GENEPI (GEnérateur de NEutrons Pulsé Intense) is an electrostatic machine accelerating deuterium (2H) ions. It delivers short (~600 ns FWHM) and very intense (~50 mA peak) pulses of ions at energy of 250 keV. The deuterium ions generate neutrons by hitting the neutron-rich target. The produced neutron energy is either 3 MeV on a Deuterium target or 15 MeV for a tritium (3H) target.
To create deuterons, a pulsed duoplasmatron source is used to ionize molecular deuterium. The source sits in a high-voltage platform held at +250 kV with respect to ground. A series of conical high voltage electrodes extracts and focuses the ion beams. The beam is accelerated up to 250 keV by a 5-stage accelerating structure. In order to control the transversal beam dimensions all along the beam line (~5 m), electrostatic quadrupolar lenses compensate the space charge effects (internal Columbian repulsion). The beam is focused on target. The entire beam line is kept under vacuum (about 10-7 mbar) to minimize interactions with residual gas molecules. A dipolar electromagnet performs the magnetic selection of the beam to transport solely D+ ions to the target.
GENEPI1 (CEA-Cadarache, 2000-2004)
The GENEPI1 accelerator was operated on the MASURCA (MAquette de SURgénératrice à CAdarache) reactor within the MUSE (MUltiplication d'une Source Externe) experimental program. This was the first experience of an installation dedicated to the study of accelerator driven reactors within the framework of the program GDR GEDEPEON (Groupement De Recherche GEstion des DEchets pour la Production d'Energie par des Options Nouvelles). This machine produced and transported deuteron beams up to 250 keV on a target, made of deuterium or tritium, in order to produce neutrons. The target was located at the center of the MASURCA reactor. The accelerator was designed, constructed and commissioned at LPSC between 1996 and 1999. After its re-assembly on the Cadarache site (first semester of 2000), the nuclear safety authorities allowed the commissioning of the accelerator and the modification of the reactor loading.The first coupling of the accelerator, using a deuterium target, to the slightly subcritical reactor was accomplished in November 2001 (MUSE 4). After this first successful coupling, the MUSE experimental program was carried out. After being trained by LPSC staff, the CEA team of Cadarache ensured the accelerator operations independentlly.A remote surveillance was perfomed by LPSC to monitor the accelerator performances and diagnose several operational accidents. This allowed the reduction of on-site interventions. The coupled operation of the accelerator and the reactor stopped in 2004 at the end of the experimental program.
Frequency modulation
The beam intensity, and therefore the neutron production, can be controlled by modifying the source operational parameters. The preferred method is to control the pulse frequency conserving their shape. In order to study delayed neutrons emitted by the reactor, the pulse frequency has to vary between two extreme values (typically 300 Hz and 4 kHz). This is done with a modulation input on the command module. The frequency decrease is not bounded but the module limits the frequency rise. This is required to maintain the current increase rate below the neutron doubling time in order to avoid reactor SRCAMS. Moreover, the slope of the average current rise is monitored on the target (logarithmic derivative) and a safety interlock drops the frequency down to its minimum in the event of a problem (human error, electrical dischrage, fast current increase).
C. Destouches et al. NIM A, vol 562, issue 2, pp 601-609 (2006)," The GENEPI accelerator operation feedback at the MASURCA reactor facility".
GENEPI2 (LPSC, since 2004)
In November 2001, it was decided to construct a second accelerator, GENEPI2, to be installed at LPSC as a part of the platform for nuclear energy research called PEREN (Plateforme d'Etudes et de Recherche sur l'Energie Nucléaire). Since the GENEPI2 accelerator had to satisfy beam specifications similar to those of GENEPI1, the structure of this second machine is almost identical to the previous one. The main difference consists in the fact that GENEPI2 can be equipped of spectrometers which slow down neutrons in order to analyse their energy. According to the experiment requirements, one can use either a light slow down block made of graphite or Teflon (placed at the first focalisation point, short beam line) or a lead slow down block (placed at the second focalisation point, long beam line) to surround the production target. Most of the design studies for the machine took place in 2002 as the same time as the first production steps. At the beginning of 2003, the duoplasmatron source was mounted and the first tests were performed. In March 2003, for the first time the beam reached 200 keV energy after the accelerator tube and the beam reached the target position in September. The blocks of graphite and lead were mounted and the first experience on a deuterium target took place at the beginning of 2004.
Beam optics
The GENEPI1 transport optics was adapted to obtain two beam focussing locations corresponding to the two target positions (short or long beam line). After the electromagnet deviation, two quadrupoles, called Q3 and Q4, insure the fist focusing point at the location of the light slow down spectrometer. The quadrupoles are followed by a simple beam pipe ended by the target (so-called short configuration). In order to reach the center of the lead slow down spectrometer, a longer beam pipe and two additional quadrupoles Q5 and Q6 are installed. In this so-called long configuration, the last quadrupole is placed about 1 m away from the target. In order to optimize the beam transport, measurements of beam profile and emittance were performed at the two target emplacements and at intermediate positions: output of the accelerator tube and downstream of the electromagnet.
Mechanics
GENEPI2 is placed in the former experimental zone A of the SARA cyclotron. The high voltage platform and granite beam for supporting the beam line are similar to those used for GENEPI1. Two cranes of 1 ton and 500 kg allow handling of the accelerator elements and light blocks. Two forklift trucks are used to pile up Teflon blocks and the two graphite cylinders. The lead spectrometer is made of blocks of 8 tons each which can be moved with the main crane of the accelerator building (bunker roof open during operations).
GENEPI-3C (SCK-CEN, Belgium, since 2009)
Continuing the studies on accelerator driven systems, LPSC developed a third accelerator for the GUINEVERE project at the SCK-CEN in Mol (Belgium). This project is based on a versatile neutron source, the GENEPI-3C machine, driving the fast VENUS-F reactor. This new accelerator is more complex than the previous two machines because it must fulfill more demanding specifications in order to insure the GUINEVERE experimental programme. On one hand, the source has to operate eihther in pulsed mode or has to produce a continuous (DC) beam with fast programmable interruptions. On the other hand, the beam line has to be inserted vertically in the core of the VENUS-F reactor (in the GENEPI1 case, the insertion was horizontal).
