On Mar 19, 3:10 am, Crown-Horned Snorkack <chornedsnork...@[EMAIL PROTECTED]
>
wrote:
> 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
hav=
e
> > > >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?
According to http://hesperia.gsfc.nasa.gov/sftheory/flare.htm,
a solar
flare can emit up to 10^20 W. This is more than three orders of
magnitude more power than emitted by the hypothetical fusion drive.
In addition, the flare will be putting a significant fraction of its
energy into x-rays due to interactions in the dense plasma at the
solar surface, while a diffuse plume of exhaust material may emit
significantly lower x-ray intensities.
> 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?
Good question. Eventually, the plume would convert its kinetic energy
into heat through interactions with the solar wind and ISM, and emit x-
rays in the process. However, this could take a long time. The
challenge of detecting an intense, localized source of x-rays is
undoubtedly different from that of detecting the emissions from a
plume extending light years in length and emitting over a period of
decades. At the moment, I do not have the tools to predict the
radiative behavior of the exhaust in detail.
Luke
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