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Alexis Devitre - internship at PPPL

The Lithium Tokamak eXperiment Beta (LTX-β) is a spherical magnetic confinement fusion device operated at the Princeton Plasma Physics Laboratory (PPPL), a US-DoE facility located in Plainsboro, New Jersey.

Lithium Tokamak eXperiment Beta

LTX-β is a singular machine in many ways:

  • The tokamak is mainly operated by graduate students and postdoctoral fellows providing a unique opportunity for young scientists to be closely involved in building and running a tokamak. For instance, it is possible to participate in the physics and engineering of vacuum and magnetic components, cooling systems, power electronics, diagnostics, data acquisition, etc.
  • LTX-β is dedicated to study the effects of lithium on confinement. Over the years, lithium – a low-Z metallic element and hydrogen-getter – has found many applications in fusion devices. For example, in the 80s, the Tokamak Test Fusion Reactor (TFTR) obtained improvements in energy confinement times (up to 50%!) by coating the inner walls of the vacuum vessel with lithium. More recently, the pacing and suppression of Edge-Localized Mode (ELM) – deleterious events present in devices with internal transport barriers – has been demonstrated in EAST (China) and DIII-D (California) by controlled introduction of lithium in plasma using piezoelectric droppers.
  • In its first campaign, the experiment produced an isomak: a so-far theoretical configuration in which temperature-gradients are suppressed. As such, the isomak carries the promise of significant simplifications of the complex tokamak equilibrium.

Can the isomak scale to reactor relevant injected powers? This is one of the main questions which will be addressed by the recently upgraded LTX-β. Another important issue is to quantify the transport of lithium in a tokamak Scrape-Off-Layer (SOL). To shed light on this concern, a piezoelectric granule dropper was commissioned. As LTX- β starts up, the dropper will be the only source of impurities in an otherwise pure-hydrogen plasma. Fast cameras will follow the vaporization and ionization of lithium particles in the upper SOL. Results from transport simulations will be compared to the measurements of lithium-ion concentrations (charge exchange recombination spectroscopy) and depositions on the walls (sample exposure probe).

My internship

My internship consisted in the commissioning of the granule dropper. I was asked to design and install a support stand in a highly-constrained environment on top of LTX- β. I also had to test out a control system to size the granules and time their drop w.r.t. the plasma discharge. This system comprises a fast camera and a laser-photodiode detector.

To complete the project, I became acquainted with computer-aided design, material solutions and manufacturers, instruments, machining, etc. This task served me to review some basic (yet crucial!) physics knowledge acquired during my bachelor, while developing a sense of the practical issues of real-world experimental physics. I also had to invest some computational physics skills in the implementation of an ablation model and transport simulations (to be completed).

On the personal side, I would note the great satisfaction of being part of the rich, diverse and prolific academic community of PPPL and Princeton University. Paraphrasing J.K. Rowling: “help will always be given at PRINCETON to those who ask for it”! Several technical and tutorial seminars were very instructive – e.g. weekly colloquia and the PPPL plasma physics graduate summer school. Volunteer at the Plasma Surface Interaction conference (PSI 2018) and attending group meetings and remote joint-experiments at EAST and DIII-D further enlarged my horizons.

While providing a complete experience in state-of-the-art experimental research, my internship on LTX-β was a unique opportunity to be part of a rich and prolific scientific community. I am in debt to the financial support of the Plasma Science and Technology department of PPPL as well as the NSTX-U group, FUSENET and the Grand-Est region of France. A world of thanks to professor Richard Majeski for supporting my participation at LTX-β, to Robert Lunsford for making me part of the dropper project, and Bob Kaita for guidance, perspectives and ideas to explore. Last and most importantly I would like to acknowledge my newfound colleagues and friends Anurag Maan, Drew Elliott, Paul Hughes and Dennis Boyle who were always there to keep me out of trouble with both my feet on the ground.

- Alexis Devitre