Neutralisation of intense ion beams at JET
Neutralization is an important issue in the design of neutral beam injection (NBI) systems for heating and fueling fusion plasmas. It has long been established that the neutralization efficiency of the JET injectors falls short of the theoretical value as determined by the expected gas target thickness and the well-known reaction cross-sections. This problem is even more acute in the recently upgraded 130 kV/60 Amp injectors at JET. An illustration shows the measured deuterium power delivered to JET as a function of extracted beam power. It has been proposed that the efficiency deficit is caused by a reduction in the line integrated gas density in the injector's neutralizer. This density reduction could be due to a number of factors including wall pumping and modification of the flow regime, but recent spectroscopic measurements of the neutralization system suggest that the density depletion is due to heating of the neutralizer gas. The mechanism responsible for this is thought to be indirect energy transfer from the beam to the neutral gas via the low temperature plasma that is formed in the neutralization process inside the neutralizer cell. This paper presents measurements of neutralization efficiency based on calorimetric data from the JET injectors. We also present some results from an experimental investigation at the JET Neutral Beam Test Bed including measurements of the low temperature neutralizer plasma. The plasma measurements are used as input parameters for a gas heating model and the outputs of this model are compared to a spectroscopic measurement of the neutralizer gas temperature. Overall, the results support hypothesis that the neutralization deficit is caused by gas density depletion due to indirect heating of the neutralizer gas.
On suitable experiments for demonstrating the feasibility of the beam-driven plasma neutraliser for neutral beam injectors for fusion reactors
Plasma neutralisers promise increased neutralisation efficiency of negative ion beams in neutral beam injection (NBI) beamlines compared with gas neutralisers. It has been suggested that, in the presence of an electron-confining magnetic cusp field along all neutraliser walls, the beam itself could ionise the neutraliser gas sufficiently to take advantage of this effect, avoiding the added complexity of external power coupling to the neutraliser. These predictions come from a zero-dimensional model by Surrey and Holmes (2013 AIP Conf. Proc. 1515 532) and Turner and Holmes (2019 Fusion Eng. Des. 149 111327). We have revisited and modified this model by introducing slowing-down energy distributions for stripped and Rudd electrons, including electron impact dissociation as an electron energy loss channel and taking into account dissociative recombination of molecular ions with electrons. Including the latter effect reduces the predicted plasma density by about a factor of four and the achievable neutralisation yield from ∼80% to 68% in the case of a negative deuterium ion beam with an energy of 1 MeV and a current of 40 A. With this revised model we estimate the expected performance of potential beam-driven plasma neutralisers (BDPN) on a variety of existing negative ion beam test facilities for NBI. Based on these results, we conclude that the most suitable proof-of-principle experiment would be a dedicated chamber, ideally of the same dimensions and with the same magnetic cusp configuration as a BDPN for the DEMO NBI, in which the plasma is not created primarily by the fast electrons stripped from the beam ions, but by electrons of similar current and energy emitted from biased filaments.
Turbine blade temperature calculation and life estimation - a sensitivity analysis
Abstract The overall operating cost of the modern gas turbines is greatly influenced by the durability of hot section components operating at high temperatures. In turbine operating conditions, some defects may occur which can decrease hot section life. In the present paper, methods used for calculating blade temperature and life are demonstrated and validated. Using these methods, a set of sensitivity analyses on the parameters affecting temperature and life of a high pressure, high temperature turbine first stage blade is carried out. Investigated uncertainties are: (1) blade coating thickness, (2) coolant inlet pressure and temperature (as a result of secondary air system), and (3) gas turbine load variation. Results show that increasing thermal barrier coating thickness by 3 times, leads to rise in the blade life by 9 times. In addition, considering inlet cooling temperature and pressure, deviation in temperature has greater effect on blade life. One of the interesting points that can be realized from the results is that 300 hours operation at 70% load can be equal to one hour operation at base load.