Development and Testing of Tungsten Coatings for the RFX-mod Device Wall

The exploitation of high plasma current regimes in the RFX-mod reversed field pinch experiment has made apparent that the graphite first wall affects plasma performance and makes the operation very much dependent on plasma wall interaction. In fact, Hydrogen retention in the graphite wall of the RFXmod device and strong influxes during plasma operation made very difficult density control especially at high plasma current regimes, explored up to the RFX-mod maximum design value of 2 MA. On the other side, in RFX-mod the same regimes revealed to be the most performing ones when density control problems are overcome by long graphite cleaning sessions or graphite coating by means of Boron or Lithium deposition [1]. These treatments require long application times (of the order of several hours) and the duration of their effectiveness in RFX-mod spans from one to tens of shots, depending on the treatment and on the plasma current. Both to enhance density control and to possibly access new operational regimes, in RFX-mod the replacement of the first wall with tungsten coated graphite is under evaluation. As a first step it is considered to insert W multilayer coated samples in the device vessel up to the first wall. Physical Vapor Deposition (PVD) and Plasma Spray techniques are considered for the coating and the samples are equipped with a set of thermocouples. The insertion location will be monitored with an infrared camera and spectroscopic diagnostics to measure the power deposition and the W ions influxes. We report on the preliminary tests that have been carried out to characterize the adherence of the W coatings on the graphite samples and the morphological characteristics of the coated layer. [1] Puiatti et al. presented at the 24th IAEA Fusion Energy Conference, San Diego, USA, October 8-13, 2012, EX/P5-01