Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fusion Summary?

  1. May 1, 2007 #1


    User Avatar
    Gold Member

    Can anyone please suggest a comparative pro/con review of the various fusion approaches for power generation? In those approaches I'd include:

    pinches (Z, focus, etc)
    laser, ion beam
    inertial electrostatic
    Beam-beam FRC

    Obviously the available information in each area is deep but Ive not found a good comparison of the physics/engineering issues against one another, or at least an attempt at using some kind of common metrics. I had in mind something like the following, in the few areas where Ive done some background:

    tokamaks, etc: thermal magnetic confinement thus limited to DD, DT fuels due to Bremstralung losses at higher temperatures required for aneutronic fuels. Therefore neutron loads a challenging problem for the toroidal geometry ...

    IEC - claims of mono energetic operation but analysis to date shows IEC quickly thermalizes and then realizes same problems as above.


    Last edited: May 1, 2007
  2. jcsd
  3. May 14, 2007 #2


    User Avatar

    Staff: Mentor

    Is this homework? One can find articles on-line about each concept.

    Parameters to compare would be efficiency [(energy out)/(energy in)], confinement times, plasma density, reaction rate, . . .

    And what does one mean by IEC quickly thermalizes? Fusion produces fast neutrons, and all neutrons will ultimately be absorbed by a nucleus or decay into a proton, electron and anti-neutrino. Neutrons passing through solid materials will certainly damage the crystal structure/atomic lattice by collisions - that is a given and there is no way around it. Materials degradation in systems using reactions producing neutrons is a significant technical problem/challenge. Even systems based on aneutronic reactions, e.g. D+He3, invariably have to deal with the fact that one can get DD reactions, half of which produce n+He3.

    the p+B system is about as purely aneutronic as one can get, but it requires high temperatures with resulting high pressures, which is a major handicap for magnetic confinment. In the inertial confinment systems, it's those darn electrons which get in the way.
  4. May 16, 2007 #3


    User Avatar
    Gold Member

    No, though my apologies for the overly broad post. I'm looking into some internal R&D work and trying to obtain a comparative high level view of the trade offs based purely on the soundness of the physics, independent of the $ and man hours invested to date, to the degree thats possible. Yes I'm well into the lit. on many of the approaches but then of course the issue in each area is that the researchers can be myopic about their particular approach: Tokamak'ers versus implosion, etc. Lidsky's "Trouble w/ Fusion" points some fingers.


    Good observables but I think those metrics illustrate my points above:
    1. They show for instance how magnetic confinement schemes completely dominate fusion discussion. Density is, what, 5-6 orders of magnitude different between implosion and mag confinement so thats not useful by itself. Confinement times (Larson), AFAICT, are not relevant to accelerator (driven) schemes like IEC or Rostoker's beam-beam http://www.sciencemag.org/cgi/content/abstract/278/5342/1419?ck=nck"
    2. $/time invested. If I want only soundness of idea and not $, do I look at Eout/Ein for 1970 Tokamaks or 1999 JET?

    IEC is basically an accelerator like beam-beam system. I dont refer to the fusion products, but the reactants. That is, the is all reactant ions are accelerated by E fields to, say, 50KeV creating a completely non-maxwellian, mono energetic system. The ideal is they stay that way unto a fussion event, but it seems to be well established that for beam-beam systems that coulomb collisions dominate ~1000:1 and thus the reactants quickly thermalize - either escaping the system on the hot end or becoming unavailable for fusion on the cold end.

    Ah, so lets get rid of (most) the electrons ;-) Non-neutral plasma work seems to be on the upswing.
    Last edited by a moderator: Apr 22, 2017
  5. May 16, 2007 #4


    User Avatar

    Staff: Mentor

    For IECs this might be of interest - http://fti.neep.wisc.edu/iec/ftisite1.htm

    I don't believe non-neutral plasmas are necessarily practical from a stability perspective.

    Inertial confinement has the problem of a cyrogenic fuel source (fuel pellet), the repeatability issue, and then net energy production vs input. Energy extraction is an issue - as well as a myriad of materials degradation issues.

    Magnetic confinement has much the same problem along with the stabilty of magnetic confinement, and limiations on plasma density, which are a consequence on the limit on the magnetic field strength, which are a consequence of the limits on currents in the superconducting magnets.

