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Description
In the FIREBALL experiments at CERN’s HiRadMat facility an electron-positron pair beam [1] is delivered through a plasma discharge with the goal of observing beam-plasma instabilities [2]. As part of the FIREBALL IV experiment a three meter long inductively coupled plasma discharge in a cylindrical configuration was designed and characterized. This is an update from the FIREBALL I and II experiments, which utilized a 870 mm long discharge [3]. The coil setup consists of two helical coils (uniform pitch of 51 mm, 93 mm diameter) which are connected in series electrically. Theses coils are mounted around glass tubes (inner diameter 58 mm) of an already existing plasma cell setup from the FIREBALL III experiment. This discharge offers the possibility of studying interaction with the plasma even for instabilities that need tens-of-centimetres propagation length to develop. The argon discharge is driven by a 6 kW radio-frequency generator combined with a matching network at 13.56 MHz. The inductively coupled plasma reaches plasma densities on the order of 10^12 cm^-3 within large regions under the coils. Additionally, the remote location in CERN’s underground HiRadMat facility required a special focus on remote control and reproducibility without in-situ interference. Operating parameters of the plasma cell, including vacuum pressure, delivered power, capacitor positions and live data from a Langmuir probe are captured and transmitted. We present measurements of the plasma density and plasma uniformity using a double Langmuir probe to define an optimal set point for the FIREBALL IV experiment. The coil setup with one electrical coil proved to create a very uniform high-density plasma, increasing the reproducibility of experimental results over preceding iterations of FIREBALL.
[1] C.D. Arrowsmith et al. Nature Communications 15 (2024) 5029
[2] C.D. Arrowsmith et al. Phys. Rev. Res. 3 (2021) 023103
[3] C.D. Arrowsmith et al. Journal of Instrumentation 18 (2023) P04008