Grape Plasma Explained

You’ve probably seen the videos of a grape — cut almost totally in half — in a microwave creates a plasma. A recent physics paper studies the phenomenon with a lot of high-tech gear and now the actual mechanism is known. [Veritasium] interviews the scientists and explains the grape plasma phenomenon in plain language. You can see the video below or read the paper directly.

Turns out the grape is about 1/10 of the microwave frequency and the refractive index of the grape at microwave frequencies might be as much as ten. A whole grape can get all the microwaves trapped inside, but two grapes — or two halves — that touch create fields strong enough to ionize the air.

The results include observations made from high-speed cameras, spectroscopy, and more. The plasma is mostly from potassium and sodium, apparently, The size of the grape matters, but it isn’t super critical because of the water content of the grape. Turns out it doesn’t even have to be a grape — hydrogel beads work, too.

The team also built models of the grape system to understand what was happening inside. The water content of the grape is high enough that the model just assumes the grapes are spheres of water.

You can actually produce useful plasma in a microwave with a little work. We doubt you’ll be cutting any material with grape plasma anytime soon, though.

24 thoughts on “Grape Plasma Explained

    1. Can you elaborate on the mechanism of frying? What damage actually happens to the magnetron?

      A magnetron is a physical resonant cavity with a filament. If it gets too hot it can crack and vent, but assuming that it isn’t scalding hot – can it be damaged by reflections?

      1. Of course it can.
        It’s a vacuum tube, albeit a weird one. During operation the anode naturally heats up from all those electrons crashing into it (and the flowing current) and if you reflect energy back into the tube, it has to go somewhere – namely the anode, at it’s the first thing said energy encounters.
        If you look at a typical magnetron for a microwave oven, you’ll that it has sizeable heasink and a fan blowing copious amounts of air through it, just to keep the anode from literally melting. It also has a BeO insulator on the output nub – because it needs the heat conductivity. Traditional white ceramic is nowhere near enough to do the job.

      1. A microwave heats to 100 degrees in minutes. If the tesicals are heated only by 1 to 2 degrees and only in the center. This would do considerable damage if your core testical temperature is that much off for hours. We are not talking here about the surface temperature. So we are talking about 100 degrees in minutes in a microwave vs. 1-2 degrees in hours in your testicals. this is easily a power factor of 1000 times less you need, to frie your fertility.

          1. It would’ve been worth more to point out that the ol’ sack gets more temperature variation between winter and summer (and when visiting a hot country from a cold one)… so thus 2 degrees is nothing to the ol sack… :p)

            As for the “a jet fuel fire can’t melt steel”…. it can melt steel if enough of it is present and there is sufficient oxygen…. Coal has been used for many a century to smelt steel…. using bellows as an oxygen (air) supply to get the heat up.
            However… in the referenced incident (911) both sides of the argument seems to all be wearing their foil hats… both the US government and the theorists, all throwing around pancake theories and alien theories….
            No-one stopped to look at the fact the building spiraled down, the building had a design flaw in the way the beams were mounted. It meant there was a continuous weak spot zipping down the building that day. at least that part is easy to know.

        1. I hope you realize that the heating effects of cellphone radiation have been tested? And that it is the surface (skin) that’s heated? If not please read up before becoming a conspiracy theorist, to many of those around nowadays.

          1. There are obviously some concurenting believes. This research clearly states that a grape sized object gets heated from the inside out.
            I mean we just didn’t knew until now, otherwise this effect wouldn’t have been unexplained or bad explained for so long .

  1. I’ve seen a bigger plasma effect, that lasted way longer
    with merrywannah “flowers”, my mate tried to speed up the drying process in a microwave

    the result was terrifying, it was bright, loud, went on for a few seconds and blew the circuit breaker
    the microwave oven was trashed, the buds were charred but still wet

    the oil in cannabis stems and seeds burns very hot and bright, the same with grape seed oil

    I’m not disputing the findings, just wondering how the described effects translate to a plate of loose buds rather than a grape

  2. In retrospect this might have to do with ball lightning.
    I would conjecture that the wind/torrential rain throws salts from the soil into the air and a standing wave forms between the cloud and ground due to all the lightning discharges. A lightning bolt “ignites” the resulting dusty plasma causing an air breakdown which persists as long as the field and materials lasts which can be in the 2 to 80 second range.

    Also explains why the microwave gets damaged sometimes: the resonance may cause excess energy to build up in the cavity and eventually (as I observed experimentally) damages the waveguide cover and arcs to the magnetron cap. The best analogy is HV arcs from a Tesla Coil sometimes causing foil to ignite despite the power being relatively low.
    Mitigation: use less power and/or a DC driver based on inverters, with active RF feedback to guard against moding, and a higher melting point waveguide cover.

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