beam for its proper injection in the RFQ (Radiofrequency Quadrupole). To maximise the accelerator reliability and the beam availability, the LEBT design is based on a compact line with a minimum of electrostatic element. It is composed of an industrial ion source, combined magnetic elements (solenoids + steerers) for the beam focussing, beam diagnostics, a fast chopper, a beam dump as well as pumping system for the residual gas regulation in the line and a global control system. The mechanical construction of the LEBT is now in progress:
- The magnetic elements are under construction,
- The beam dump design is achieved with thermal simulation,
- A system of 4 independent slits is defined; it will enable to tune the transvers profile of the beam,
- Beam diagnostics will enable to measure the profile and the emittances in both transverse directions.
The SCK-CEN is responsible for the chopper design and its construction. The global control system is developed by a private company in partnership with SCK-CEN and the LPSC. An experimental area was implemented to welcome the LEBT at LPSC. It will fully be characterised at Grenoble before being sent to Belgium for its coupling with the RFQ.
The understanding of high intensity beam dynamics at low energy is a key point to enable its good transports through the LEBT. In this energy range, the beam physics is dominated by non-linear effects due to the space charge field induce by the beam on itself. This field has defocussing effect on the beam, which induce a halo growth. However, the beam can also ionise the residual gas inside the line, with the consequence of creating ion/electron pairs. These generated secondary particles can either be trapped or repelled by the proton beam; depending on their charge. Particles with an opposite charge to the beam (in our case electrons for the proton beam) will gathered inside the beam potential well which can therefore be considered as a kind of plasma. The beam defocussing, due to the proton/proton repulsive effect, can therefore be minimised.
In this way, the MYRRHA LEBT will enable to study the physics of low energy hadron beams. It is crucial point to optimise the beam transmission and behaviour in the following parts of the accelerator. These studies would also be useful for all the actual high power hadrons accelerator projects (ESS, SPIRAL2, IFMIF-EVEDA, etc).