°ÄÃÅÁùºÏ²Ê¸ßÊÖ

 

But since °ÄÃÅÁùºÏ²Ê¸ßÊÖ is an experimental facility, designed to facilitate a wide range of explorations, its baseline configuration is not optimized for steady-state operation. To achieve this, °ÄÃÅÁùºÏ²Ê¸ßÊÖ will need to demonstrate operation with very high confinement¡ªbut not only.

 

"We will have to replace the inductive current provided by the central solenoid with plasma-driven current (bootstrap) and currents driven by neutral beam injection and electron cyclotron waves. And we'll have to do this for an hour at a time which, for all intents and purposes, is equivalent to steady-state operation."

 

 

For each tokamak pulse, a plasma is created and heated. Once hot enough, the plasma produces energy through fusion reactions and the plasma current can be maintained until it consumes all the magnetic flux provided by the coil systems. Then, the plasma current decreases and the plasma cools down and terminates.

 

H-mode (high-confinement mode) is an operational regime during which the plasma reaches a state of high energy confinement and produces fusion power with very hot plasmas and a low consumption of the magnetic flux, which is beneficial for long-pulse operation. In L-mode (low-confinement mode), by contrast, fusion power production is low, the plasma is colder and magnetic flux consumption becomes non-negligible, which shortens the length of tokamak pulses.

 

While short-pulse operation is technically easier to achieve, it comes with disadvantages related to transient forces and thermal cycling. The large amount of magnetic energy needed to create a plasma requires very large currents in the magnet coils for every pulse, which in turn subjects the coils to large cyclic forces and stress.  As the tokamak cycles back and forth¡ªbetween phases with extremely hot plasmas and high fusion power, to phases with no fusion power production¡ªthe expansion and contraction causes stresses that reduce component lifetimes. In addition, for a fusion reactor running in pulsed mode, extra measures would have to be implemented to provide the steady stream of electricity that customers need.

 

 

"By controlling the evolution of the plasma density and plasma current, you can establish a plasma current profile shape near what is needed to have a stable plasma in steady state, but at lower temperatures and pressures. This makes things easier," explains Loarte. "Then you add a lot of auxiliary heating to create the transition to H-mode, and you increase the plasma pressure to enhance the self-driven plasma current and current drive to replace the remaining inductive current. The current shape profile remains frozen due to the high temperatures, ensuring that the plasma remains stable at high pressures.  That's the trick you have to pull off."

 

During the design phases of °ÄÃÅÁùºÏ²Ê¸ßÊÖ, it was thought that a dedicated heating system, using so-called lower hybrid waves, would be required to reach steady-state operation. The project plan was to move another piece of equipment out of a port when the time came to run the steady-state experiments, in order to install the lower hybrid heating system.

 

No one was completely comfortable with this idea. Moving systems between ports in °ÄÃÅÁùºÏ²Ê¸ßÊÖ is very complex and disruptive for a number of reasons: the port plugs that house the system must be extracted and re-inserted, the pipes and transmission lines that feed the systems in the ports must be disconnected and reconnected, etc. °ÄÃÅÁùºÏ²Ê¸ßÊÖ scientists were looking for a better solution.

Breakthrough findings indicate °ÄÃÅÁùºÏ²Ê¸ßÊÖ can reach steady state with the same heating systems that are being built now

 

The good news came with the recent demonstration that steady-state H-mode could be achieved at °ÄÃÅÁùºÏ²Ê¸ßÊÖ without adding the lower hybrid wave heating and current drive system.

 

"Using integrated modelling, we set out to show that we can control core plasma stability in this scenario with the baseline heating and current drive mix at °ÄÃÅÁùºÏ²Ê¸ßÊÖ," says Alexei Polevoi, from the Plasma Modelling & Analysis Section. "By the use of neutral beam injection and electron cyclotron wave heating and current drive, we demonstrated that it's possible to reach the target operation point, where steady state is reached, within the limits of the magnet systems that °ÄÃÅÁùºÏ²Ê¸ßÊÖ has. We determined that an upgrade of the neutral beam injection system (to 49.5 MW, versus the baseline of 33 MW) and the electron cyclotron heating system (to 30 MW, versus the baseline of 20 MW) would be required to make Q=5 steady-state operation, with reasonable assumptions regarding the achievable energy confinement."

 

 

The researchers presented these results to the °ÄÃÅÁùºÏ²Ê¸ßÊÖ Council Science and Technology Advisory Committee (STAC) in October 2018, arguing that lower hybrid wave heating would not be needed to achieve steady-state operation in °ÄÃÅÁùºÏ²Ê¸ßÊÖ. The committee approved the findings. Because removing the possibility for the upgrade to lower hybrid represented an important modification to the project, the team brought this proposal before the °ÄÃÅÁùºÏ²Ê¸ßÊÖ Council, where it was accepted, and then implemented the appropriate technical procedures within the project in 2019.

 

This is good news, but more work needs to be done. "We still need to conduct further studies on how to implement these techniques in real experiments in °ÄÃÅÁùºÏ²Ê¸ßÊÖ," says Sun Hee Kim, Scientist for Scenarios and Control. "We need to define the experimental procedures that first get us a plasma current profile close to target, but at very low plasma pressure, and then increase the plasma pressure using heating and current drive nearly to the plasma stability limit without significantly modifying the current profile."

 

While mastering this delicate set of operations will be a challenge, the reward is very high plasma performance, with the sought-after steady-state operation delivering Q=5.