The IFMIF prototype accelerator
The IFMIF prototype accelerator

The IFMIF injector ; from left to right: Source, accelerator tube and low energy transport line.

As part of the wider approach to ITER, IRFU is responsible for delivering the French contribution to the International Fusion Material Irradiation Facility (IFMIF) project which will enable the testing of materials developed for future nuclear fusion installations. The initial Engineering Validation and Engineering Design Activity (EVEDA) phase will consist of the construction and characterization of a full scale prototype of the low energy section (up to 9 MeV). This will then be used to validate the design of such a machine.

 
The IFMIF prototype accelerator

Beam characterization in progress.

World record for a deuteron injector

 

The first component of the IFMIF-EVADA accelerator, the injector, has been designed and constructed by SACM teams in collaboration with the SIS. This injector was installed in the hall of Building 126 at CEA Saclay in early 2011. The injector consists of an electronic cyclotronic resonance (ECR) ion source installed on a high voltage platform capable of delivering 100 kV, together with a grounded transport line fitted with solenoids, electrically driven pumps and diagnostic systems. The entire system is installed in a concrete radiological protection bunker.

 

Following delivery and acceptance of the various systems and conditioning of the injector, a beam of hydrogen ions was generated and guided along the transport line at energies of between 50 and 100 keV in both pulsed and continuous modes. More recently, the first deuteron beam has been extracted at an energy of 100 keV with a duty cycle of 1% (10 ms – 1 Hz). The duty cycle was progressively increased to 10%, 30% and 50% before entering continuous mode. During these stages, the beam was measured and analyzed using diagnostic systems specially developed for this application.

 
The IFMIF prototype accelerator

Emittance of a pulsed beam at 140 mA - 100 keV.

During November 2012, in the presence of delegates from the Japan Atomic Energy Agency (JAEA) and the European Fusion for Energy group (F4E), the IFMIF-EVEDA injector achieved almost all of the required performance criteria, both in terms of the beam characteristics and the machine and personnel protection systems. The control system was also approved. The results obtained represented a world record for this type of beam. In continuous mode, the beam power reached 17 kW at the exit from the source (170 mA of deuterium ions at 100 kV), and the D+ ion current reached 140 mA, i.e. 14 kW, immediately behind the radio frequency quadrupole (RFQ) feed horn. In pulsed mode, the emittance measurements carried out with beams of between 100 and 150 mA showed that the beam quality met with the requirements at the entry to the RFQ accelerating cavity.

 

On completion of these tests, F4E, JAEA and the project manager decided to transfer the injector to the Rokkasho site in Japan in early 2013. The injector will be the first accelerator component to be installed at the Japanese site. The installation will be carried out by a team from JAEA with the active participation of CEA personnel. The beam is expected to go live again in the late summer of 2013.

 

 
The IFMIF prototype accelerator

General view of the IFMIF/EVEDA accelerator cryomodule.

The cryomodule

 

The cryomodule of the IFMIF prototype accelerator is intended to accelerate the deuteron beam exiting the RFQ from 5 MeV to 9 MeV. It consists of eight superconducting cavities operating at 175 MHz, equipped with power couplers that continuously transfer 70 kW of power to the beam. Eight superconducting solenoids are fitted between each cavity to focus the beam. The Spanish laboratory and collaboration partner Ciemat is responsible for the solenoids and their current feeds.

 

Prototype niobium half-wave resonator (HWR) cavities were delivered to the CEA at the end of 2010. A number of additional studies involving interconnected RF and thermal simulations were carried out at the same time as the first radio frequency tests in a vertical cryostat. In addition, new mechanical studies have been carried out with the aim of ensuring that the cavity complies with the Japanese regulations for equipment under pressure. Following these studies, the design of the frequency tuning system was changed to a system based on the elastic deformation of the cavity, similar to the technique used in SPIRAL2. A prototype cavity compatible with this type of tuning system was qualified in December 2012, demonstrating a level of performance well in excess of the specifications. The accelerator field achieved 8 MV/m (specification: E = 4.5 MV/m), and the nominal quality factor was obtained (specification: Q0 = 5 x 108 at 4.5 MV/m).

 
The IFMIF prototype accelerator

Prototype power coupler.

At the same time, a number of modifications to the cryomodule were made, including an increase in the length of the cryomodule, optimization of the pumping circuit, and changes to the alignment system. The detailed design of the cryomodule was released following these changes.

 

The call for proposals for the design and construction of the set of power couplers was issued in 2009 and the contract was awarded to the American company CPI. SACM was responsible for overseeing the design, construction and characterization of the components, and prototypes were delivered to the CEA in June 2012. They were then prepared for use in the cleanroom and shipped to Ciemat for the RF power couplers to be conditioned.

 

Manufacture of the production couplers is due to begin during 2013 following the RF conditioning of the prototypes, together with the various cryomodule components including the vacuum vessel and thermal screens, helium circuit, magnetic shielding and supporting frame. The procurement of the niobium is currently under way with a view to beginning manufacture of the production HWR cavities.

 
The IFMIF prototype accelerator

Beam density along the IFMIF-EVEDA accelerator.

Simulations of the beam dynamics

 

The deuteron beam from the IFMIF accelerator features two characteristics that have never been achieved before; a power of 5 MW and an intensity of 125 mA in continuous mode. Operation at these levels poses new challenges in the design and optimization of the beam dynamics. These include a required beam loss of less than 10-6 times the total beam intensity, non-linear dynamics resulting from strong space charge forces, difficulties relating to the installation of sub-assemblies and diagnostic systems due to a highly compact machine structure. With these constraints in mind, a specific strategy for adjusting the accelerator to achieve minimum beam losses has been implemented through simulation, especially in relation to those sections of the machine in which the beam energy exceeds 5 MeV.

 

Initial studies into the beam dynamics have been carried out separately for each accelerator sub-system including the low energy line (LBE), the radio frequency quadrupole (RFQ), the medium energy line (MBE), the superconducting linac, and the high energy line (LHE). This is followed by a simulation of the entire IFMIF-EVEDA accelerator, from the exit from the ion source to the end of the high energy line (beam stop).

 
The IFMIF prototype accelerator

Cumulative beam density for 500 machines with randomly applied errors.

Simulations including the static and dynamic errors for each of the various components of the machine are carried out in order to validate the design and adjustment of the IFMIF-EVEDA accelerator. The static errors include errors in the alignment of the various accelerator components and focusing errors. In this case, errors in the beam trajectory may be corrected by deflectors associated with beam position monitors located at strategic points along the length of the machine. The dynamic errors arise from vibration phenomena or rapid variations in the power supplies. The time constants of these errors are too fast to be corrected. It has been determined that the beam losses remain acceptable, even assuming worst case error values.

 

 

Radio frequency test station

 

The IFMIF-ESS bunker occupying a floor area of 18 m x 5 m in the hall of Building 126 at CEA Saclay will house the RFQ and cryomodule tests. Biological protection is provided by the 1.5 m thick walls. The installation is supplied with 800 kW of electrical power and 80 m3/h of cooling water at 22 °C. The bunker will be fitted with a 200 kW RF source operating at a frequency of 175 MHz.

 

 


 

 

Last update : 06/22 2018 (3360)

 

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