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Register a new userImproved Criterion for Sawtooth Trigger and Modelling
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We discuss the role of neoclassical resistivity and local magnetic shear in the triggering of the sawtooth in tokamaks. When collisional detrapping of electrons is considered the value of the safety factor on axis, $q(0,t)$, evolves on a new time scale, $tau_{*}=tau_{eta} u_{*}/(8sqrt{epsilon})$, where $tau_{eta}=4pi a^{2}/[c^{2}eta(0)]$ is the resistive diffusion time, $ u_{*}= u_{e}/(epsilon^{3/2}omega_{te})$ the electron collision frequency normalised to the transit frequency and $epsilon=a/R_{0}$ the tokamak inverse aspect ratio. Such evolution is characterised by the formation of a structure of size $delta_{*}sim u_{*}^{2/3}a$ around the magnetic axis, which can drive rapid evolution of the magnetic shear and decrease of $q(0,t)$. We investigate two possible trigger mechanisms for a sawtooth collapse corresponding to crossing the linear threshold for the $m=1,n=1$ instability and non-linear triggering of this mode by a core resonant mode near the magnetic axis. The sawtooth period in each case is determined by the time for the resistive evolution of the $q$-profile to reach the relevant stability threshold; in the latter case it can be strongly affected by $ u_*.$
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In large hot tokamaks like JET, the width of the reconnecting layer for resistive modes is determined by semi-collisional electron dynamics and is much less than the ion Larmor radius. Firstly a dispersion relation valid in this regime is derived which provides a unified description of drift-tearing modes, kinetic Alfven waves and the internal kink mode at low beta. Tearing mode stability is investigated analytically recovering the stabilising ion orbit effect, obtained previously by Cowley et al. [Phys. Fluids (29) 3230 1986], which implies large values of the tearing mode stability parameter Delta prime are required for instability. Secondly, at high beta it is shown that the tearing mode interacts with the kinetic Alfven wave and that there is an absolute stabilisation for all Delta prime due to the shielding effects of the electron temperature gradients, extending the result of Drake et. al [Phys. Fluids (26) 2509 1983] to large ion orbits. The nature of the transition between these two limits at finite values of beta is then elucidated. The low beta formalism is also relevant to the m=n=1 tearing mode and the dissipative internal kink mode, thus extending the work of Pegoraro et al. [Phys. Fluids B (1) 364 1989] to a more realistic electron model incorporating temperature perturbations, but then the smallness of the dissipative internal kink mode frequency is exploited to obtain a new dispersion relation valid at arbitrary beta. A diagram describing the stability of both the tearing mode and dissipative internal kink mode, in the space of Delta prime and beta, is obtained. The trajectory of the evolution of the current profile during a sawtooth period can be plotted in this diagram, providing a model for the triggering of a sawtooth crash.
New analytical expressions for parallel transport coefficients in multicomponent collisional plasmas are presented in this paper. They are improv
13MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neo-classical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced alpha particle stabilisation for instance, this ancillary sawtooth control can be provided from > 10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the magnetic axis, the efficacy of ECCD and ICRH schemes significantly increases, the negative effect on the fusion gain is reduced, and off-axis negative-ion neutral beam injection (NNBI) can also be considered for sawtooth control. Consequently, schemes to reduce the q = 1 radius are highly desirable, such as early heating to delay the current penetration and, of course, active sawtooth destabilisation to mediate small frequent sawteeth and retain a small q = 1 radius.
In 2007 a prototype of a new analog Sum-Trigger was installed in the MAGIC I telescope, which lowered the trigger threshold from 55 GeV to 25 GeV and led to the detection of pulsed gamma radiation from the Crab pulsar. To eliminate the need for manual tuning and maintenance demanded by that first prototype, a new setup with fully automatic calibration was designed recently. The key element of the new circuit is a novel, continuously variable analog delay line that enables the temporal equalization of the signals from the camera photo sensors, which is crucial for the efficient detection of low-energy showers. A further improvement is the much larger trigger area consisting of a fully revised configuration of overlapping summing patches. The new system will be installed on both telescopes, MAGIC I and II, enabling stereo observation in Sum-Trigger mode. This will significantly improve the sensitivity in the very low energy regime of 20 to 100 GeV, which is essential in particular for detailed pulsar studies, as well as the observation of high-redshift AGNs and distant GRB events. Here we like to present the results of functionality tests of a fully working prototype and the basic design of the final system.
Reliable predictions of light charged particle production in spallation reactions are important to correctly assess gas production in spallation targets. In particular, the helium production yield is important for assessing damage in the window separating the accelerator vacuum from a spallation target, and tritium is a major contributor to the target radioactivity. Up to now, the models available in the MCNPX transport code, including the widely used default option Bertini-Dresner and the INCL4.2-ABLA combination of models, were not able to correctly predict light charged particle yields. The work done recently on both the intranuclear cascade model INCL4, in which cluster emission through a coalescence process has been introduced, and on the de-excitation model ABLA allows correcting these deficiencies. This paper shows that the coalescence emission plays an important role in the tritium and $^3He$ production and that the combination of the newly develop
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