Can vacuum be used to store flowing electrons?

Question

Since electricity can flow through vacuum can it also be used to store electricity? Though it wont remain a vacuum in strictest form.

Answer 1

The title of the question is:

Can vacuum be used to store flowing electrons?

Free charged particles like electrons can of course be stored. And of course, this needs a vacuum, since otherwise they would not be free for very long. To store them, they have to be forced on a somehow circular shaped path. Magnetic fields are suitable for this, and they have one interesting property: They deflect moving charged particles, but they don't change their energy.

The simplest method is a homogeneous magnetic field, where electrons would fly on perfect circles. The faster the particles, the larger the circle. The stronger the field, the smaller.

A more advanced example of this are particle accelerator rings like the Large Electron Positron Collider (LEP) at CERN. This was a ring of 27km circumference, where electrons (and positrons) were accelerated to an energy of 104.5GeV. This can be understood as acceleration by flying through the field of a capacitor charged to 104.5 billion volts. In the beginning, while the ring is filled with packets of electrons / positrons, they are not yet accelerated, just stored. And after acceleration to the desired energy, they are again not accelerated any more, and again just stored in the ring.

Today, LEP has been replaced by LHC, which does more or less the same with protons. (And 7 trillion volts.)

Another interesting way is to use a magnetic bottle, which consists of an non-homogeneous magnetic field:

Source

The charged particles move on helical paths ans revert their direction when reaching the "bottle necks". However, for a given field configuration, this only works for a limited energy range of the electrons.

Can vacuum be used to store energy in the form of flowing electrons?

(This seems to be the actual question...)

In short: No.

The number of electrons that are "stored" in simple magnetic fields is quite low, so it's no useful energy storage. Also the LHC (I have no data about LEP) "stores" only about 700Megajoules in the moving protons. A typical household consumes this in less than a month, so it's again not that much energy compared to the large machine.

There are two two reasons why storing energy this way doesn't make sense:

1. You need to generate the magnetic fields to guide the electrons on their track, and since the electrons repel each other, you also need fields to focus them again. Permanent magnets are not suitable since you need to be able to adjust the fields. But electromagnets need electrical power, even superconducting systems will consume some power somewhere. The energy stored in the electrons would be consumed in (fractions of) seconds by the electromagnets. (By the way: In the LHC, the power stored in the magnetic fields is abozt 10x the energy stored in the particles)

2. Each accelerated charged particle radiates electromagnetic waves. (That's what antennas do). Also an electron on a circular track accelerates permanently (i.e. changes direction), and the waves are called synchroton radiation. (Again: LHC looses about 8MW by this) So, even if you neglect the power consumed by the magnets etc, the moving electrons will loose their energy over time.

The only option is to store resting electrons. There are again inefficient power-consuming methods (e.g. for positrons which should not touch matter) or capacitors.

But you asked for moving electrons...

Answer 2

The answer is yes you can, though not in very large quantities.

The term for electrons stored in vacuum is "space charge" and it commonly builds up around a thermal emitter (cathode) in a vacuum. If they are emitted from a cathode in isolation. the result of their emission is a positively charged cathode, whose weak attraction holds (most of) the space charge in place.

Alternatively if there is a positively charged "anode" nearby the space charge will accelerate towards it to form a useful current between cathode and anode.

You can interpose a negatively charged "grid" around the space charge, so that the repulsive force from the "grid" contains the space charge, cutting off the anode current - and then control the charge storage and anode current by varying the voltage on the "grid".

This was demonstrated in 1903, and at one time, there was quite a large industry based on this idea.

Some manifestations of this idea remain : using electromagnetic fields to concentrate space charge in packets travelling from cathode to anode, such that the charge is effectively stored only for the transit time, and that storage time forms the period of the electromagnetic field.

Klystrons and magtetrons fall into this class, though the period of a magnetron may not be the sort of storage time you had in mind...

Answer 3

Yes, you can store electrons in a vacuum, but it takes a lot of power to keep them there because of their mutual repulsion. It's a whole lot easier to store them in a conductor (e.g., metal), because the positive nuclei of the atoms counteract the tendency of the negative electrons to fly apart. See Superconducting Storage Ring.

Answer 4

The existence of Alfvén waves, the Spheromak, and the confirmed functioning of Stellerator seem to indicate that it is possible.

As others have pointed out you lose energy to bremsstrahlung. This should mean that compact storage is less efficient, but there is a factor caused by lag from the speed of light which I do not fully understand beyond the intuition that the size of the circuit might not be eliminated from the equation even though it is a magnetic loop.

Answer 5

You can make a capacitor that has vacuum between the plates instead of some dielectric. Such capacitors are used for very high voltages. So, yes, you can store some small amount of energy in a vacuum.

Answer 6

Due to static electrical repulsion, electrons in a 'near vaccuum' would be hard to contain, as they would be constantly 'pushing away from' each other.

However, if you placed anough positively charged ions in said vaccuum, you could possibly (using electrical &/or magnetic field[s]) coerce the electrons to obit the ions in such a way as to form an atom (or a sort).

Assuming you could sufficiently control the path(s) & pattern(s) of these electron orbits, it would theoretically be possible to use the new atom as an 'atomic flywheel' in much the same manner as a 'superconductor flywheel' energy storage device (just with vaccuum in place of lead & LN2)