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I've read about the concept of a laser induced plasma channel where air can be ionized to form a plasma using laser light. Can this be done or is it hypothetically possible to ionize air with microwave radiation? I know that microwaves don't have enough energy in electron volts to cause air to become ionized but what if the microwaves are generated with a high voltage? Can these microwaves have enough energy to cause ionization?

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  • $\begingroup$ Do you want to know if the electromagnetic energy of microwaves can cause ionization of air, or if voltage at the microwave frequency can cause ionization. It's really two different questions. My answer applies to the former, because air ionization is a voltage phenomena and doesn't matter if its microwave frequency voltage, or low frequency (e.g., 60 Hz) voltage. $\endgroup$ – Bob D Aug 4 at 20:01
  • $\begingroup$ Tom, see my update. $\endgroup$ – Bob D Aug 4 at 20:24
  • $\begingroup$ In further support of my answer, see the following link: hyperphysics.phy-astr.gsu.edu/hbase/mod3.html $\endgroup$ – Bob D Aug 4 at 23:08
  • $\begingroup$ Related: physics.stackexchange.com/q/311225/59023 $\endgroup$ – honeste_vivere Aug 5 at 14:02
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Microwaves at typical strengths produced by typical equipment can not ionize individual neutral atoms in the air the same way that individual UV or X-ray photons can.

This is because the energy of individual photons at microwave frequencies are way lower than the ionization potential of atoms.

The two caveats are

  1. They can get a hold of a random free electron in the air (of which there are always some) and slam it into other atoms and cause more ionization if the power is high enough and lead to plenty of ionization or even breakdown as described in this answer.
  2. If the power is really really absurdly high, then microwaves can certainly field-ionize neutral atoms the same way a high DC electric field can. However this would be on the order of volts per Angstrom. For DC fields this is done with sharp needles, for microwaves in free space you'd need an extremely high power field to get the order of a volt per Angstrom. However you could use the needle again (similarly to what's described in this answer), or perhaps even a fork.
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  • $\begingroup$ The first is taking an already free electron. The second is still a high voltage effect. In neither case are microwaves the primary cause of ionizing matter $\endgroup$ – Bob D Aug 6 at 18:47
  • $\begingroup$ @BobD My first sentence is carefully written to make that clear, and I've labeled the two enumerated scenarios as "caveats" for just that reason. What happens is what happens; your "primary cause" distinction is more of a lawyers term than a Physics concept. $\endgroup$ – uhoh Aug 6 at 21:17
  • $\begingroup$ You are entitled to your opinion but I respectfully disagree $\endgroup$ – Bob D Aug 6 at 21:52
  • $\begingroup$ Your “lawyer” comment aside, a microwave photon is a microwave photon and what it’s not is capable of ionizing matter. Period $\endgroup$ – Bob D Aug 6 at 22:19
  • $\begingroup$ @BobD Once again. my first sentence states that up front, and the enumerated scenarios are labeled as "caveats". $\endgroup$ – uhoh Aug 6 at 22:23
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Yes, you can achieve air discharge in the atmosphere using high power microwave radiation. The mechanism is avalanche ionization (https://apps.dtic.mil/dtic/tr/fulltext/u2/a172227.pdf)

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    $\begingroup$ Just to be clear, the breakdown (ionization) of air, or any insulating material for that matter, is a voltage phenomenon. That voltage can be at any frequency, including microwave frequency. But if it not the electromagnetic energy (photon) of the microwaves that is causing the breakdown, but the magnitude of voltage. $\endgroup$ – Bob D Aug 4 at 19:58
  • $\begingroup$ @BobD : I have not heard about electromagnetic radiation without voltage. If it is not electromagnetic energy of the microwaves that causes the breakdown, what is the source of the energy required for breakdown? (I hope it is obvious that air has higher energy after breakdown.) If your reasoning were correct, it would have been applicable to laser radiation as well, as visible light cannot cause one-photon ionization of air. $\endgroup$ – akhmeteli Aug 4 at 20:27
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    $\begingroup$ Also, just to be clear, for avalanche ionization it is absolutely essential that there be pre-existing ions to initiate the avalanche process (or that these are generated via some form of multiphoton ionization, which I don't really think is feasible with microwaves). If the gas is completely made up of neutral atoms and molecules, avalanche ionization cannot happen at any reasonable voltage. $\endgroup$ – Emilio Pisanty Aug 4 at 20:30
  • $\begingroup$ Voltage initiates the avalanche effect. The intensity of the discharge and the potential damage it can do is due to the energy available. Air can be ionized by means of an electrostatic discharge which involves very little energy. Or it can be ionized by a power frequency source with the possibility of a large follow through current and energy dissipation. $\endgroup$ – Bob D Aug 4 at 20:34
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    $\begingroup$ @akhmeteli I would add, as I indicated in my updated answer, that I am not disagreeing with you. I am just concerned that some people may come away from this thinking that microwave radiation is ionizing radiation, which it is not. $\endgroup$ – Bob D Aug 4 at 20:36
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Can these microwaves have enough energy to cause ionization?

No.

As you yourself pointed out, the energy level of photon in the microwave frequency range simply does not have enough energy to ionize atoms and molecules.

As for voltage, a magnetron already uses several thousand volts to generate microwaves. Household microwave ovens use a transformer to step up 120 vac.

UPDATE:

This update in relation to the answer given by @akhmeteli. I'm not disagreeing with the answer but feel it is very important to stress that the ionization of air referred to is due to high voltage, and not the magnitude of the electromagnetic energy of a microwave.

The reason I want to stress this is because there is already much confusion on the part of people regarding ionizing vs non-ionizing radiation. Microwave radiation is non ionizing radiation. I don't want readers to be left with the impression that microwave radiation is ionizing radiation.

Hope this helps.

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  • $\begingroup$ Microwaves can cause hydrogen dissociation from my first hand experience so the argument that a single microwave photon does not have enough energy is not convincing. $\endgroup$ – my2cts Aug 4 at 20:42
  • $\begingroup$ Probably by "high voltage" you mean a strong electric field. High intensity microwaves deliver just that. $\endgroup$ – my2cts Aug 4 at 20:46
  • $\begingroup$ @my2cts Yes by high voltage I meant a strong electric field. Yes high intensity microwaves deliver that. My only point is that does not make microwave radiation ionizing radiation. As to your first comment, I'm not familiar with hydrogen dissociation, but recall hearing it has something to do with very high temperatures. $\endgroup$ – Bob D Aug 4 at 20:57
  • $\begingroup$ No high temperatures are involved. This happens at room temperature. $\endgroup$ – my2cts Aug 4 at 21:14
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Rather than revise my previous answer (as it has been accepted) this will expand upon it.

I believe the confusion as to whether or not “air can be ionized using microwaves” is due to confusion between cause and association.

Microwave radiation can be associated with the ionization of air. But the ionization of air is not caused by the interaction of microwave radiation with air.

An example of association is the arcing that can occur in a microwave oven when metal objects are placed in the oven not designed for use with it. Induced high voltages cause the air to ionize and break down. This is a high voltage phenomenon. This association does not mean microwave radiation is ionizing radiation.

The ionization potential of air is about 14 ev. The energy of a 2450 MHz microwave photon is only about $10^{-5}$ ev, far below that needed to ionize air.

Hope this helps

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If you put a metal object in a microwave oven and switch on the power you will see sparks, hence microwaves can cause ionisation of air.

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