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Neutron monitors for Wendelstein 7-X

Fusion experiment with PTB measuring equipment launched

PTB News 1.2016
31.03.2016
Especially interesting for
  • energy generation

The Wendelstein 7-X fusion experiment, which recently went into operation at the Max-Planck-Institut für Plasmaphysik (IPP) in Greifswald, is intended to demonstrate the suitability of stellerator-type fusion facilities for the production of energy. At the end of 2015, a helium plasma was generated for the first time. Producing plasmas with deuterium, in which neutrons are generated by nuclear reactions, is planned for a later date. Within the scope of a cooperation agreement, PTB together with IPP developed a monitoring system which measures the total neutron radiation generated by the machine during operation.

Neutron monitor (left) during calibration with monoenergetic neutrons in the reference radiation fields of PTB.

The neutron monitors are used to measure neutron radiation during the operation of Wendelstein 7-X (W7-X), in particular to establish that the annual limit of neutron production permitted by the operation authorization will not be exceeded. In addition, important information about physical processes taking place in the fusion plasma will be obtained by measuring the time-resolved neutron fluence rate. When the neutron monitors are correctly calibrated, they enable the measurement of the fusion rate and of the energy generated by the fusion plasma. They also allow conclusions to be drawn, e.g. on the ion temperature of the plasma.

Essential criteria for the design of the three monitors installed at the facility include the lowest possible energy dependence of the monitor response and a directional dependence adapted to the main angle of incidence of the neutrons. These requirements ensure that the influence of scattered neutrons is kept low. A further design criterion is the ability to measure the neutron production rate over a dynamic range of at least five orders of magnitude. These requirements have been accounted for by means of a set-up which conforms to the principle of the “precision long counter” from 1966: the monitor design consists of a moderator made of polyethylene, which surrounds several detectors with different sensitivities to thermal neutrons. The design was optimized by means of neutron transport calculations.

The three monitors were calibrated in the reference radiation fields of PTB. In addition, the behavior of the various detectors in the monitors was examined at very high count rates using the highintensity neutron field of PTB. It could be demonstrated that the proportional counters used (types 3He and BF3) can each be operated with count rates of up to 2 · 105 s-1, and the ionization chambers with cathodes made of 235U with count rates of up to 1 · 106 s-1. These counters make it possible to measure the neutron production rate of W7-X in a range from 1011 to 1016 neutrons per second with a time resolution of 5 ms and a relative statistical accuracy that is better than 15 %.

Before W7-X went into operation, the neutron monitors were calibrated on site – as specified in the operation authorisation which had been applied for. For this purpose, a 241AmBe radionuclide neutron source was used. The source strength had previously been determined at PTB and is traceable to primary standards. The source was placed on a rail system installed inside the plasma vessel and moved along a path which corresponds to the average location of the plasma in the vessel. The calibration factor was obtained from the time-dependent count rates of all detectors; for the most sensitive proportional counters, this factor was determined with a relative statistical measurement uncertainty of better than 1 %. To determine the total measurement uncertainty of the calibration factors during plasma operation, it is in addition necessary to take systematic effects into account such as the complex plasma volume and the neutron energy distribution from the plasma which differs from the one of the calibration source used. To better understand these interdependences, further investigations based on detailed neutron transport calculations will be carried out.

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Contact

Andreas Zimbal
Department 6.4 “Ion and Neutron Radiation”
+49 (0)531 592-6530
andreas.zimbal(at)ptb.de

 

Scientific publication

B. Wiegel, W. Schneider, F. Grünauer, R. Burhenn, H. Schuhmacher, A. Zimbal: Monitoring of the neutron production at the Wendelstein 7-X stellarator. Radiat. Prot. Dosim. 161, 326–330 (2014)