JET's recent experimental programme, with its high power and Trace Tritium Experimental (TTE) campaigns, has focused progressively more on producing plasmas of reactor relevance. Such a programme requires major progress in many detection techniques, in particular in the fields of "burning plasma diagnostics" "i.e." neutron, alpha particle, He ash, and Isotopic Composition measurements. In the last years it has emerged very clearly that the diagnostics for these quantities can also provide very useful information about crucial physical aspects of great reactor relevance.