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Runaway electrons

  1. May 18, 2013 #1
    Was reading about runaway electrons in magnetic fusion. Had me thinking what would be the benefit/cost of engineering a device which creates a beam of reletivistic electrons. What would be the best way to extract energy from this system and does it even matter the speed they are traveling?
  2. jcsd
  3. May 18, 2013 #2


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    For fusion, one wants to minimize the heat/energy of the plasma, in order to avoid unnecessary radiative losses. Plasma temperatures would be on the order of 10 to 50 keV, or perhaps up to 100 to 200 keV, depending on the species.

    Fusion requires nuclei of a minimum energy in order to induce fusion, so heating of nuclei is critical. However, in a neutral plasma one cannot heat nuclei without heating electrons. Also, it is easier to impart energy to electrons, and heating electrons can heat the nuclei, but that is less desirable. Pushing electrons up to relativistic energies would unnecessary.

    A fusion reactor requires an optimal balance of plasma heating to maximize reaction rate while minimizing radiative and leakage losses.
  4. May 20, 2013 #3
    Currently there is a lot of research into plasma accelerators. Experiments using plasma accelerators have been able to create GeV electron beams over a distance of a few centimeters, where traditional rf accelerators do so over distances of tens of meters. Most of the research is into the plasma wakefield accelerator. To my knowledge, its operation is based on a principle unrelated to the runaway effect.

    While true, I'm not sure what this has to do with runaway electrons? During disruptions conditions can occur where the plasma accelerates the high energy tail of the electron distribution to relativistic energies. These beams are a concern because they can cause serious damage the first wall. I think the OP's post was asking if we could design an accelerator that uses this effect for applications unrelated to fusion.
  5. May 20, 2013 #4
    I have not known about the wakefield accelerator, but yes you are correct I was trying to understand if the phenomena of runaway electrons could be exploited in some ways to generate energy, or simply from physics principles what could one do in harnessing the energy of a relativistic electron (or group of such). Since electricity is moving electrons, and after a few runaway electrons could there be enough potential of pure positive ions which are remnant in a magnetically confined plasma such that we could use? Basic physics questions really
  6. May 20, 2013 #5
    The most powerful pulse electron accelerators are the so-called induction linacs. Good example is "Advanced Test Accelerator" (ATA) http://accelconf.web.cern.ch/Accelconf/p83/PDF/PAC1983_2970.PDF producing 10 kA current of 50 MeV electrons (the rest mass of electron is about 511 keV and so 50 MeV is rather high relativistic).
    But usage of electron beam for heating plasma (background plasma) is less useful. As this is typical condition for development of Electron-electron two-stream instability.

    But in frame of HIF (heavy ions fusion) program people are going to use induction linacs for acceleration of Cesium ions (current about 40-50 kA) which then have to be focused by propagation through plasma column and then to hit the target (hohlraum).

    The inventor of induction linacs is Nick Christafilos http://www.hep.princeton.edu/~mcdonald/accel/christofilos.pdf
    Last edited: May 20, 2013
  7. May 20, 2013 #6
    Although this is very interesting, my question is as follows: How does one go from high energy, relativistic electrons and use them for power generation? Edit* also which of the Heavy ion fusion experiments look best to you?
    Last edited: May 20, 2013
  8. May 20, 2013 #7
    For acceleration of electrons you should spend energy: some form of energy converts into kinetic energy of electrons.
    Then for power generation (production of electricity) you should convert kinetic energy back into electricity.
    Taking into account that efficiency of any energy conversion cycle is always lower than 1, you will spend more energy than gain.
    As far as I know, today's application of powerful electron accelerators is the production of X-rays used for investigation of fast processes http://accelconf.web.cern.ch/accelconf/l06/PAPERS/TU1003.PDF
    In another thread I mentioned the possibility to use powerful pulse electron beam for focusing of ion beams moving coaxially with different velocities with such a parity of speed that collision energy is enough for fusion. Such a beam configuration (three beams: two ions' moving at the same direction and electrons moving oppositely) would produce the so called "halo-layer" of high energetic particle and also if to use neutronic fuel components will also produce neutrons.
    Neutrons can be used for e.g. breading of tritium, while "halo-layer" can be used for creation and very effective heating of plasma thus allowing us to refuse very impractical for commercial fusion reactors Neutral Beam Injection.

    Heavy Ions Fusion is a mature approach with rather strong theoretical team being sponsored by DOE but its development will be even more expensive than TOKAMAK program.
    Last edited: May 20, 2013
  9. May 20, 2013 #8
    The NDCX-II was built from the linac you showed me, they have aquired parts from all over. There is not much information coming out on the experiment but so far I think its complete.
  10. May 20, 2013 #9
    My question arose from thinking about electron "runaways". In general, what if I could make a beam of relativistic electrons? Forget what the energy cost was, how would I at least try to extract electricity out of a beam of electrons is there any way? Try to put a potential somewhere or try to "steer them" into a channel of high amp current.. ?
  11. May 20, 2013 #10
    Acceleration consumes energy, while deceleration by idea should produce energy.
    So, you need certain field configuration decelerating those relativistic or non-relativistic charged particles (e.g. electrons).
  12. May 20, 2013 #11
    Neutralized Drift Compression eXperiment (NDCX-II) is only the small part of very extensive Heavy Ions Fusion (HIF) program.
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