Presentation to Committee on a Strategic Plan for U.S. Burning Plasma Research

Nat FischProfessor and Associate Chair, Department of Astrophysical Sciences
Director, Program in Plasma Physics, Princeton University
Associate Director for Academic Affairs, PPPL
April 11, 2018

Trying for Upside Potential in Controlling Fusion
My Background:25+ years of recruiting graduate students
40+ years as nuclear fusion researcher -- trying to reach the upside

Preliminaries -- an educational perspective: Program the non-programmatic
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A healthy field needs to attract top, creative students.
Any strategic plan, particularly at this juncture in the field, must insist on this.
Programmatic tasks recruit students who want well-defined career paths.
Holding out the upside, uncharted potential of fusion is often needed to recruit top students.
What does this mean? While pursuing mainstream ideas, strategically plan for balance.

Upside potential for Magnetic Confinement Fusion
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Hot ion mode: Ti > Te
Steady state
Natural ejection of high-Z impurities (differential confinement)
Natural ejection of slow α-particles (differential confinement)
Disruptions not destructive
Direct energy conversion
Advanced fuels
Engineering compatibility

Double analogy: In MFE, mirror is to tokamak; as in ICF, Z is to NIF.
Toroidal MFE and laser-driven ICF are winning, elegant, scientific solutions.
Open-B systems and capacitor-driven ICF are more engineering compatible.
The further advanced, the more apparent is the importance of engineering compatibility.
One more thing: The reactor development path, if it is to span many decades,
ought optimally to have intermediate applications.
H. P. Furth, Sci. Am. (1995)

Examples:1. Steam engines: Hauling coal from coal mines (replacing reluctant horses).
2. ICF: Stockpile stewardship, and discovery science in extreme regimes of density-temperature.
3. MFE -- Open-B and other rotating devices: Legacy nuclear waste and other separations.

One Upside: “α-Channeling”

Power Flow in aFusion Reactor

Waves
α-par/cles

Tail ions

Normal Power Flowelectrons

Fuel ionsGet Hot Ion Mode: Ti > Te
75% of α power to ions ⇒ Pf →2 Pf

D + T → He 4 + n

Outline of the Argument
1. Rotating magnetized plasma confinement may enable upside potential to fusion.
2. Central to rotation is creating radial potential, by moving charge across field lines.
3. This might be accomplished by alpha channeling, which has other desirable features.
4. There are many unanswered basic scientific questions regarding cross-field charging.
5. Many of these questions can be posed and answered most easily in linear devices.
6. Linear, rotating plasma devices can be used for separations.
7. This suggests a fundamental physics program with high upside potential for magneticfusion, with intermediate applications in other areas, both curiosity and applications
driven, and, at least in the linear limit, relying upon relatively easy to build devices.

Reactor designs around Aries I operating point
no channelingchanneling

cdP

75%

75%

Ti(keV)Te(keV)

20
20

15
15

20
12

15
12n(1014 cm-3)

1.2

1.8

1.8

2.1

τi(s)
τe(s)

2.0
1.0

2.0
0.7

2.0
0.3

1.0
0.5

Pf (W cm-3)

4.7

6.1

10.9

9.7

make virtue of low electron heat confinement time
Fisch and Herrmann (1994)
Snyder, Herrmann and Fisch (1994)

Extracting Free EnergyFavorable Diffusion Path

Motivated byWong and Ono, 1984

fueling

Tokamak center

extraction

Tokamak periphery

Advantages of Alpha Channeling
1. Hot ion mode gives about 30% cheaper COE, compared toaggressively designed reactors, due to increased reactivity atgiven confined pressure (and free current drive).
2. Impurities removed and plasma fueled automatically.
3. Ion transport might eventually be tamed, but maybe not electrontransport, so having ions hotter than electrons reduces heat loss.
4. Present data base of the top tokamak confinement and heating
results supports hot-ion mode operation only.
5. Deleterious instabilities stabilized by removal of α free energy.

Diﬀusion Pathsv⊥

Bω/kyv y → vy + Δv yxgc → xgc + Δv y ΩΩ ≡ eB m

Vy=ω/ky

ΔE = mvy Δv yΔExgc → xgc +mΩ ωxzyky

Fisch and Rax, 1992

TFTR Experiment: Surprise Loss Rate Dependence on Prf
With Prf = 3.2 MWWith Prf = 1.0 MW

Beam (black), neutrons (blue), & losses (red)Energy diffusion coefficient of lost particles : Dε ~ 25 MeV2/sec !!
Figures from Herrmann Thesis (1998)
Comment: a very under appreciated experimentHot-ion Mode RF-Driven Tokamak
Vision: 400 MW or more re-circulating RF; RF is first-order physics
Present best results extrapolate to Hot ion mode

1. Top performance results to date achieved in hot ion mode
2. Ion heat transport might be controlled but not electron heat transport
Upside to hot ion mode -- confidence in extrapolation and 30% on COE

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RF energy channeled from alpha particles.
Fusion reactivity can be doubled in hot ion mode.
RF current drive fueled by alpha channeling.
Ash removal. Fueling.
Expedited by possible resonant “ringing” of tokamak.
Electron heat can be poorly confined.
Less free energy to drive instabilities.

