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Thermonuclear plasma turbine powerplant?

  1. Jul 13, 2007 #1
    I know that confinement and stability are a big issues when it comes to harnessing fusion power in a controlled fashion, but what if we could do away with the stability constraint? (sorry if this is completely unfeasible, I don't really know very much about plasma and discharge physics).

    My idea is to confine the plasma within a magnetic tube and apply heat to it whilst it accelerates out of the tube (much like many proposed plasma rocket schemes). I assume that as long as the plasma is confined within the tube at a critical density and temperature (whilst accelerating towards the "outlet"), the plasma will fuse and produce a net energy surplus.

    The superheated hot gases could then be allowed to mix and dissipate with air in a large air breathing turbine assembly (possibly with a forward compressor stage) to drive a large turbine and produce power.

    Crude I know, but is it feasible? I feel that such a scheme would encounter mostly engineering problems, rather than scientific problems which are showstoppers for most other fusion power plant designs. Given a large enough turbine and confinement area, I can't see why this isn't possible.
     
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  3. Jul 13, 2007 #2

    Astronuc

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    Fusion plasma normal operate in near vacuum conditions. The particle densities are very low ~ 1014 /cm3.

    If one could produce such a tube, then one might simply use direct conversion rather than injecting a hot plasma into a turbine or heating zone for input into turbine.

    And one would need a compressor to compress gas/air, which would then be heated and expelled through a turbine in order to produce mechanical energy, which presumably would be used to drive a generator.
     
  4. Jul 27, 2007 #3
    I guess the point I'm getting at is, on a big enough scale, if the stability constraint can be removed, then surely the energy can be harnessed through relatively crude means?

    Another hypothetical: What if an uncontrolled (explosive) thermonuclear device were detonated within a very large turbular enclosure filled with air. One end could feature a parabolic reflector and the other end would be open to allow the high pressure gases to escape the enclosure through a large turbine.

    Provisions could be made on the parabolic end to allow the tube to "breathe" after the detonation so that the pressure wave and subsequent vacuum caused by the detonation does not cause counterflow and backspin the turbine. This could also feature a forward compressor stage.

    Surely if the enclosure were large enough to withstand the stresses of the detonation (a mere engineering and logistics problem) then the energy can be harnessed.
     
    Last edited: Jul 27, 2007
  5. Jul 27, 2007 #4

    mgb_phys

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    Yes, once.
    You could have the containment fail, heat the air in the room and have a turnbine on one of the doors to capture the energy as the blast went through it.
    I suppose you could also use an H Bomb and a large number of windmills.
     
  6. Jul 27, 2007 #5

    Astronuc

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    Big enough would mean something like a km or so.

    H-bombs are designed to destroy structures, really large areas such as cities. There is no practical way to harness the energy of a thermonuclear device, because the surrounding structure would be subject to a catastrophic impulsive load. Notice that for underground tests, the ground heaves! And large cavities develop and the ground/rock around the cavity melts!

    That is why we prefer Controlled Thermonuclear reactions.
     
  7. Jul 27, 2007 #6

    Morbius

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    Astronuc,

    Correct you are. Exploding an H-bomb and trying to "catch" the energy is
    totally infeasible. As you stated, there's no way that some mechanism
    smaller than a city designed to capture this energy is going to survive - it
    will be vaporized by that amount of energy.

    However, if one can make the H-bomb smaller so that it can generate an
    amount of energy that we can deal with - then THAT appears to be
    feasible. That, of course; is what ICF - Inertial Confinement Fusion or
    "laser fusion" is all about. In this case, the "H-bomb" would be a little
    tiny fusion pellet - about the size of a BB. That's the scale of the size
    of H-bomb that we can "handle".

    The problem is although the physics of the H-bomb does scale down to
    small size; the physics of the only device currently capable of igniting
    the H-bomb; the A-bomb; doesn't scale down. A-bombs only come in
    sizes of very big, and bigger.

    So one has to dispense with the A-bomb and use something else to ignite
    the fusion reaction. That's what the laser is for. However, to ignite a
    fusion pellet the size of a BB; you need a laser as big as a sports stadium.

