How was the recent $\rm 59 MJ$ JET fusion record (Dec 2021) different from previous records? Did they simply induce a stronger current (meaning more heat) in the plasma? Did they manage to compress the plasma more? Or did they change DT pressure or something else?

 A: Some of the previous DT records and the new record are compared in the plot below and nicely summarized during the first 20 minutes of this video: 
In the above plot, the highest power shot was a short pulse, which is not as interesting for reactors since we plan to operate them for long periods at a time. The much more interesting record to beat was the more modest but also more reactor-like scenario of 22 MJ (long and steady-state grey line).
Beating this record wasn't easy since JET was modified to make it more relevant for ITER and future power plants, which added some uncertainty. For example, the plasma facing components were changed from Carbon (which traps Tritium fuel in the wall) to Beryllium and Tungsten. Tungsten can cause energy loss by Bremsstrahlung if it builds up in the plasma, so strategies were developed to keep it away from the plasma with a blanket of Argon or Neon gas between the plasma and the wall, and flush out any Tungsten by tuning the amplitude and frequency of so-called edge localized modes (ELMs). The details are still being analyzed and written up. In the big picture, this record was achieved with large leaps in computational plasma physics tools, which allowed an accurate prediction of DT plasma performance, and diagnostic capabilities, which allowed measurements needed to control and fine-tune performance.
