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ENEA - Fusion division

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Frascati Neutron Generator (FNG)

FNG was designed for conducting neutronics experiments in the framework of the research activity on controlled thermonuclear fusion. The neutronics design of blanket and shields of the next-step fusion devices requires verification that the neutron cross section data used in the calculations are as accurate as possible and confirmation that the calculation tools used to transport the neutrons are as reliable as practical. To ensure that these criteria are met, a suitable experimental activity (benchmark experiments) is necessary. These activity is carried out using FNG.

FNG uses a deuteron beam accelerated up to 300 keV impinging on a tritiated target to produce a nearly isotropic 14 MeV neutron output via the T(d,n)α fusion reaction.

A mixed beam of atomic and molecular deuterium ions is produced by a duoplasmatron ion source (2) and then analysed by a 90° bending magnet (4). Only monoatomic deuterium ions are sent into a uniform gradient accelerating tube (5). The beam is then focussed onto the target (8) by means of a magnetic quadrupole triplet (7). The ion source, the bending magnet and associated electronics and power supplies are held at high voltage potential and are housed in two different high-voltage terminals. The quadrupole triplet, the vacuum pump station and the target are grounded. The control signal and the current/voltage reading are transmitted to the control room through glass fibers.

FNG schematic: top viewEM analyses on ITER-FEAT

Under beam bombardment, some tritium is released from target into the vacuum system. A vacuum exhaust tritium removal system is used, housed inside a glove-box, just near the pumping station.

FNG schematic: side view EM analyses on ITER-FEAT

When required, deuterated target can be mounted, to produce 2.5 MeV neutron through the D(d,n)3He fusion reaction. In this case the neutron output is about one hundred time less than the 14 MeV output due to the lower cross-section of the D-D reaction.

The present operation performances of FNG are gathered in the table below:

Present FNG characteristics
Beam energy (keV) 230
D+ beam current at the target (mA) 1
Beam spot at the target (mm) 10
Minimum distance from neutron source to expose samples (mm) 4
14 MeV neutron intensity on solid angle of 4π(s-1) 1 x 1011
Tritiated target neutron output half-life time (h) ˜ 25
   
2.5 MeV neutron intensity on solid angle of 4π(s-1) ˜ 1 x 108
   
Neutron output monitor method associated particle

FNG is housed in a large shielded hall (11.5 x 12 m2 and 9 m high) and the target is more than 4 m far from walls, floor and ceiling. This was done to reduce as much as possible neutron background coming from neutron reflection. Also the target holder was design very light to reduce the contamination of the spectrum due to neutron scattering.
[see large photo of FNG laboratory (304KB)]