Re: Why does a tokamak need vacuum?

Bruce Scott TOK wrote:
>
> Graham Cowan wrote:
>
> >Hoping someone in the field doesn't mind renipping
> >a non-bud that has been independently discovered
> >and found wanting many times ...
>
> It is in fact sometimes discussed but only concerning exotic regimes
> (see below).
>
> >Why does a tokamak need vacuum?
> >Perhaps because particle beams need to be injected.
> >But often the heating is by induction, right?
>
> The Ohmic heating scenario is old and since the early 1980s the (for
> each time) modern tokamaks all use auxiliary heating. Usually with
> neutral particle beams but also sometimes solely with some sort of
> resonance heating.
>
> >What if it were instead filled with high-pressure helium.
> >No mobile electric charges there.
>
> Main problem: interaction with the rest of the plasma. A high-density
> neutral layer invades the plasma to the point where it is ionised. If
> the energy cost of that ionisation is high enough (e.g., the neutral
> density is high enough), the temperature crashes and you get a radiation
> collapse. In the literature this is called a MARFE.
>
> >Then a circular array of chunks of DT ice, self-ionizing,
> >would be dropped.
>
> [...]
>
> Sometimes we want to quench a tokamak discharge with a ``killer pellet''
> before it disrupts, saving the materials from the energy dump a
> disruption causes. Then this method is used :-)
>
> >With more pressure, 1 kbar, maybe we simultaneously bring upon
> >ourselves a first-wall problem, and solve it:
> >2 m of really thick helium is our first wall.
>
> The exotic idea for the first wall is liqiud lithium. The plasma is in
> direct contact and the temperature is still very high. Different from a
> neutral (ice) boundary. The lithium is supposed to flow down,
> continually stripping the outer plasma layer. This is talked about and
> superficially studied, some people believe in it, but it is a fringe
> idea.
>
> In any such scenario, the energy cost of ionisation must be considered.
> The liquid lithium scenario tries to avoid the problem by carrying that
> outer layer away. But the high pressure gas boundary would invade the
> plasma and then its ionisation energy cost is unavoidable.
>
> Always remember: a fusion plasma is a **low** energy density
> configuration.
>
> --
> ciao,
> Bruce
>
> drift wave turbulence: http://www.rzg.mpg.de/~bds/

Oh, by "radiation collapse" you must mean the plasma gets cool
enough to start emitting blackbody radiation.

I see now that this would drop the temperature quickly,
and heating it up from near zero, under high pressure all along,
would entail going through temperatures where it shines a lot.
Can't do it.


-- Graham Cowan, former hydrogen fan
http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.html
boron as energy carrier: real-car range, nuclear cachet
.

Relevant Pages

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  • Re: Why does a tokamak need vacuum?
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