Temperature at electron beam hot spots

[floris]

Dear all,

In a post by Carl Willis (viewtopic.php?f=11&t=4338#p28104), the heating at the chamber walls was calculated. The calculation involved a high fusor power of 1200 watts. This was the calculation:

E = S * T^4 (Stefan-Boltzmann law)
T = 4th RT OF[ (1 / (5.67 * 10^-8 W / m^2 / K^4)) * 1 / 0.08 m^2 *1200 W]
T = ~ 720 K

Now, I understand that this is the heating at the entire inner surface of a cylindrical chamber wall. My concern is the heating effect of the electron beams escaping the inner grid. According to Carl, the temperature would rise about 100 to 200 K higher at the places where these beams hit the walls. Is there anyway to calculate this extra heating effect on different kinds of materials?

Also, the electron beams could be deflected by a magnetic field from the more delicate parts, such as a viewport. Are there any other ways of blocking the electron beams? Maybe a mesh cathode or a shield of transparent material could be used?

Regards,
-Floris Braakman

[paulriley]

Heat in = Volts x amps of the grid. Most Fusors have negligiable extra energy from the fusing ions, so the energy into the walls will equal the electrical energy input. (Once in equilibrium the grid will neither absorb nor emit energy)

[paulriley]

Am I right in thinking that electron emission (and hence energy loss) is proportional to the Grid temperature? water cooled grid = lower loss.
What about Grid material anyone done any work to reduce electron emission by better material?

[Richard Hester]

Most of the electrons are generated by impact ionization and multiplication in the gas fill. If the inner grid gets hot enough to start emitting significant numbers of electrons, you are in trouble. You would only get to this sort of temperature with a tantalum or tungsten grid. A stainless grid will have melted long before this point.

[Richard Hull]

You may readily assume that 100% of the input enegy will wind up as heat at the outer shell eventually. This includes the inner grid heating which, will through conduction, heat the insulator and flange and shell, ultimately.

Short wave radiation escape losses via x-radiation through the shell and particulates like neutrons through the shell are like .000000008%.

The fusor, as an electrical loss device, may be looked at as a simple electrical space heater returning all input energy to the shell and external body components in a virtual 100% efficient electrical to IR energy conversion.

The total energy output of a fusing fusor is always over unity (over 100%) due to the fusion contribution as I have posted here many, many, times before.

Richard Hull

[floris]

I understand that all of the energy input and even more as the result of fusion will be converted to heat at the outer shell. My concern, however is the case of high-power fusors. In these devices, electron beams will escape the inner grid and strike the outer shell. This will create a heating effect which is not at all homogeneous across the shell surface, there will be hotter spots at the places where the electron beams strike the shell.

What I would like to know is what the temperatures at these hot spots can be. How can this be calculated?

-Floris Braakman

[Adam Szendrey]

If i'm right this should only be a concern if you use a glass or plastic chamber (or a glass viewport, since these materials will most likely melt, and the result is a nasty leak). Such electron/ion beams can be deflected by a magnet at the viewport, and a dense metal mesh right before the viewport (on the vacuum side ofcourse) would increase safety.
The temperature at such a "hotspot" is determined by the acceleration voltage if i'm correct. But i don't know how to calculate it. Why do you want to calculate this temperature?

Adam

[Richard Hull]

The heating affect of the electron beam is a function of BEAM current (number of electrons) and impact kinetic energy of each ( function of acceleratory voltages). Measuring individual beam currents for an amorphous or soft source that subdivides into multiple beams can be quite hairy and can be most easily accomplished via thermocouple reductions (beam by beam) if the acceleratroy voltage is know to some degree of accuracy.

[Jon Rosenstiel]

Floris,

Don't know if this will help or not....

The highest temperature I've seen on my fusor shell at an electron beam "hot spot" was 185C. That was with an input power of about 750W. Move a centimeter or two away from the "hot spot" and the temperature drops to 100 to 110C range.

Jon Rosenstiel