Will Fuller

Will Fuller

Experimentalist, Engineer, and Adventurer

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Designing A Probe Head For Extreme Environments

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Over the course of my PhD, I have had the opportunity to spend the past few years working closely with the team at MAST-U. MAST-U is a spherical tokamak based at the UKAEA Culham campus in Oxford. The machine is home to one of the most intense man-made environments. Maximum core temperatures of around 3 keV (34 million °C), an ultra-high vacuum (~10-6 mbar), electrically conductive plasma with high currents up to 1 MA, and neutron rates on the order of ~1014 high-energy neutrons per discharge. How do you design a probe to survive these conditions?

MAST-U

Firstly, minimising the time the probe spends in contact with the plasma helps survivability. The probe is mounted on a compressed air driven reciprocating arm capable of moving the probe up to 9 cm at a speed of 0.45 ms-1 (~1 mph).

RP system

Secondly, careful design considerations bearing in mind the environment it will regularly operate in. A good summary can be found on my poster here.

The main factor is the materials used for the probe. The probe is protected with a ceramic shell made from Boron Nitride. BN has a layered hexagonal structure similar to graphene, and shares similar material properties to graphene, notably, high temperature tolerance (~1500 °C), low thermal expansion, and good vacuum compatibility. Unlike graphene, it is not electrically conductive, perfect for an electrical probe inside a plasma. Another benefit due to the layered structure of BN, is if the probe spends too long in the plasma, the top layers burn off, leaving the rest of the shell intact.

Turbulence Probe Head

Maintainability is key. These probes are custom build in-house at UKAEA (by yours truly) and this means they may not always work properly, so the ability to easily repair and upgrade the probe is vital. What better way to understand the maintenance problem then to get hands on with existing probe. One of the most frustrating things I learned was that the graphite probe tips are super fragile (the worst was after hours of wrestling the shell on, the wires twisted the tip and it snapped just as I thought I finished 😒). So we designed several ways to reduce the stress on the tips. Thicker tips for a start (apart from the top), preventing the wires from twisting the internal stack, finally, reducing the number of unique sized tips so we could order more spares.

Mach Probe Head

One thing we didn’t anticipate, to make sure the probe is vacuum compatible, it needs to have all of the moisture baked out of it, so the finished probe head was put into a vacuum oven and heated to 200 °C for 6 hours. Disaster strikes! The internal PEEK components expanded by 0.7 mm, splitting the brittle ceramic shell in half! Version 2 now has smaller diameter internal components leaving a gap of 1.5 mm, plenty of room to expand… this probe is aiming to be installed ready for experiments next year.

Split shell cap

Why did we build this? To explore the outer edge of the plasma to better understand the intermittent plasma bursts (called filaments) in the edge, bringing heat and particles from the core onto the walls of the machine. This reduces performance, and can cause damage to the machine over time. You can find more detail on the layout of the probe tips for the Turbulence Probe Head we build here.

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2nd Year Physics Electronics Lab

Undergraduate course, The University of Warwick, Department of Physics, 2019

Demonstrated for 2nd year undergrads safety in the lab, encouraged them to apply what they had learned in lectures to real-world electronics.

Details of the experiment

Students were to follow a lab script and build a basic AC to DC variable bench top power supply. Next they were to improve DC supply using an amplifier and diode circuit to smooth the output. Finally, for extra-credit, they were tasked with building the same system/circuit with a more advanced chip that did all of the previous work in one small package.

Time Requirements

I was required to run one lab per week, for 3 weeks per group, then mark their lab book within 2 weeks of handing in. Overall I dealt with 6 groups in total, each containing 20+ students.

Experience

This demonstration position helped improve my communication skills and lead the students to figure the answer on their own, without explicitly explaining everything. I learned the joy of teaching those who want to be taught. Working in a team of demonstrators allowed us to better manage the needs of the students and direct our attention to where it was needed most.