The sensors are divided into two groups; one group (DFD, LP, MWR, and VAC) is installed in a case house on the top of the second stage of the deployed boom, while the other group (HF, VLF and IMP) is installed in another case housed on top of the first stage of the boom. The sensors of each case are assembled in one package in order to make it as small as possible. Shield cases are used to protect the sensors from interfering with each other. The antennas for the VLF, the HF and the IMP are extendable and initially restrained by a wire on the surface of the case, which is cut later on. After the METS experiment, the antennas are detached by being cut at the roots of the antennas and thrown away from the case. This is unfortunately unavoidable because their complete retraction (necessary before retrieval of the SFU) would require a complex mechanical system. Neutral Gas Plume (NGP) The frequency of collisions at the altitude of the experiment (about 400 km) between ionospheric electrons and neutral particles will not be large enough to effectively heat the plasma with the microwave. The neutral gas plume (NGP) injects a neutral gas of nitrogen into the ionospheric plasma in order to artificially enhance the collision frequency for ohmic heating. The gas is released via a simple system (blow-down type) which is composed of a gas tank (904 cc), an electrical valve, a flow regulator and a Laval nozzle. The nozzle has an aperture of 0.9 mm in diameter opening out into a 10 degree cone towards the focal point of the microwave. The spherical gas tank and the electrical valve must withstand the gas pressure
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