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

'Break Even' with the Tokamak

  1. Dec 8, 2006 #1
    I have tryed to look into ITER and tokamak reactors latest updated information online, and cannot find any real specifics on it's latest progress. Does anyone know of where we are with producing sustained fusion for energy purposes, through currently using the tokamak?

    Is it possible to reach 'break-even' with the tokamak design considering it uses very large current, and therefore electric power to produce the sort of conditions (temperature in this case) to create fusion?

    Why specifically does the tokamak appear to be the design currently in primary consideration which will produce commercial fusion for energy?
  2. jcsd
  3. Dec 9, 2006 #2
    I usually try to stay up to date with this website:
    Also, you can try looking at the journal subscriptions of your library; they may have one that deals with the latest research going on in the world.

    ITER is being built to determine if breakeven is possible.

    To produce a lot of energy, many reactions will be needed. The easiest way to do that is through confining a lot of plasma and giving it plenty of opportunity to fuse with something else. Inertial confinement could produce a lot of energy, but it would be all at once. Dealing with a tokamak, where you have some control over the reaction rate, would be easier.
  4. Dec 9, 2006 #3


    User Avatar
    Science Advisor
    Dearly Missed


    You have control with inertial confinement too.

    A single ICF capsule or pellet is only going to put out a given amount of energy.
    As long as you can absorb one pellet's worth of energy at a time - there's no
    problem with control.

    Dr. Gregory Greenman
  5. Dec 12, 2006 #4
    I was thinking that if ITER has been underway for sometime, and that they have already poured a large amount of funding (financial), research and time into the project - that producing artificial fusion is a theoretical possibility. Im sure that they would have 'crunched the numbers' before building the reactor.

    Im not really sure whether the tokamak's design, using large current to heat the plasma, is the best way to achieve 'break-even'. I was just looking at a few other sites online which claim that at the moment, more energy has to be put into the system - then what they are able to get out of it. This raises questions, at least in my mind, about the overall approach and design of the tokamak reactor.
  6. Dec 12, 2006 #5


    User Avatar
    Science Advisor
    Dearly Missed


    You can be sure that ITER has been extensively modelled using computer simulation.
    You don't embark on a project like ITER without "crunching the numbers" as you put it.

    There are basically two methods under consideration for fusion - and they are at
    opposite limits of what I call the "Lawson spectrum". As you may know, there is a rule
    of thumb as to what conditions are necessary for break-even fusion; called the Lawson

    The Lawson criterion states that the product of the particle density and the confinement
    time has to be greater than some threshold. For a given product, there are two extremes.
    In the low density limit, one can have a low density plasma, and confine it for a relatively
    long time. That is the regime that magnetic fusion operates in.

    At the other limit, the high density limit; one can have a highly dense plasma but very
    short confinement time. That is the regime that inertial confinement fusion lives in.

    Within the inertial confinement limit; there are two schemes, laser fusion and fusion using
    pulsed power techniques. Laser fusion is being explored at Lawrence Livermore National


    and the Laboratory for Laser Energetics at the University of Rochester:


    Pulsed power techniques are being researched by Sandia National Laboratory:

    http://zpinch.sandia.gov/ [Broken]

    "Arcs and Sparks" close-up:
    http://zpinch.sandia.gov/Z/Images/z.jpg [Broken]

    Dr. Gregory Greenman
    Last edited by a moderator: May 2, 2017
  7. Dec 15, 2006 #6
    According to the torodial tube, closed ended geometry of the tokamak, it appears as if the plasma is desired to be contained for a long period of time, giving low densities. This would then suggest that the large currents (in the millions of amperes) employed by both fields (torodial & poloidial) for confinement and heating would be required to be operational for a significant amount of time. This also adds problems to trying to acheive break-even.
  8. Dec 15, 2006 #7
    Note: Perhaps only in the millions of amperes with the heating field, im not sure about the confinement. But regardless, they use large amounts of electrical power.
  9. Dec 15, 2006 #8
    I think heating the plasma to the range of tens of keV (I do not know what current would be required), you will have a characteristic confinement time of about five seconds. After that the plasma will hopefully create its own heat from the reactions.
  10. Dec 18, 2006 #9
    I think that they have already achieved fusion reactions inside the reactor - but for only a very short period of time. Obviously there has been no net gain of energy from the plasma (no breakeven, as far as im aware) but they have nevertheless been able to obtain energy from fusion reactions.

    This site at the ITER claims the "JET has produced 16 MW for 1-2 s, heating injected plasma at 24 MW." http://www.iter.org/a/index_faq.htm" [Broken] According to this link, this is the closest that anyone has acheived to breakeven.

    But the problem of keeping the pressure on the plasma (electromagnetically) so that they are able to achieve the required density, as well as continual confinement and perhaps if needed, additional heating, would appear to stand in the way of breakeven.
    Last edited by a moderator: May 2, 2017
  11. Dec 18, 2006 #10


    User Avatar
    Staff Emeritus
    Science Advisor

    That is the problem in a nutshell - maintaining pressure and temperature of the plasma, which affects the reaction rate, and keeping it stable. Beyond breakeven, the goal is to produce excess energy efficiently and be able to extract for a useful purpose, e.g. electricity.
  12. Dec 19, 2006 #11
    I'm quite intriguided as to why exactly ITER has decided to use the tokamak design as the basis to "demonstrate the technical feasibility of fusion power", especially when it has several countries involved as partners, and a large amount of joint funding and time has been, and is to be, invested in the project.