    Beam systems, including injected beams into magnetic confinement, have the drawback of beam energy efficiency and scattering of the beam by the plasma.
  6. May 16, 2007 #5


    User Avatar
    Gold Member

    Thanks for your summary

    Yes U. Wisc. seems to be IEC central HQ. The Rider[15] & Nevins [16] papers listed http://fti.neep.wisc.edu/iec/inertial_electrostatic_confineme.htm" [Broken] in the overview section are most frequently cited stakes to heart of IEC, though I dont know that there is any experimental verification of the losses they claim.

    Can you say more about that? I see that external mag confinement alone wouldn't work against the resulting space charge but why not w/ electric accelerators or a combination? Penning/Paul Traps? BTW a non-neutral conference:

    This is an area where it appears to me that the basic idea is unsound. The external mag. field doesn't really confine anything, especially not heavy ions (Brillouin limit inversely related to mass). It only retards the inevitable random walk to escape the system. On the other hand an E field based recirculating accelerator or trap is actually confining the ion, theoretically forever.
    Yes, no question its going to scatter. The interesting question for me is (in a recirculating system) how fast does the scattering take place (loss power) relevant to the fusion power.
    Last edited by a moderator: May 2, 2017
  7. May 17, 2007 #6


    User Avatar

    Staff: Mentor

    Well, in a fusion plasma, one does not want heavy ions, which don't fuse and which cause much greater energy losses as a function of Z (nuclear charge and number electrons in neutral atom). Higher Z nuclei in plasmas lead to greater losses due to bremsstrahlung, recombination, electron-electron collisions (including ionization).

    The external magnetic field does confine in Tokamak or other devices, but it is short term due to instabilities, which are exacerbated by non-uniformities in the plasma temperature distribution/ion/electron densities.

    Non-neutral plasmas have the disadvantage of Coulomb repulsion in addition to temperature-related pressure. In theory, the dynamic (acceleration of the nuclei) in an IEC overcomes some of the pressure limitations, but by virtue of the continuity equation (rule, law) - what goes in, must come out, otherwise pressure will increase with matter accumulation, particularly in the number of particles does not change, i.e. if one does not get A + A = B, but rather A + A = B + B.
  8. May 17, 2007 #7


    User Avatar
    Gold Member

    I meant heavy D or T ions relative to the electron mass; the electron being much easier to confine as a result.

    Granted a theoretical non-neutral IEC has Coulomb (and kinetic?) pressure to handle, but it has the advantage of having ~all the plasma mass being at reaction energy (at the bottom of the potential E well); vice a thermal, neutral Tokamak plasma where a) only the ions in the high energy tail of the Maxwellian distribution can practically fuse and most of the lower energy plasma mass is just along for the ride, and b) the electrons radiate significant Bremsstrahlung.

    Last edited: May 17, 2007
  9. Jun 8, 2007 #8
    Are you refering to wire grid IEC devices? I think so. Polywell IEC by Bussard over the last 11 yrs shows virtual cathode & potential well formation with no real electron losses, minimal to near zero maxwellian thermalization of ions. 100,000 times better output in relation to input than any previous Fusor type device.

    Dr Bussards former assistant Tom Ligon, hangs here:
    http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=5367&start=916&posts=928 [Broken]

    A schematic look at electron & ion flow in DR. Bussards WB6 IEC fusion reactor.


    Google tech talk by Dr Bussard

    http://video.google.com/videoplay?docid=1996321846673788606&q=bussard [Broken]
    Last edited by a moderator: May 2, 2017
  10. Jun 10, 2007 #9


    User Avatar
    Gold Member

    I'm familiar with Bussard and the Polywell design, watched the video, read the papers, (including the ones from the early 90's by Krall), got the T-shirt. None of it documents measuring 'minimal to near zero ...'. What is your source for this claim? All of the physics I know of says that any beam - beam collision of ions will quickly thermalize.
    Last edited: Jun 11, 2007
  11. Jun 11, 2007 #10
    Since WB6 ran in late 2005, I am not aware of Krall or Rider writing about the topic. I don't understand citing 15 to 20 yr old papers to refute R&D done in late 2005.:rolleyes:
    If Dr Bussards claim's are accurate.... does that suggest he solved the fuel ion thermalization? Would he have gotten increasing neutron counts if his fuel was thermalizing? Though Bussard does need to duplicate the results outside of the D.O.D. publishing embargo.

