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The paper generally outlines a design of new multichannel optical diagnostics for the COMPASS (COMPact ASSembly) tokamak. The final design of a port plug for a multispectral diagnostic tool is introduced and its evolution taking into consideration spatial and other technical restrictions is reviewed. A similar solution of both focusing optics and optical signal diversion will be used for a new Thomson scattering (TS) system, which is specified in the second part of the article. Main parameters of the TS system, such as laser energy and regions of observation, were determined with respect to both electron temperature and density profiles and required ultrahigh spatial resolution at the plasma edge.

Quasi-stationary H-mode plasmas have been produced on MAST and submitted to the ITPA H-mode confinement database. The MAST data are expanding the database by a factor of 2.2 in inverse aspect ratio e = a/R and by a factor of 2.5 in thermal toroidal β. The MAST energy confinement times broadly agree with the IPB98(y, 2) scaling law. Merger with the published international database shows that the energy confinement time in MAST supports an e dependence consistent with the subset of conventional shaped cross-section tokamaks when represented in engineering parameters. In dimensionless parameters, MAST indicates that the collisionality dependence might be stronger than in the IPB98(y, 2) scaling. MAST data link e and β scalings and, as a result, the proposed β independence would imply smaller e exponent. L-mode confinement exceeds the value given by the current L-mode scaling, suggesting a stronger e dependence than predicted. In H-mode, the working particle transport in the core is dominated by the Ware pinch and the neutral beam source.

The development of reliable H-modes on MAST, together with advances in heating power and a range of high spatial resolution diagnostics, has provided a platform to enable MAST to address some of the most important issues of tokamak stability. In particular the high beta potential of the spherical tokamak is highlighted with stable operation at beta(N) similar to 5-6, beta(T) similar to 16% and beta(p) up to similar to2. Magnetic diagnostic evaluation of the global beta parameters is independently confirmed by kinetic profile data. Calculations indicate that the beta(N) values are in the vicinity of no-wall stability limits. Studies of neoclassical tearing modes (NTMs) have been extended to explore their effects and develop avoidance strategies. Experiments have demonstrated that sawteeth play a strong role in triggering NTMs-by avoiding large sawteeth a much higher beta(N) value has been reached. The significance of NTMs is confirmed, with large islands observed using the 300 point Thomson scattering diagnostic, and locking of large n = 1 modes frequently leading to disruptions, which become more rapid at low q(95). The role of error fields has been explored. H-mode plasmas are also limited by edge localized modes (ELMs), with confinement degraded as the ELM frequency rises. However, in contrast to the conventional tokamak, the ELMs in high performing regimes on MAST (H-IPB98Y2 similar to 1) appear to be type III in nature. Modelling using the ELITE code, which incorporates finite n corrections, identifies instability to peeling modes, consistent with a type III interpretation. It also shows considerable scope to raise pressure gradients before ballooning type modes (perhaps associated with type I ELMs) occur. The calculations show that narrow pedestals can support much stronger pressure gradients than might be expected from simple n = infinity ballooning calculations. Finally sawteeth are shown to degrade confinement by similar to10-15% in particular cases examined. They are observed not to remove the q = 1 surface in the cases where snakes are present-various physics models of the sawteeth are now being explored. Thus research on MAST is not only demonstrating stable operation at high performance levels and developing methods to control instabilities; it is also providing detailed tests of the stability physics and models applicable to conventional tokamaks, such as ITER.

The letter reports the first use of laser ablation of trace impurities into a reversed field pinch (RFP) similar to that used in tokamaks, but selecting different impurity elements appropriate to the confinement times of present RFPs. The ionization rates from helium-like carbon and boron are sufficiently slow to allow measurements of particle confinement times from line intensity decay rates of the near UV transitions. A zero-dimensional treatment, incorporating impurity ionization and confinement, is used to interpret the temporal behaviour of the line intensity. The results obtained yield typical particle confinement times of about 1 ms.

A set of time-skewed analog-to-digital converters (ADCs) in combination with a complex finite-impulse response (FIR) filter has been utilized to directly sample and demodulate RF signals. This architecture is referred to as a complex analog-to-digital converter (CADC) B.W. Tietjen. The clock signals driving the ADCs are staggered in time such that the system achieves an effective sampling rate commensurate with the carrier frequency of interest, while each ADC need only run at a rate greater than the immediate bandwidth of interest. The output of each ADC is fed into the complex bandpass filter, which suppresses the input signal's image frequency to produce I and Q data. The data rate out of the filter is equal to the ADC clock rate; thus the RF band of interest is aliased down to baseband or a low IF band. Hardware implementations of the CADC architecture have been achieved for both UHF and S-Band. The measured values of signal-to-noise ratio (SNR) and image rejection for both prototypes closely match expected results. Furthermore, by using the CADC to down-convert the RF band of interest to a low IF band, a second digital filter can be used to easily suppress any harmonics introduced by the ADC hardware. This provides superior spurious-free dynamic range (SFDR) and increased SNR performance. This technique is demonstrated using the UHF prototype. An automatic filter-tuning algorithm is also implemented to adaptively balance the amplitude and phase of the ADC channels composing the CADC. This technique allows the CADC filter to operate as expected despite hardware imperfections.