Description of capability
REBCO superconducting material could enable a better Gyrotron device for Electron cyclotron heating in Nuclear fusion plants, especially in tokamaks and stellarators.
One caveat: according to Richard J. Temkin, MIT isn’t sitting on any magical inventions for how to make gyrotrons significantly better than they are today. Moreover, the four major suppliers of Gyrotrons are well entrenched, so it will be difficult to disrupt the gyrotron industry.
Background
There are four major ways of heating a plasma in a tokamak:
- Electron cyclotron heating (EHC)
- Ion cyclotron range of frequencies (ICRF) heating
- Lower hybrid current drive (LHCD)
- Ohmic heating
Ohmic heating stops working at plasma temperatures above ~2 eV because plasmas become less resistive as they heat up. ICRF is not ideal for a power plant because of heat challenges, and because the components must be in close proximity to the plasma.
Why do we need a better gyrotron for fusion?
ECH is preferred for a fusion power plant. You control the plasma by moving the RF beam to targeted parts of the plasma. However, new tokamak designs like the UK’s STEP Spherical tokamak or CFS’s ARC tokamak, as well as any Stellarator (like Type One Energy), will require a new kind of gyrotron to produce ECH:
- The cyclo frequency of electrons is determined the magnitude of the magnetic field only. Thus, the gyrotron frequency must be 28x the value of the magnetic field. That’s ~350 GHz for an ARC tokamak. Today’s gyrotrons can’t really drive at these ultra high frequencies.
- Need commensurately high power density
- Need higher wall plug efficiency (currently only ~30%)
- Need durability
- Need a reliable supply chain:
- The major supplier to date has been GyCom, a Russia-based company
- Other established players (Varian?) seem to be exiting the gyrotron business
Designing this ‘better gyrotron’ for fusion will require new innovations, including:
- The use of REBCO superconducting material for higher-field High-temperature superconducting magnets inside the gyrotron
- RF physics
- Power handling and power electronics
- Modern manufacturing approaches, including to cool or replace the collector, which gets blasted during operation
To be clear, SPARC doesn’t need a gyrotron. And ARC tokamak could definitely use a gyrotron, but it is not strictly necessary, which is a good thing since the device doesn’t exist yet.
Bridge 12 has an ARPA-e GAMOW project to build a 250 GHz at 1 ms pulse length 1 MW of power with 65% efficiency.
Key people
Technology Readiness Level (1-9)
Needs that this could potentially address
- Energy-efficient generation of high-power RF for fusion (currently only 30-40%)
- Long-lived gyrotrons. Right now, gyrotrons burn out after a few hours of total use.