    Bussard ran WB6 4 times, each time at increased drive conditions, each test yielded higher neutron counts. If Bussards WB6 results are duplicated, then Kral, Rider, etc might want to write anew.

    Bussard touches on fuel thermaliztion in the google vid & Tom Ligon covers thermalization in response to Riders paper of 1995, here:
    http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=5367&start=901 [Broken]

    If the ions would live sufficiently long and make enough passes without fusing, they would no doubt thermalize, or at least lose energy. This will also happen if the density is allowed to get too high. The key is achieving a density profile where high-energy collisions occur in the center at fusion energies, very low energy (and thermalized) collisions occur in the high-density ion turn-around zone near the magrid, and very few other collisions occur. A narrow range of optimal ion lifetime versus fusion density exists for any given size of machine and set of operating parameters (too high, thermalize, too low, too few fusions). Larger sizes are required for that optimal density to produce net power.

    The thermalized turn-around zone is a key to killing two of Rider's objections. Rider believes A) the plasma will maxwellianize, and B) because it is maxwellian, some of the ions will upscatter in energy by collisions and so be able to exit the potential well complety. The outer collision zone does have maxwellian properties, but at very low energy levels, so it essentially removes the scatter in the energy of the fuel ions on every pass, essentially resetting the population to all near zero kinetic energy. Thus, thermalization keeps the machine from thermalizing.

    Rider thinks the machine will have excessive bremsstrahlung losses. Bussard and Krall counter that the central region Rider thinks will cause bremsstrahlung due to mutual repulsion is actually a convergence zone for both electrons and ions. The electrons are making a virtual cathode that attracts ions, the ions make a virtual anode that attracts electrons, and the whole zone is never all that far from a neutral plasma. Control of virtual anode height controls the bremsstrahlung problem.

    All of this says you need very good control of the ion population. WB6 and that puff gas system did not offer such control ... I suspect the system put in roughly the amount of gas needed, initially, but the amount continued to rise. Fusion occurred in the short period during which the right density profile existed. A successful machine will need to hold that condition.

    It may be that it will occasionally be necessary to stop the reaction and clear the machine of junk gas, which may include fusion products in a power reactor.

    Rider's final objection was dead-on right ... for HEPS, PXL-1, and WB-5. Cusp losses of electrons in a box machine are too high. They are irrelevant in a magrid machine. Electrons lost out the cusps come right back in.
    Last edited by a moderator: May 2, 2017
  12. Jun 11, 2007 #11


    User Avatar
    Gold Member

    No, that won't fly here on PF. I asked for a citation, and you talk 3rd hand about more 'R&D'.

    So you say. Citation please.

    Link from this forum to an enthusiast on another forum? No good here. Citation please.

    I don't see any sense in this. The coulomb collision / fusion collision ratio does not change with density. This suggestion is akin to manipulating a light dimmer for effect: "If the light is too hot/drawing too much power Ill turn it down. And, if the light is too dim I'll just turn it up. There! Its bright and no heat."

    No, you don't get to merely assert which area will be maxwellian and which will not.

    Convergence zones? Bremsstrahlung power in a D-D or D-T plasma is proportional to the electron density and the electron energy, thats it. Its worse w/ the higher Z fuels.
    [tex]P_{Br} [\textrm{Watt/m}^3] = \left[{n_e \over 7.69 \times 10^{18} \textrm{m}^{-3} }\right]^2 T_e[\textrm{eV}]^{1/2} [/tex]

    To reduce B. power then you either reduce the density or keep the electrons cold. Period. You don't get to waive this a way buy talking convergence zones.
    Last edited by a moderator: May 2, 2017
  13. Jun 11, 2007 #12


    User Avatar
    Science Advisor


    You got THAT right. Hot electrons are the "death knell" of ANY fusion plasma.

    Even in HIGHLY convergent plasmas; like those found in ICF.

    Once you couple strongly to the radiation field, which is what electrons do - then
    you might as well just blow a hole in the side of your plasma - you have a BIG energy

    Dr. Gregory Greenman
  14. Jun 11, 2007 #13


    User Avatar
    Gold Member


    Agreed for any neutral plasma thats optically thin to Bremsstrahlung. I'm interested in some non-neutral and degenerate plasma ideas for just this reason. For instance:

    "Confinement Of Pure Ion Plasma In A Cylindrical Current Sheet"

    "Beyond the Brillouin limit with the Penning Fusion Experiment"

    "Aneutronic fusion in a degenerate plasma"
    Last edited by a moderator: Apr 22, 2017
  15. Jun 14, 2007 #14
    My apologies for my behavior, being a noobie with a cut & paste post.

