Prof. Kim Molvig - MIT OpenCourseWare · 2020. 7. 10. · April 20, 2006: 22.012 Fusion Seminar...

April 20, 2006: 22.012 Fusion Seminar (MIT)

Prof. Kim Molvig


D-T Fusion DD--T Fusion T Fusion

MeVnTD 6.17++→+ αMeV5.3 MeV1.14

• What is GOOD about this reaction?– Highest specific energy of ALL nuclear reactions– Lowest temperature for sizeable reaction rate

• What is BAD about this reaction?– NEUTRONS => activation of confining vessel and resultant

radioactivity– Neutron energy must be thermally converted (inefficiently) to

electricity– Deuterium must be separated from seawater– Tritium must be bred


Consider Another Nuclear Reaction Consider Another Nuclear Reaction Consider Another Nuclear Reaction

MeVBp 7.8311 +→+ α• What is GOOD about this reaction?

– Aneutronic (No neutrons => no radioactivity!)– Direct electrical conversion of output energy (reactants all

charged particles)– Fuels ubiquitous in nature

• What is BAD about this reaction?– High Temperatures required (why?)– Difficulty of confinement (technology immature relative to

Tokamaks)


DT Fusion – Visual PictureDT Fusion DT Fusion –– Visual PictureVisual Picture

Figure by MIT OCW.


Energetics of FusionEnergeticsEnergetics of Fusionof Fusion

r

VkinE

MeVVNuc 50−≅−

KeVRR

eVTD

Coul 4002

≅+

≅

409,076.1,10368.1,50200,95.45 542

321 ==×=== − AAAAA

QM “tunneling” required . . .

Empirical fit to data

Coefficients for DT (E in KeV, σ in barns)

2


Tunneling Fusion Cross Section and ReactivityTunneling Fusion Cross Section and ReactivityTunneling Fusion Cross Section and Reactivity

Gamow factor . . .

Compare to DT . . .


Reactivity for DT FuelReactivity for DT FuelReactivity for DT Fuel

Figure by MIT OCW.

(σν)

[x10

-16 c

m3 /s

ec]

T1 (KeV)

8

6

4

2

00 50 100 150 200


Reactivity for proton-Boron FuelReactivity for protonReactivity for proton--Boron FuelBoron Fuel

4

3

2

1

00 100 200 300 400 500 600 700

(σν)

[x10

-16 c

m3 /s

ec]

T1 (KeV)

Figure by MIT OCW.


Comparison ReactivitiesComparison Comparison ReactivitiesReactivities

4

3

2

1

00 100 200 300 400 500 600 700

(σν)

[x10

-16

cm3 /

sec]

T1 (KeV)

(σν)

[x10

-16

cm3 /

sec]

T1 (KeV)

8

6

4

2

00 50 100 150 200

Figure by MIT OCW.

Figure by MIT OCW.


Availability of FuelAvailability of FuelAvailability of Fuel

• Protons? • => Overwhelmingly available in

seawater (deuterium extraction not even required!)

• Boron?• Widely found in nature as alkali or

alkaline borates or as boric acid (Boron11 constitutes 80.2% of the natural abundance)


Power Density ComparisonPower Density ComparisonPower Density Comparison

p - 11B has almost 3 times the alpha energy of DT, so even with ½ the reactivity it produces LARGER alpha heating than DT => in that sense self-sustaining fusion is easier to maintain if high temperatures can be stably confined.

Example Numbers:

Compare to losses (Bremstrahlung):

Whoops!


How to beat Bremstrahlung losses?How to beat How to beat BremstrahlungBremstrahlung losses?losses?

• Some Radiation can be recovered via wall aborption and conversion

• But really must Run at lower electron temperature:

• Example:

• Still marginal Power balance

ie TT <

3

3

/20

/16

23585

cmWattsP

cmWattsP

KeVTKeVT

Fus

B

i

e

=

=

==


ITER Comparison for ReferenceITER Comparison for ReferenceITER Comparison for Reference

What losses dominate in this Tokamak scheme?


Requirements for Aneutronic FusionRequirements for Requirements for AneutronicAneutronic FusionFusion

• Magnetic Pressure must balance particle pressure for confinement:

• High beta plasma•• Highly efficient direct energy

recovery system (High recirculating power levels)

TeslaBnKeVnB

TnTnBiiee

10&101@2488

)(8

152

2

=×=≈

+>

π

π

ie TT <


Field Reversed Configuration (FRC)Field Reversed Configuration (FRC)Field Reversed Configuration (FRC)

Diagram removed for copyright reasons.

See Figure 1 in Rostoker, N., A. Qerushi, and M. Binderbauer. "Colliding Beam Fusion Reactors.“ Journal of Fusion Energy 22, no. 2 (June 2003): 83-92.


QuestionsQuestionsQuestions

• What about “negative” moving ions?• What about electrons?


Formation ScenarioFormation ScenarioFormation Scenario

• Form cold target plasma in axial guide field• Blast target plasma with proton and Boron

beams at high energy• This provides fuel, initial heating and confining

current (electrons confined by positive electrostatic potential)

• Assume power balance all works out and one has a “win” that is self-sustaining thermonuclear fusion

• Collect energy (direct electric conversion) from escaping alphas


FRC FormationFRC FormationFRC Formation

Figure removed for copyright reasons.

A different view of the FRC configuration.


Problems?Problems?Problems?

• Do you recognize this geometry from schemes presented during semester?

• It’s an elongated Tokamak tiped sidewise with no toroidal magnetic field!!??

• What gross stability issues would worry you?• Kink & Sausage modes (toroidal varieties) – gross

MHD instability like Z-pinch• Proposed to be “solved” via high energy, large orbit

ions – system surely NOT MHD -- and possibly feedback control

• Efficiency requirements for ion beam systems, alpha and radiation energy recovery are daunting

• Some FRC properties demonstrated experimentally but BIG scale ups in all physical parameters will be required


The ChallengeThe ChallengeThe Challenge

Prof. Kim Molvig - MIT OpenCourseWare · 2020. 7. 10. · April 20, 2006: 22.012 Fusion Seminar...

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