To ‘B’ or not to ‘B’ That is the question Jeff Freidberg Presented by Dennis Whyte MIT - PSFC

What technology are we advocating? MIT is advocating high field as the best path forward for tokamaks This should not be a surprise We have been advocating this for decades Almost all agree that high field helps all magnetic fusion concepts, not only tokamaks BUT – Obtaining high fields (B0 = 10 T) using superconducting magnets has been a pipe dream. Technology has just not been available Are we just beating a dead horse?

NO!! New REBCO superconducting tapes have been developed over the last few years

What’s so good about REBCO superconductors? Consider YBCO At a higher temperature This allows a switch from helium to hydrogen coolants – a big win For comparison: REBCO – higher fields REBCO – higher temperatures REBCO – comparable current densities REBCO – a game changing technology

Order of magnitude improvement in all 3 axes of superconducting volume

How does REBCO change the game? Improved plasma physics Higher field allows higher current – Kink limit Higher current allows higher beta – Troyon limit Higher current allows higher density – Greenwald limit Higher current leads to higher tau – H-mode scaling Higher field leads to increased LHCD efficiency – smaller Plasma physics becomes ‘boring’. This is good for fusion energy!

There’s more Improved technology High field leads to compact reactors High field leads to high power density High field leads to Walmart priced reactors, not Neiman Marcus Super important High field leads to an affordable development path High field leads to a quicker development path

The development path is crucial If each step along the development path is too expensive and takes too long to build Fusion will never happen Neither government nor private funders will provide support This is the current tokamak path based on ITER This is the current stellarator path based on W7-X

What are the specific physics and engineering problems and how does high field help?

Steady state – the Achilles' heel of tokamaks? A fusion reactor must be a steady state device – engineering reliability After half a century of fusion research we still haven’t produced a high performance steady state tokamak Neutral beam CD ok now but probably not in a reactor Best bet is RF CD, probably lower hybrid – highest relative efficiency Still absolute efficiency is low Good confinement wants high I, low CD efficiency wants low I A fundamental mismatch

Possible solutions Tailor profiles, improve transport, so that Problem: reliability depends on the good will of the bootstrap elves Improve LHCD efficiency – How? High field lowers - this is good Inside launch raises B which lowers - this is good Potential gains in LHCD efficiency about a factor 2 This may be just enough

Another possible show stopper Heat load on the divertor No solution exists today that has been demonstrated experimentally Long legged divertors may be the answer but this needs to be tested High field hurts and helps the divertor problem Hurt: Compact, high power density increases heat load Help: Expected high density increases radiation and reduces heat load Recent studies (e.g. Reinke et al) have show that Help > Hurt Strong need for more experiments

Tokamak development path is too expensive ITER speaks for itself High field leads to compact, high power density reactors These can be built on a shorter time scale compared to large, low power density reactors This is true at every step along the development path How can we prove this?

Proof by example #1 World record for plasma pressure Alcator C-Mod: Compare JET:

Proof by example #2 A pulsed ignition experiment ITER: Compare FIRE:

Are we really working on the critical items for fusion to succeed Are we really working on the critical items for fusion to succeed? Our list… Develop practical REBCO magnets Improve LHCD efficiency (e.g. high field inside launch) for reliable, realistic non-inductive scenarios with acceptable recirculating power Continue to improve confinement understanding High power density, long legged dissipative divertor experiment

The world’s tokamak fusion program is Summary Paraphrase Wall Street bank situation after 2008 financial collapse The world’s tokamak fusion program is “Too big to succeed”

Proof by example #3 A steady state pilot plant ARC: Compare ITER: Similar cost but steady state pilot vs. pulsed burning plasma!