Uses of RF Waves in Magnetic Confinement Fusion Devices
seeking ever increasing control of plasma
1970's: Heat Plasma to Thermonuclear Temperature:

Ion Cyclotron, Lower Hybrid, Electron Cyclotron Waves
1980’s: Drive Mega-amps of plasma current

LHCD, ECCD, MiCCD current drive
1990's: More detailed positioning of plasma current

Use LHCD, ECCD to control of NTM, sawteeth, plasma current profile
1990’s: Exploit coupled diffusion of particles in velocity and position“α- channeling effect”

Trend to “phase space engineering”
Detailed control of rf-induced fluxes in 12-D
Select particles in 6D velocity-configuration space
Select flux vector in 6D velocity-configuration space

/"%"+.$%&'$()*#+,&-+*%.
!"0"+.$%&'$()*#+,-+*%.

!"#$%&'$()*#+,&-+*%.

Poloidal Magnetic Field produces swirling to stabilize drift
But so does radial electric field!

driftMethods of Producing “Rotational Transform”
B
Magnetic:

J||Parallel current induced by pushing charge along field
Electron current is impeded by collisions with ionsE⊥

BElectric:

Perpendicular voltage induced by pushing charge across field
But collisions do not restore charge neutrality(caveat: field lines could move)Wave-driven Rotating Torus
Rax, Gueroult, and Fisch (POP, 2017)

Rotational transform produced by radial electric field
Waves generate radial electric field (α-channeling effect)
Pick radial potential negative MeV – direct conversion

Two questions:
1. How much stored energy?
2. How much dissipation?
Answers
1. Stored energy small – quick release not damaging!
2. Dissipation small for skinny torus.
Some Advantages:
a. No runaways
b. Possibility of natural hot ion modec. Possibility of natural impurity control
Caveat: Many things to check:
Perpendicular conductivity (in commonality with centrifuges)
Stability and transport
Major radial force balance [Ochs and Fisch (POP, 2017)

Impurity Concentration Effect in transverse B fieldni (x) = nb (x)⎛ Zi ⎞⎜⎜ Z ⎟⎟b⎠⎝

Taylor, 1961

What about in a potential, like gravity, or centrifugal forces?

⎡n (x)e⎣ a

Φa /kT

1/Z a⎤⎦~ ⎡⎣nb (x)eΦb /kT

1/Zb⎤⎦~ ⎡⎣nc (x)eΦc /kT

1/Zc⎤⎦

à interesting effects in rotating magnetized plasma.
Φ ~ mgy; Φ ~ mΩ2 r 2 2

Curious de-Mixing in Potential Field
relevant to (rapidly) rotating plasma

m, q
M, Q

M>m

B
m/q > M/Qg

m/q < M/Q

Kolmes, Mlodik, Ochs, & Fisch (2018)

Differential de-confinement of particles of intermediate mass and charge
Example: p-B11 fusionRotation regime for pushing alpha particles to periphery
Kolmes, Ochs, and Fisch (POP, 2018)

Legacy Waste at Hanford(from US nuclear weapons program)

Single shell tanks constructed in 1944

Waste treatment plant in 2005

• 177 tanks contain 54 million gallons of high level
waste with 194 MCi total radioactivity• Tanks are decades past planned lifetime. Decades
remain until they are fully processed.
River Protection Project System Plan, Revision 5 (2010)

Three important applications
for high-throughput mass
separation that could exploit
large atomic mass differences

Gueroult, Rax and Fisch, PPCF (2018)
Gueroult, Rax and Fisch, J. Cleaner Production (2018)

Plasma mass ﬁlters can exploit “mass gap” toseparate bulk elements from radioac/ve ones
Water Washed Solids Inventory at Hanford

MassGap

B. Cluggish, “Technical Overview of Archimedes Filter.” Archimedes Workshop at General Atomics, 2006.

Archimedes Demo was built to prove concept at scale

B. Cluggish, “Technical Overview of Archimedes Filter.” Archimedes Workshop at General Atomics, 2006.

Centrifugal Confinement Fusion or Separation
Mirror coils
Fc
Fc||EElectrodesΩB

Magnetic field linesReplace electrodes
with α channeling
For insulated ends,
potential
maintained against
perpendicular
conductivity

Centrifugal forces (mass dependent)
push charged particles from: low radius region
to high radius regions

One Possible Improvement:
Magnetic Centrifugal Mass Filter
Centrifugal force on heavy ionsovercomes the magnetic mirror force.

Centrifugal force is not sufficient
to confine energetic light ions.

A. J. Fetterman and N. J. Fisch (2011)

MCMF Advantages

Confinement condition only depends on mass

1. Output streams collected axially over a smaller area2. Plasma source can be on field lines
3. Works much better on large mass differences (less proliferative)

Summary of the Argument -- a Program for “Differential” Magnetic Confinement
1. Rotating magnetized plasma confinement may enable upside potential to fusion.
2. Central to the rotation is creating a radial potential, by moving charge across field lines.
3. This might be accomplished by alpha channeling, which has other very desirable features.
4. But there are many unanswered questions – and many curiosities – particularly in crossfield charging and transport – either by waves or through collisional effects.
5. Parallel plasma conductivity well demonstrated – not so perpendicular conductivity.
6. Linear, rotating plasma devices can be used both for separations and nuclear fusion.
7. Many fundamental physics (conductivity and differential transport) and technology
questions (radial potential) can be posed and answered most easily in linear devices.

8. This suggests a fundamental physics program with high upside
potential for magnetic fusion, with intermediate applications inother areas, both curiosity and applications driven, and, at least in
the linear limit, relying upon relatively easy to build devices.