    The difficulty for me is that heating plasma by using very large currents seems like an un-viable approach, especially to achieving 'break-even'. - I just need some clarification on this from others who may have a clear understanding in this area of nuclear engineering.

    Note: Quote taken from http://www.iter.org/" [Broken]
    Last edited by a moderator: May 2, 2017
  13. Dec 19, 2006 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    The nuclear aspects (fusion) are relatively straightforward. The main problem is achieving uniform heating and confinement conditions which is a challenge in plasma physics. Large currents perhaps provide the most uniform (circumferentially that is) for heating the plasma since microwaves and neutral beams must enter the plasma locally at the outer surface and are then attenuated.

    Then there is the matter of extracting the energy produced by fusion - e.g. as the plasma expands.
  14. Dec 19, 2006 #13
    As opposed to what? Are you asking why stellarators or inertial confinment with lasers was not chosen as the ITER basis?
  15. Dec 20, 2006 #14
    Sort of - in a way.
    Perhaps the problem of the continual confinement of fusion - at least in the nature as proposed by the tokamak is an unrealistic expectation.

    The problem I see is that natural fusion reactors, as for instance the sun, are able to confine and sustain continual plasma reactions through their gargantuan mass, and therefore inherently powerful gravitational fields. But to try and artificially reproduce this, using specifically the tokamak's approach of a closed torodial geometry, appears to create problems, at least to achieving 'break-even. An extended period of applied pressure (to continuously achieve a "uniformly" required density) as well as confinement requires large currents to be operational extensively. This leads to even more "energy being put into the system", as opposed to the amount extracted.

    Also sustaining the plasma for extended periods of time seems to provide more problems. Many web site sources mention that a large proportion of instability within the plasma appears to occur during longer confinement times. Altought this appears to be a theoretical and not practical observation, as current reactors have only achieved fusion for a few seconds, this would however validate the idea of plasma instability increasing with time.

    There are some links through the ITER website which show this to be the case http://www.iter.org/paramchoice.htm" [Broken]
    Last edited by a moderator: May 2, 2017
  16. Dec 20, 2006 #15
    Yes, but look at the energy required on the atomic scale. The energy put in is on the order of KeV and what you get out is MeV. Instabilities arise during the time when the plasma starts to heat itself from its own reaction. I believe once this stage is passed, heating the plasa won't require as much energy. I don't know how much energy is required for the magnets, though.
  17. Dec 20, 2006 #16


    User Avatar

    Staff: Mentor

    Well, speaking of alternatives to Tokamaks, how many of you remember the old MFTFB architecture?

    http://www.psfc.mit.edu/library1/catalog/reports/1980/83rr/83rr021/83rr021_full.pdf [Broken]

    Basically it was a straight tube with reflecting magnets at the ends. I got to see it in construction at Lawrence Livermore Labs many years ago. It was completed on time and on budget, and then mothballed by the Regan administration. Rats. Those reflecting magnets on the ends were way cool to see in person.

    Hey Morbius -- wicked picture!
    Last edited by a moderator: May 2, 2017
  18. Dec 21, 2006 #17
    I mentioned in a previous post that the JET fusion reactor, "the world's largest nuclear fusion research facility" http://www.jet.efda.org/" [Broken].

    According to http://www.engnetglobal.com/tips/convert.asp?catid=12", 1 MW-H is equivelant to 2.25*10^28 eV.

    It is diffucult to find the total input power into a torus, either by JET or ITER, however, ITER have claimed that a heating power alone of 110 MW is attainable ("through radio frequency, ion cyclotron, electron cyclotron, 1 MeV negative ions" ect.) http://www.iter.org/Heating.htm" [Broken]. This information is sketchy since it is efficiently limited and summarised, but I believe that the confinement (control) phase is included in this input estimate.

    It appears true that the primary source of instability of the plasma is caused by it undergoing fusion and as so releasing large quantities of thermal energy in nuetrons, gamma rays and also the more stable nuclei products. However, confining this product plasma for extensive periods would, by my observations, add more problems to trying to achieve 'break-even'. Although this appears to be more a theoretical observation rather than a practical one, since fusion reactors have only been able to sustain fusion for a few seconds (according to my understanding), this would however validate the idea of plasma instability increasing with longer confinement periods.

    Perhaps this extended confinement period (on the product plasma) should be removed since significant instabilities arise causing noticeable degradation to the integritiy of the fusion cycle in the reactor (i.e. it's ability to sustain and control subsequent fusion reactions and remain operational). I agree with initiating, and then controlling and sustaining the reaction whilst fusion is occuring, however not in the manner as suggested by the design of the tokamak. Other methods within inertial confinement appear promising to initiate fusion, and so perhaps should be combined with magnetic confiment methods to control it.

    However, subsequently releasing the products from the system once the thermal energy has been extracted may have more desireable results than trying to produce a uniformly stable and self-sustaining closed ended reactor.
    Last edited by a moderator: May 2, 2017
  19. Mar 26, 2007 #18


    User Avatar
    Gold Member

    Please help me confirm my understanding of Lawson here: it applies to the case where one is trying to 'ignite' the plasma, that is you want it do generate enough energy under pressure and time conditions such that it can sustain itself without the application of additional energy to the system. So then the Lawson criterion does not apply to driven fusion concepts such as the old Hirsch-Farnswirth fusor idea of the colliding beam ideas, that is plasmas not in thermal equilibrium. In these cases the plasma is never intended to ignite, rather power is obtained purely in terms of some power out = density of ion species * ion cross section - energy; this is true independent of time of operation.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook