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A new triple diaphragm-based trigger system (Fig. 3.8-9) has been constructed to obtain high precision pressure control in magma fragmentation experiments at temperatures up to 900°C. The first diaphragm above the high pressure-high temperature autoclave is stabilised by the elevated pressure in the adjacent chambers. We keep the pressure difference between the first chamber and the autoclave below 50 bars and use it primarily as a thermal shield for the second diaphragm, which facilitates longer run duration. Using this method it is possible to precisely trigger the decompression of the autoclave even at pressures above 200 bars, with <1% pressure error.

Since the processes of magma fragmentation and therefore of volcanic eruptions are largely controlled by volatiles, it is important to control the water content of the melt during the experiments. In this context, the new trigger system contains another advantage in that the thermal dwell period can be prolonged, allowing equilibrium volatile contents to be obtained. Furthermore, a new autoclave designed for fragmentation experiments up to 500 bars has recently been installed. Preliminary experiments are being performed to test the new sample design in the previously obtained P-T range.
 

Fig. 3.8-9: Schematic diagram showing the high pressure-high temperature autoclave and the gas-trigger system for precise pressure control.