We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10 ÿ9 mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10 ÿ13 mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1:2 10 9 positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 10 6 positrons were compressed to a density of 2:6 10 10 cm ÿ3 . This is a factor of 6 improvement over earlier measurements.

Cold antihydrogen atoms were produced by mixing cold samples of antiprotons and positrons. The temperature of the positron plasma was increased by controlled radio-frequency (RF) heating, and the antihydrogen production was measured. Formation is observed to decrease with increased temperature but a simple power law scaling is not observed. Significant production is still present at room temperature

Antihydrogen is formed when antiprotons are mixed with cold positrons in a nested Penning trap. We present experimental evidence, obtained using our antihydrogen annihilation detector, that the spatial distribution of the emerging antihydrogen atoms is independent of the positron temperature and axially enhanced. This indicates that antihydrogen is formed before the antiprotons are in thermal equilibrium with the positron plasma. This result has important implications for the trapping and spectroscopy of antihydrogen.

COMPOSITESThe objective of this study was to adapt ultrasonic and radiographic techniques for the inspection of wind turbine blades and to compare the obtained results. The measurements performed show that radiographic techniques are capable of reliably detecting a number of structural defects within the blade. The adapted air-coupled ultrasonic technique, using Lamb waves, proved to be the most promising in terms of implementation as it only requires access to one side. However, the novel combination of contact pulse-echo and immersion techniques using a moving container of water identified the shape and size of defects better. Through comparisons of images obtained using both radiographic and ultrasonic techniques, different defect properties can be identified. Hence, the best results are achieved when both techniques are combined together.

An observation of a clear dependence of antihydrogen production on positron plasma shapes is reported. For this purpose a plasma control method has been developed combining the plasma rotating-wall technique with a mode diagnostic system. With the help of real-time and nondestructive observations, the rotating-wall parameters have been optimized. The positron plasma can be manipulated into a wide range of shapes ͑aspect ratio 6.5ഛ ␣ Շ 80͒ and densities ͑1.5ϫ 10 8 ഛ n Շ 7 ϫ 10 9 cm −3 ͒ within a short duration ͑25 s͒ compatible with the ATHENA antihydrogen production cycle.