On 29th October, UKAEA announced that after 7 years of preparations the MAST-U tokamak had achieved its first plasma. MAST-U stands for the Mega-Amp Spherical Tokamak Upgrade. Pretty self-explanatory, right? It’s a tokamak which is spherical and designed to create plasma with currents on the order of megaamperes. From the U for Upgrade, you’d assume it’s just a case of improving current equipment from the original MAST, simple right? Nope, almost 90% of the machine is new! So it’s practically a new machine, with tons more functionality than the original MAST.
You may have heard of other non-spherical tokamaks like JET, which has set records for fusion power. So you couldn’t be faulted for asking the question ‘why make a tokamak spherical?’ Well, spherical tokamaks come with some major advantages. Due to the smaller ‘hole’ in the centre of the plasma, it’s believed we can get higher power generation at a lower cost compared to normal tokamaks. This is because the magnetic field-generating coils are closer together so the field is naturally stronger and less electricity is required! In addition, the reactors are generally lower in cost as they can be built smaller.
The history of spherical tokamaks is longer than most at Culham. In 1990 UKAEA built START (the Small Tight Aspect Ratio Tokamak – an informative name) which was the precursor to MAST. MAST was built over the course of 3 years and operated from 1999 until 2013. The tokamak conducted key research into edge-localised mode (ELM) mitigation and the effect of high beta (the ratio of plasma pressure to the pressure from the confining magnetic field) on plasma, and its effect on the rate of fusion.
In 2007, MAST was operating at its engineering limits and big questions still remained for fusion research. Prime among these was how to handle the exhaust plasma produced in the reaction and how to dissipate that heat. Not to mention the ever-tantalising question of what happens if we put even more energy into it. The proposal was made and it was agreed that MAST would shut down, beginning its journey to upgrade. The challenge: introduce coils capable of higher current (130 kiloamps in the case of the toroidal field coils), raise the operational time to 5 seconds instead of 2, and add an off-axis neutral beam injector to investigate plasma current profile control and mitigation of instabilities.
Most daunting of all, a novel divertor concept not seen anywhere in the world: the Super-X configuration. An area at the top and bottom of the tokamak capable of accepting plasma exhaust and, through the application of additional magnetic field coils, spread the plasma over a wide area to dissipate heat more effectively. Why have one configuration though when you could have many! By changing the magnetic fields, we can investigate standard (here ‘double null’), Super-X and snowflake divertor modes. These are all candidates for the plasma exhaust solution: how to dissipate heat from the 100–150 million-degree plasma without melting the bottom of the reactor.
Magnetic field lines for the potential MAST-U divertor modes. Source: MAST-U Research Plan, November 2019.
The upgrade has experienced its fair share of hiccups during construction and commissioning. We’ll say no more about the source of the mysterious vacuum leak or the elusive earth fault (both now resolved). Big projects are bound to have problems though. Just look at NSTX-U or a football stadium. The difference being MAST-U has ~19km of cabling and fits in a 16m x 19.2m room.
MAST-U is a testament to the dedication of engineers and scientists to build the bridge fusion desperately needs in its journey toward commercialisation. Seeking to learn more about heat dissipation in the hottest place on earth and whether a change in geometry can potentially shave acres of land off the final power plant size.
The MAST-U hall. Source: IOP
Meet the graduates
The MAST-U graduates Mike, Jimmy, Sarah, Mourmour, and Ellen joined the MAST-U project in 2018 and 2019. Their work has contributed to putting MAST-U over the finish line, with their efforts in commissioning since April vindicated by first plasma this week. Here's how they've been helping.
Ellen Wright, 2nd-year Graduate Electrical/Electronic Engineer:
I started the graduate scheme last year in the Electrical Engineering department, our team works across many projects at UKAEA but most of my work has been on MAST-U and MAST-UE (MAST-U Enhancement).
For MAST-U I have been involved in the plasma control system (PCS), I have been documenting the PCS interface, assisting with some new control algorithms and am working on converting a simulation that could be used to test the system without having to connect it to the machine. I have also been lucky to be involved in some cable termination work, a good opportunity to practise practical skills! All of this work has been very enjoyable and rewarding.
This was an exciting time to join MAST-U in the final stages of commissioning, I am looking forward to seeing operations and everyone's hard work paying off and facilitating important research (and also all the future inevitable problem-solving opportunities!).
Mourmour Man-Friel, 2nd-year Graduate Mechanical Engineer:
I started the graduate scheme as a graduate mechanical engineer in the MAST-U operations team on September 2019. Before lockdown my role was mainly to support the activities going on in the Load assembly team; during lockdown I’ve been working on MAST-U Enhancement Chilled Coolant System (CCS) for the centre column and assisted in writing a literature review for STEP.
The Load Assembly team is responsible for MAST-U vessel internal structures, vessel supports & coils as well as some of the supporting systems. My role varies from producing calculations and supporting documents to assembling components and inspecting the vessel for faults. I had the opportunities to do the following: leak testing on the vessel; help in producing a report on condensation levels in the block house (the room MAST-U is contained in); assembling piezo valves and leak testing them.
I found that opportunities like this really help me to understand concepts. I’m very excited to go back onsite and to see the machine running, as well as carry out work required for MAST-UE CCS project because it is something we’ve been working on during lockdown.
Sarah Parry-Wright, 2nd-year Graduate Electrical Engineer:
I am Sarah Parry-Wright a Graduate Electrical Engineer working in the Power Supplies Group on MAST-U. I did an MEng in electrical and electronic engineering at Swansea University and started with UKAEA last April. My work here has involved the installation and upgrade of the radial field power supply, which powers the P6 coils controlling the vertical position of the plasma in the machine. I have been heavily involved in the commissioning of most of the coil supplies we have for MAST-U and am currently working in the control room with the operations team working towards getting ready for our 1st plasma.
My work has been very exciting and fulfilling throughout my time at UKAEA and I have learned so much while thoroughly enjoying working with an amazing team.
Michael Robson (Brother Nature), Graduate Mechanical Engineer:
As a graduate mechanical engineer (with a background in physics), I’ve been working on getting diagnostics into the blockhouse (our equivalent of JET’s torus hall) and ready for operations. Working with a varied team I’ve been organising not only the mechanical side of bolting a camera to the vessel but also the electrical, networking and data acquisition of the diagnostic as a complete system. Many of the diagnostics I work on are spectrometers that separate light into its constituent wavelengths. This data tells us about what material is in the plasma (like impurities) as well as how much there is and how it’s moving.
Alongside this work I’ve been involved in a graduate project designing a test cubicle for the electrical safety course and organising a tour of STFC Harwell’s facilities for the graduates.
The most difficult thing I’ve encountered while helping with commissioning is the awful puns a member of the data acquisition team comes up with.
Jimmy Measures, Graduate Software Engineer:
As a graduate software engineer, I’ve worked on many different projects, from designing and creating data analysis tools to work on MASTU’s plasma control systems. The majority of my work has focused around writing and testing code for the Vertical Controller (or Z Controller as its more commonly known), which controls the power supply to one set of coils. This in turn changes the shape of the magnetic field and is used to alter the vertical position of the plasma and elongation. This device will hopefully enable more scientific research into the plasma’s behaviour during fusion.
I have also worked on a graduate project to create a site-wide digital logbook. The aim is to prevent the loss of notes and make collaborating easier. On the side of working at UKAEA I also created and organised Grad’Lympics (with a lot of help from other graduates). A 5-month long competition of 14 events, from football to a quiz, to determine which out of the Oxford or Abingdon graduates were best. To find out who won please refer to previous blog posts from March 2019.
Written by Jimmy Measures and Mike Robson