On 19 m=E4rts, 00:09, Luke Campbell <lwc...@[EMAIL PROTECTED]
> wrote:
> On Mar 18, 11:58 am, jdnic...@[EMAIL PROTECTED]
(James Nicoll) wrote:
>
>
>
> > In article
<c5edc0c1-efe7-47ca-9abd-400864338...@[EMAIL PROTECTED]
>,
>
> > <sigidu...@[EMAIL PROTECTED]
> wrote:
> > >On Mar 18, 7:01 pm, jdnic...@[EMAIL PROTECTED]
(James Nicoll) wrote:
>
> > >> might have a peak power output of 5^16 Watts (plus whatever
> > >> for ineffeciencies).
>
> > >> Could we spot this at 4.3 LY? Would we recognize it
> > >> for what it was?
>
> > >Cheap dumb answer: the power output of the Sun is 4 x 10^26 watts.
> > >So, this is about 10^-10 as bright as the Sun.
>
> > >One way to phrase the question: can we detect a one-ten billionth
> > >change in luminosity?
>
> > >As for absolute magnitude, Alpha Centauri is about as bright as the
> > >Sun (it's a bit brighter actually, but this is BOTE). Its visual
> > >magnitude is just about 0.0. So, your Centuarian torch****p would
have
> > >an absolute magnitude of about +25.
>
> > >I'm thinking no, we wouldn't see it.
>
> > OK, this probably has a flaw in it somewhere: The shuttle
> > jet is what, 3000K? And ISP scales as the square root of temperature,
> > so if the ISP in this case is 500,000 or about 1000 times greater
> > than the shuttle's ISP, then the temperature should be somewhere
> > around 3 billion degrees.
>
> > If I run that through Wein's law, I get a peak at about
> > 10^-12 m. That's gamma radiation, right?
>
> The optimum temperature for igniting a D-T fuel mixture is 1.6E8 K,
> which corresponds to 13.6 keV. This means the radiated
> electromagnetic radiation will peak at 2.82 x 13.6 keV =3D 38.4 keV,
> which is in the x-ray part of the spectrum.
> However, fusioning gas in a reactor or pulse drive will be optically
> thin for pretty much any kind of engineering you can imagine. This
> means it will not be in thermal equilibrium with its electromagnetic
> radiation field and thus will not emit any kind of blackbody
> spectrum. The primary radiation will be x-ray bremsstrahlung, with
> energies roughly of the same order as the temperature (so between
> about 10 to 30 keV for D-T fusion). The bremsstrahlung power per unit
> volume goes as the ion density times the electron density. Thus the
> fusion exhaust plume will radiate the most energy while it is still
> relatively dense. If it has not radiated most of its energy by the
> time it has expanded significantly, it may well essentially stop
> radiating, except from collisions between the particles in the plume
> and the solar wind (or ISM, as the case may be). In addition, the
> process of allowing the hot propellant to expand against a magnetic
> nozzle will cool the exhaust, exchanging thermal energy for kinetic
> energy of the bulk flow, which will also reduce the radiation
> emitted. On the other hand, the rocket exhaust signature would not
> now be near the peak of the solar emission spectrum - stars emit far
> fewer x-rays than they do visible or IR or near UV photons. This will
> greatly reduce the background for detection.
>
> For other fusion fuels, the fuel temperature during fusion will more
> likely be set by the requirement of maximizing fusion energy delivered
> to the fuel compared to the bremsstrahlung losses from the fuel. For
> D-D, this will put the fuel temperature at around 500 keV, or 5.8E9 K,
> and for D-3He at 100 keV, or 1.2E9 K. Again, the radiated photons
> will be x-rays, although a bit harder x-rays.
>
> Other kinds of fusion reactions will always lose more energy to
> bremsstrahlung than they gain due to fusion, so these kinds of fusion
> reactions cannot take place in a fuel that has a well defined
> temperature. You can get around this by using techniques that keep
> the ions with a significantly larger energy than the electrons, such
> as polywell fusors. In this case, the maximum exhaust velocity will
> be given by the energy of the fast fusion products as they exit the
> reaction volume - an average of 2.9 MeV for the alphas from p-11B
> fusion (12E6 m/s), or a 3.6 MeV alpha particle (13E6 m/s) and 14.7 MeV
> proton (53E6 m/s) for D-3He fusion. It is unclear what the actual
> temperature of such an exhaust plume would be, if it exists at all,
> since although the particles would have a high energy they might be
> nearly non-interacting and in a highly non-thermal distribution, and
> might have a rather small distribution of energies from each other.
> It could the radiated energy is mostly diffuse bremsstrahlung gamma
> rays from the plume interactions with the solar wind. Since stars put
> out relatively few gamma rays, this gamma ray emission from the plume
> might be fairly detectable.
>
> Luke
Can you put numbers to the x-rays and gammas emitted by stars?
The Sun does emit a bit more x-rays than the Wien law blue edge of
blackbody radiation. There are things like corona, sunspots, flares,
quiet and non-quiet Sun...
How would a fusion rocket compare with the x-ray background of quiet
Sun? With ordinary flares?
We do know that Proxima Centauri flares. What about the activity, if
any, of Alpha Centauri A and B?
Also, what is the angular resolution of x-rays and gammas - would we
see a x-ray source between Alpha Centauri A and B as distinct?
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