    We are currently building such a huge laser at Lawrence Livermore
    National Laboratory; it is called the NIF or National Ignition Facility.

    The movie; "Laser Bay Flyover" gives one an idea of how truly massive
    this laser is. In some of the scenes, you can see people moving about;
    which gives you an ideal of the scale of this truly massive facility.

    http://www.llnl.gov/nif/project/mm_flyover_lg.html

    Although it is HUGE; that is what it takes to get even a little BB-sized
    "H-bomb" up to nuclear fusion conditions. [ As the text accompanying
    the movie explains; the movie only shows Laser Bay #2 or only HALF
    of NIF. It's twin Laser Bay #1 is in the other half of the building. ]

    http://www.llnl.gov/nif/programs/index.html

    http://www.llnl.gov/nif/project/news_status.html

    Other NIF movies are available at:

    http://www.llnl.gov/nif/project/lib_movies.html

    I don't think many really appreciate the magnitude of what one has to
    do to get even a very small amount of material up to fusion conditions.
    Fusion conditions are not just a small step from the temperatures and
    pressures of other Earth-bound machines. Even the conditions in outer
    layer of the Sun; is not hot enough for fusion. The fusion reactions in
    the Sun go on only in the interior.

    Dr. Gregory Greenman
    Physicist
     
    Last edited: Jul 27, 2007
  8. Jul 28, 2007 #7
    You've preempted my next question, which is can the device be scaled down.

    This facility is very impressive. What are the energy densities required to fuse a small pellet (say 1 cubic centimeter)?

    Do you have an idea of the smallest pratical fission device that could be used to initiate uncontrolled fusion? If the process could be scaled such that the enclosure required is even a km long, it's possible this could be practical.

    I'm a firm believer that any and every human endeavour can be disseminated into a base energy cost. Surely the energy surplus afforded by such a device could outweigh its initial energy cost in assembly and materials. Controlling radioactive discharge could be problematic, but that's another engineering problem.

    I imagine that if small fission initiated fusion devices could be loaded regularily in an automated fashion, energy could be pulsed into the large turbine assembly. Its great rotational inertia could act as a low pass filter to produce base load generation rather than huge pulsed energy transients.


    Would a compression stage improve the efficiency of such a design? In conventional gas turbine setups it clearly does, but this scenario isn't mere heat addition, as there's a distinct blast (pressure) wave created by the device. I'm finding the thermodynamics unintuitive.
     
  9. Jul 28, 2007 #8

    Morbius

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    parsec,

    The smallest POSSIBLE fission device will destroy a city!!! In order to have
    a fission device, there has to be a "critical mass". Fission devices don't work
    until you have this minimal critical mass. However, a fission device based
    on even the minimal critical mass has the energy to destroy a city.

    You can't trap and contain that much energy.

    If you are talking about a mechanical compressor - NO!!

    Mechanical compressors are orders of magnitude TOO WEAK to do
    anything in terms of compression for nuclear fusion.

    In inertial confinement fusion, it is the radiation that compresses the
    fusion fuel via ablation of the outer layers. The radiation can compress
    hydrogen gas to densities exceeding that of LEAD!!! No mechanical
    compressor can do anything like that. In this realm, don't think about
    doing things "mechanically".

    In the nuclear realm, you are talking about forces and energies that are
    MILLIONS of times greater than mechanical forces and energies.

    Compressors and turbines are of ZERO use here - they would just be
    destroyed by the high temperatures and pressures. A turbine exposed
    to a nuclear blast isn't going to capture energy - it's just going to be
    turned into a gas - vaporized. No material can withstand the temperatures
    and pressures and maintain any structural integrity.

    Dr. Gregory Greenman
    Physicist
     
    Last edited: Jul 28, 2007
  10. Jul 28, 2007 #9
    I meant a compressor used in a brayton cycle context, not so much to get closer to the pressures and temperatures required for fusion, but to increase the amount of energy extracted from the dissipative medium (in this case air) after the heat addition process. (akin to the compression that happens in internal combustion engines before fuel addition and combustion. without this compression, no useful work would be extracted from the heat addition process)

    I am curious as to whether harnessing a fusion blast wave would require a compression stage, much like conventional internal combustion engines do, assuming that the hot gases expand to ambient temperature and pressure aft of the turbine.