    By hot electrons, do you mean when electron density is high as well as energy? Ala Bremsstrahlung ? (sp?)

    Would electron energy at the center of the potential well (in a device such as WB6), be low energy but high density?


    In the google video, From 55:09 to 55:17 RWB comments that they repeated the experiment 4 times. I know, crap as a cite, but it what it is.



    EMC2-0891-04, and EMC2-1291-02

    I was informed that these pertain to the HEPS device, a closed box design with large electron losses, from the DARPA days.
    Last edited: Jun 14, 2007
  16. Jun 14, 2007 #15


    User Avatar
    Gold Member

    I mean the temperature, or kinetic energy of the electron in eV. Typically Bremmsstrahlung becomes significant > 10KeV as per the above equation. There is X amount of radiation for each electron (on average), so more electrons = more radiation.

    Ok, what were the results each time? Ive only seen the figure of 3 neutrons quoted for one experiment.
  17. Jun 16, 2007 #16
  18. Jun 17, 2007 #17


    User Avatar
    Gold Member

    The electrons would be there w/ the high speed ions and collisions /w the ions will heat up the electrons.

    I don't think the paper you list is the relevant reference. The earlier one addresses the howto for 'cold' electrons:
    "Bremmstrahlung Radiation Losses in Polywell Systems", Bussard, '92
    http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=A257646&Location=U2&doc=GetTRDoc.pdf [Broken]

    Ive been through it once and it warrants more time. At 1st look I've got two problems:
    1. Everything up front seems to be standard plasma beam physics calculations: running the energy and particle density through the geometry of the system. To simplify the approach he initially disregards the ion collisions. Then finally on page 5 just before equation 11 he gets back to it:

    He then proposes that, yes, the electrons will be heated in the core by the ions but that they will be cooled on the edge and that the two areas offset each other yielding a stable electron temperature. Ok, interesting idea. My 1st impression is that this is particle description of a heat transfer engine and as such its governed the thermodyamics (carnot cycle) and hence can't be very efficient. Thats a quick take - I don't really know. In any case this is a point of questionable physics and requires some detail description of why /how it can work. Bussard simply refers to 'Analysis of these processes shows the stable up scattered core energy...'. Come on. Addressing this point is the entire key to answering the question of radiation losses. This is the paper to show that and he says 'Analysis of these processes shows ...'? Furthermore this is problem number one to address in the lab starting back in '92; you don't even need to waste time w/ D or T fuels - just put any low Z ionized gas in there and measure your core electron temperature to verify this cold electron theory, or better yet measure the radiation level vs density if its high enough. Just answer this one question and Bussard et al will have much more serious attention. You don't need a 50 page paper, just a clearly documented note verifying cold electrons via test.

    2. The ratio of fusion power to Bremm. power in equation 8 is shown to depend on density. Bremm. power and fusion power are directly dependent on density so that the two would cancel out in the ratio. So I'm not following this; I need another look. [EDIT, scratch this: I was confused by the density symbol ne in the paper and the hand written (dang these old papers ;-) nu e, two different things]

    Last edited by a moderator: May 2, 2017
  19. Jun 21, 2007 #18
    An single ion is introduced into the device. The ion sees the potential well, shoots thru the well. Once past the well it slows (cooling?), turns, again seeing the well.


    Right, 3 neutrons at the detector. That may be the 12.5kV, mentioned by Tom Ligon here:

    http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=5367&start=16 [Broken]

    Yes, though a nice 50-100 page paper would be OK too, right?
    Last edited by a moderator: May 2, 2017
  20. Jun 21, 2007 #19
    Edit: I was told table 2 shows Brem-losses as 1/32nd ?
    Last edited by a moderator: May 2, 2017
  21. Jun 21, 2007 #20


    User Avatar
    Gold Member

    Last edited: Jun 21, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Fusion Summary?
  1. Fusion for students (Replies: 4)

  2. Fission or fusions? (Replies: 1)

  3. Questions on Fusion (Replies: 28)

  4. Fusion or Fission? (Replies: 16)

  5. Fusion power (Replies: 2)