Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

In tokamaks and stellarators, measurements of electromagnetic fluctuations in the presence of resonant particle drive, including fusion-produced α's, reveal the excitation of Alfvén eigenmodes (AE), related under certain conditions to a degradation in the fast-particle confinement. The balance between the drive and the background damping is investigated using active diagnostic systems to excite and measure the AE spectrum in terms of frequencies and damping rates. At JET, saddle-coil antennae drive low toroidal mode number (n < 4) AE in the range 30-500 kHz, including toroidal AE, kinetic AE, elliptical AE and global AE. Conditions for weak damping (γ /ω damp < 1%) are identified. Low-n AE appear to be strongly damped (γ /ω damp > 1%) during the creation of the magnetic X-point. In the presence of resonant fast particles, information on the instability drive is obtained: low-n modes are found to be stable in the presence of NBI with v /v A < 1. Fast ions generated by ICRH are observed to produce a drive for P ICRH > P thresh , with 2.5 MW < P thresh < 5 MW; under these conditions, intrinsically driven TAE and EAE are clearly observed in the magnetic fluctuation spectra, with no measurable effect on the plasma performance.

We present the real-time VME system used to detect and track MHD instabilities, and particularly Alfvén Eigenmodes, on the JET tokamak [J.Wesson, Tokamaks, 3 rd edition, (Oxford Science Publication, Oxford, 2003), p.617]. This system runs on a 1kHz clock cycle, and allows performing a real-time, unsupervised and blind detection, decomposition and tracking of the individual components in a frequency-degenerate, multi-harmonic spectrum, using a small number of input data which are unevenly sampled in the spatial domain. This makes it possible to follow in real-time the detected modes as the plasma background evolves, and measure in real-time their frequency, damping rate, toroidal mode-number and relative amplitude. The successful implementation of this system opens a clear path towards developing real-time control tools for electro-magnetic instabilities in future fusion devices aimed at achieving a net energy gain, such as ITER [J.Wesson, Tokamaks,3 1) Introduction.The problem of blind and unsupervised real-time detection of the different components in a multiharmonics spectrum using a small number of input data which are un-evenly sampled in the spatial domain is now becoming one of the main aspects required for machine protection and the control of plasma discharges in thermonuclear fusion experiments, and a wealth of literature is available on this subject (for some examples see Chapter3 and Chapter7 and references therein in Ref. [1] and Chapter2 and references therein in Ref. [2]). The method routinely used for this analysis involves sampling of a (relatively) small set of magnetic and non-magnetic signals, often containing some spatial periodicities so as to enhance or eliminate detection of certain components when the input signals are processed appropriately. A real-time algorithm then runs, and a global alarm is generated which may trigger a feedback control mechanism under certain specified conditions. The main drawback of this method is that it can only detect modes when they have become unstable, i.e. when they may have already had some, possibly detrimental, effect on the plasma background parameters.Conversely, an innovative method has been employed for quite some time now on the JET tokamak [3], which combines active excitation of magnetic field perturbations with a very small amplitude at the plasma edge (maximum intensity max(|B DRIVEN |)~0.1G, i.e. 10 5 times smaller than the typical value of the toroidal magnetic field in JET, B TOR~( 1-3)T) with synchronous real-time detection of the driven perturbations. This is the so-called Alfvén Eigenmodes (AEs) Active Diagnostic (AEAD) system [4,5], of which the real-time Alfvén Eigenmodes Local Manager (AELM) constitutes one essential, moreover worldwide unique component. This diagnostic system allows the real-time detection and tracking of the driven modes, usually magneto-hydrodynamic (MHD) Eigenmodes supported by the plasma, when still stable, i.e. when the modes have a positive damping >0, and have not yet caused any effect on the plas...

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