    Why does fission require a critical mass? I always thought it required critical density, after which more neutrons are are generated and captured than allowed to escape, causing a self sustaining chain reaction. This being mass dependent seems unintuitive.

    Given a large enough structure and enough dissipative medium, surely it's not IMPOSSIBLE. This is merely logistics. The energy can be trapped and contained so long as it is allowed to dissipate before interacting with the frame and walls of such a structure and the turbine. As long as the intense local energies and pressures can be converted into large mass flow rates of high velocity gases at much lower temperatures and pressures, a conventional turbine should be able to extract useful energy. This may be hugely impractical though, as you suggest.
     
    Last edited: Jul 28, 2007
  11. Jul 28, 2007 #10

    Morbius

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    parsec,

    NOPE - it's mass!!! It's a combination of density AND geometry. Suppose
    I have a long thin "pencil" of uranium that has been compressed to your
    critical density. It is still going to leak neutrons out the sides.

    It's the combination of density AND geometry that gives you the term
    "critical mass" not "critical density"

    http://en.wikipedia.org/wiki/Critical_mass

    You need a building with linear dimensions of MILES - miles east/west,
    miles north/south and miles up/down. That's what you are going to
    have to do to contain the nuclear fireball.

    Show me how we are going to build something many miles high without it
    collapsing under its own weight.

    Even if you could collect this energy - how are you going to store it.

    Enough energy is released to meet your demand for a significant time;
    but all this energy is going to be released in less than a second. So you
    will have to store this energy until it can be used. How are you going to
    store that amount of energy? Are you going to use 100X the world's
    supply of lead to store this energy in a battery? At some point you hit
    real limits - you can't use more lead than exists on the planet. You can't
    have structures that can't support themselves because materials don't
    have as much strength as you'd like....

    IMPOSSIBLE is a much, much better word than "logistics" in this case.

    Dr. Gregory Greenman
    Physicist
     
  12. Aug 13, 2007 #11
    I've done some reading on laser ICF. The lasers that have and are being constructed are quite impressive in size and scale.

    Has any work been done into controlled fission initiators for ICF? I know that there are some nuclear weapon designs that use cylindrical beam guides to implode a fusion device using x-rays from a fission device.

    It seems that energy density is a limiting factor in attaining successful ICF ignition using lasers. Hohlraums are used to attain much higher photon energies (X-ray) at the expense of efficiency. Would it be possible to create fission criticality for a short transient to generate the energy densities required for fusion?

    I'm imagining a spherical fissile device with beam halls to channel hard x-rays onto the target. The device could be brought in and out of criticality for a short period of time (mechanically) within some moderative medium, to lessen the destructive effects of the sharp power release.

    In such a scenario, the energy released would not be on the scale of a typical fission weapon that is designed to release energy until it blows itself apart or attains subcriticality through some other mechanism.
     
  13. Aug 13, 2007 #12

    Morbius

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    parsec,

    Fission reactions DON'T give you X-rays - they give you GAMMA rays and neutrons.

    NEITHER gamma rays nor neutrons are very good for compressing and heating a plasma;
    in comparison to X-rays.

    The way you make X-rays with fission is to allow the fission energy to make an object
    VERY, VERY HOT - like MILLIONS of degrees - that's how you get X-rays from fission.
    That's what atomic bombs do to drive hydrogen bombs. But that's a ONE SHOT scheme -
    objects heated to MILLIONS of degrees don't survive.

    Dr. Gregory Greenman
    Physicist
     
  14. Aug 13, 2007 #13
    I was under the impression that the difference between X-rays and Gamma rays is their creation mechanism. I always remember some gamma rays having similar photon energies to X-rays. It's been a while since my physics undegrad though.

    Is it possible to use wavelength doubling optics to create X-rays from Gamma rays?
     
  15. Aug 13, 2007 #14

    Morbius

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    parsec,

    Gamma rays are the results of nuclear transitions - that is nucleons in the nucleus change
    state, and release energy due to the strong nuclear force. That is the genesis of gamma
    rays.

