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December 15, 2015

Wendelstein 7-X Stellarator

After 9 years in construction, the stellarator (nuclear-fusion machine) in Greifswald generated plasma for the first time on December 10, 2015. The reactor had been gradually ramped up over the past 12 months. The event was given extensive coverage in the media. Ganter Instruments specially developed sensors for temperature and strain measurement for the project, which are otherwise unavailable on the market.

1. A5-1CR e.bloxx Measuring Module for Logging Low Temperatures

Special Cernox and TVOl cryogenic sensors were used to measure low tempera­tures beginning at 3 Kelvin. These sensors have special characteristics with a resistance of approx. 3,000 to 8,000 Ohms at 3 Kelvin dropping to approx. 30 to 100 Ohms at room temperature.

2. A101IPP Measuring Module

Special low temperature strain gages with half bridges were used for measuring the strain on the coil bodies. The knowledge gained from the series of tests resulted in continuously new requirements being placed on the measuring amplifiers during the course of this cooperative project.

Since the strain gages were located in a strong magnetic field with values up to 6 Tesla, carrier frequency technology was used initially. Here, however, the microphonics effect resulted in vibration in the measuring lines (alternating current in magnetic field). Moreover it was also necessary here to minimize the power consumption on the half bridges. The solution was a pulsed bridge excitation voltage with special filters to guarantee soft on and off switching and thus a low dU/dt ratio.

The test series was nearly completed when it became apparent that the special alloys at the power feedthroughs into the plasma chamber resulted in thermo­electric voltages at the contact points, which reached nearly the level of the strain gage signal voltages themselves. The objective was then to compensate these thermo­electric voltages to ensure pure stress measurement. The measuring cycle already established was supplemented to include additional measurement of the thermoelectric voltage during the strain gage measuring pauses when the excitation voltage was switched off.

3. A107IPP Measuring Module

Modified A107IPP Q.bloxx modules are used at approx. 1,000 other Pt 100 measuring points.

The space required is also an important factor for a number of the 2,000 mea­suring points. With a width of 25 mm, the A101IPP Q.bloxx top hat rail module offers two and the A107IPP Q.bloxx module four measuring channels.

The modules are connected in groups to a number of Q.gate test controllers over a high speed serial RS485 interface. Here all data is logged synchronously, provided with a time signal and sent on to a server over a common instrument network.

The measuring systems are presently being installed; this trend-setting fusion reactor is scheduled to be put into test operation in 2014.

Use of nuclear fusion as a new, environmentally friendly source of energy could prove to be a turning point in the discussion regarding increasing energy requirements in emerging economies, the finite nature of fossil fuels as energy sources and the problems involved with CO2 emissions.

www.ipp.mpg.de

Measurement Variables

Measurement in Numbers

Strain and temperature measuring points

> 2000