The high neutron fluxes of burning plasmas will be an essential source of information to assess the performance and properties of ITER discharges. Therefore, neutron diagnostics are expected to play a key role in ITER and the possibility of testing them in advance is one of the reasons for the great emphasis placed on them during the recent JET Campaign which used trace levels of tritium in deuterium discharges. In the area of neutron counters, a significant effort is being devoted to the search for good detectors which can survive the harsh environment of a fusion device. This involves an international co-operation between EFDA parties and Russian Federation Institutes.

Silicon detectors provide accurate information on neutron fluxes but show severe limitations in their operational life when operating in the harsh environment present in a tokamak. This is a matter of concern for their use as 14 MeV neutron monitors in ITER, due to the expected high neutron fluences.

There are several approaches which could be alternatives to the more standard silicon technology. The large band gap energy (5.5 eV) and the high breakdown voltage (≈ 10 7 Vcm-1) make diamond the most promising among the different proposed alternatives to silicon for fast and low-noise sensors. In the past, and also in the Trace Tritium Experiments (TTE) of 2003, Natural Diamond Detectors (NDD) have been used as 14 MeV neutron detectors. During these experiments Chemical Vapour Deposited (CVD) diamond detectors were also tested successfully (Figure 1 shows the diode installed on JET). A particular advantage of the CVD diamonds with respect to natural diamonds is their low cost and the possibility of producing large surfaces of variable thickness ranging from a few microns up to more than 1 mm.

For several years high quality polycrystalline CVD diamond films have been produced at the Faculty of Engineering, Rome “Tor Vergata” University. Since 1998 collaboration with Association Euratom-ENEA Frascati (Italy) has been established to study the use of CVD diamond films as fast neutron detectors. The comparison between CVD diamond and standard silicon detectors shows an excellent correlation (Figure 2).

In the future, it is proposed to install new and more sensitive detectors to be used during the JET Experimental Campaigns of 2005 for the measurement of the total neutron flux and to provide additional information for the design of a neutron camera for ITER.

This work shows yet again that the tritium capability of JET is a key asset for progressing reactor relevant issues on the route to ITER.

For more information see our EFDA website:

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editors: Federico Casci, Doris Lutz-Lanzinger

graphic design: Karen Jens

© M.Q.Tran (EFDA Leader) 2004.

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