    X-rays are the result of electron transitions - that is electrons surrounding the nucleus
    change state and release energy due to the Coulomb force. Since the Coulomb force is
    a MILLION times weaker than the strong nuclear force; the energy of the transitions is
    less.

    Gamma rays don't behave much like waves - they behave more like particles.

    You can't optically reflect, refract, frequency, or wavelength double in an optical
    fashion with gamma. Optics relies on the properties of the electrons as charge
    carriers of the Coulomb force to be able to move and in essence "short out" the wave,
    for example - which is how you get reflection.

    There is NO analogous reaction for gammas - so you can't do optics with gammas.
    The wavelength is just too small for the electrons to do for gammas what they do
    in the case of X-rays. You don't have the "collective effects" with gammas; so
    envision them more like neutral particles.

    Dr. Gregory Greenman
    Physicist
     
    Last edited: Aug 13, 2007
  16. Sep 28, 2007 #15
    Plasma Propulsion

    There are a number of projects that have been underway for quite some time ragarding plasma propulsion. My favorite is being developed by AdAstra, based in Texas and Costa Rica and is called VASIMR. It just recently received an increase in funding from NASA.

    http://www.adastrarocket.com/vasimr.html

    It contains the plasma with magnetic fields and heats it with RF and lasers.

    This project is a plasma rocket, but there is plasma turbine project somewhere out there as well.

    Usarian
     
  17. Mar 10, 2008 #16
    Parsec,

    In response to your original suggestion, which I believe has not yet been fully addressed, I believe you are generally correct. In fact, what you suggest, in my opinion, is the most efficient scaleable method for using the hot plasma generated by some (yet technologically elusive) sustainable small-scale continuous fusion source. Such a setup would function very similar to a current Brayton cycle gas turbine, in which the compression and turbine sections would be largely unchanged, but with the combustors being replaced by some kind of plasma stream injectors. Due to the extreme temperatures of this plasma, the conduit which transfers this plasma from it's fusion reactor source to the turbine would probably need to use electromagnetic confinement, and the mass flow rate of plasma would be many orders of magnitude smaller than the air mass flow rate through the turbine in order to reach the desired 3,000 F (1600 C) or lower air temperatures. The whole cycle can be made even more efficient by then using the exhaust gasses from the turbine in a heat recovery steam generator to drive a steam turbine, similar to present day combined cycle gas/steam turbine power plants.

    In fact, I don't see a reason that the turbine part of this whole thing would need to be scaled up more. I think a current 200MW-300MW modified gas turbine would do just fine. As Dr. Greenman pointed out, it is the generation of this plasma from thermonuclear fusion that is the difficult part, as getting a sustained reaction in a small enough to control portion is very difficult.
     
  18. Mar 10, 2008 #17
    The crux of my hypothetical involved a gas turbine design facilitating the controlled diffusion of explosive fission initiated plasma into an air stream. This hot mixture, like you suggest, would be used to drive a turbine.

    Apparently it's impossible to build a detuned fission device capable of initiating fusion on a manageable scale. I don't know what the smallest explosive fission device is (either a primitive standalone fission weapon or a modern/refined fusion initiator), but I would have thought that something with a yield of less than a kiloton would be able to be used as a fusion initiator and contained within a structure (either directly in the gas turbine flow stream, much like a conventional combustion chamber, or injected into the flow stream from elsewhere like you suggest).

    I would have thought given a big enough structure, enough thermally opaque gas (to stop the structure from overheating) and large enough flow rates, this would be possible... but apparently it isn't.
     
  19. Mar 25, 2008 #18
    Holy crap thats so cool. How do they stop the plasma from melting everything??
     
  20. Mar 25, 2008 #19

    Astronuc

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    The plasma is confined by a magnetic field, and the charged particles (ions and electrons) are kept away from the solid walls by the magnetic pressure (it's actually a little more complicated than that statement).
     
  21. Mar 25, 2008 #20
    I think the way where going at fussion is wrong yah the SUN use massive amount of heat and pressure to initiate fusion but its really is a pain to do it on earth so why not work on lowering the activation energy to fuse the two nuclei so that you do not need to constrain the massive temps that you have to input to reach fusion?
     
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