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

2. 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?

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

4. 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

5. Order of magnitude improvement in all 3 axes of superconducting volume

6. 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!

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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?

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

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

16. 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

17. 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”

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