About science; About hydromagnetics

This is about science produced by the California Institute of Technology in cooperation with station KPCC in Pasadena California. The programs are made available to the station by national educational radio. This program is about hydro magnetics with host Dr. Albert Hibbs and his guest Dr. Alex Bratton all here now is Dr. hit about a century ago or to be precise in the year 1873 the great British scientist James Clark Maxwell succeeded in working out a set of mathematical equations which bears his name. Maxwell's Equations describe the relationship between electricity and magnetism. It was for the purpose of connecting these equations of Maxwells with Newton's laws of physics that Einstein developed his theory of relativity. Maxwell's equations have been the basis for the design of electrical and electronic equipment ever since they were originally published. Throughout the subsequent years it's been demonstrated over and over again that all

electrical and magnetic field follow the rules which Maxwell laid down. And yet some of these phenomena are so complicated that even though we have the equations which describe them we cannot work out the answers. Even modern computers though they were designed with the help of Maxwell's equations can't solve those same equations in certain very complicated cases. And one of the most spectacular examples of such a complicated case occurs regularly on the surface of the sun. Here we can see sunspots and solar flares and prominences those great sheets of flame that shoot up hundreds of thousands of miles from the sun's surface. As far as we can tell these two things are related. We know that the solar flares are ionized gas and that means they conduct electricity. We know that the sunspots are highly magnetized and we're certain that Maxwell's equations apply to this activity on the Sun's surface and yet no one has been able to

solve the equations for this case. The best we can do is to make a few rough approximations and set up some experiments in which we try to reduce this sort of relation between electrical and magnetic fields to easier dimensions inside a laboratory and simpler more easily control shapes. One such experimental program is going on at Cal TX jet propulsion laboratory under the direction of Dr. Alex Bratton Oh. Dr. Bratton all received this training at Washington and Lee University and at the University of California at Berkeley where he graduated with a degree of Doctor of Philosophy and physics in one thousand fifty two. His initial work was in the field of nuclear physics but in 1954 he became interested in this problem which goes into the general general heading of Magneto hydrodynamics this means problems which concern the interaction of electric and magnetic fields where a hot ionized gas is present to carry electric currents and in turn to be moved by the

magnetic field. He joined Caltex Jet Propulsion Laboratory in 1961 has been working there since on both the experimental and theoretical aspects of a subject which he calls hydro magnetics. And Alex I know that you're working with a very strong electric currents and very Parlophone magnetic fields. And you describe your work as hydro magnetics. I understand where the word magnetics comes from but what do you use the term hydro. Well I door hydro comes from the great high door which is really means water I suppose or liquid. And in modern usage it means fluids or even gases as long as the. Particle nature of the gases ignored or behaves like a continuum through a fluid. So that this is involved with what to what you're doing is more than just a magnet. Magnetism electricity.

Well yes the dynamics of fluids have the equations the dynamics of fluids have to be combined with Maxwell's equations and this combination is what we known as Magneto hydrogen Mannix or hydro magnetic. I see so it's the presence of a moving fluid in the magnetic and electric field that introduces this particular complication. Yes and it's a considerable complication because fluid dynamics itself is complicated. But what what does your equipment that you're working with in this experimental set up what does it look like. Well let's think of a copper disk about a foot in diameter with two holes in it and up through these two holes are two copper rods which are insulated with glass tubing and at the top of the rods we mount another copper dish just like the lower one and there the two are about five inches apart. So now the whole spaces in between is filled with gas at very low pressure.

Now let's imagine a current running up these two rods to the top plate and then returning to the bottom plate through the gas. That's that's the initial situation. So I guess as they're going to have to climb back down to access to conduct the current Always thing is in some sort of a tank yeah it's in a vacuum tank and glass and we can see in it it's kind of fun to watch. So then the the central central thing is just the conductors the plates and the gas that's that's the ingredients and then a big power supply to run a large amount of current through it and the power supply is just something you plug into the wall or what well you have an automatic plug it into the wall but and after we process what we get out of the wall we end up with a stored amount of electricity in a huge capacitor bank.

And with this switch of sorts and when we close the switch the current goes and it rises up to something like two three hundred amperes to 300000 to 300 thousand amperes. But I would say just for comparison the amperage in a hundred watt light bulb. In our houses about Want to Hampson. That's right. So a considerable amount of amperage help. How long does it take for this huge pulse of current to charge its way through your setup of the copper tubes and plate. Not very long I don't know how much we could light up the city of Pasadena this way at last for it rises for 10 microseconds and then it declines and then it oscillates a little while but the interesting part is during the rising part the first 10 microseconds right then and when this current builds up then during this early 10 microseconds I suppose that this is a time in which a strong magnetic field is created since it's a changing current to create a

field. That's correct we have two currents and they're both on both the two rods and they're both running in the same direction and around a current rod or current conductor. There are lines of force in the form of circles. So since this is easier to rise in the magnetic field takes on sort of a a a a shell a cylindrical shell shape around the two rods that's correct and a outer part or part of the shell as is the current flowing back to the lower plate and the. Reaction to the current and the magnetic field is an outward force which drives the gas outward an expanding cylinder actually has a shock way. So that the current goes up through the two rods around each of them there is a magnetic field created. The current returns back down through the gas through the gas at the outer edge of this field.

And while enduring this 10 microsecond time rise time the holes these two cylinders expand very rapidly and collide at the center of the machine. And an interesting thing happens there were not so much interested in the shocks as we are and what happens at this particular point in the not how do you observe a magnetic field. Do you have some magnetic probe you can use or is there something visible that happens at the same time so you know we take fast. Pictures as fast photography in a couple tenths of a microsecond. We can take a picture. What is your glow from the shock which grew from the top view looks like a pair of rings that are expanding and running together. Looking down the top of the songs yes it is a glow caused by the current going down to the ionized gas yes. Yes exactly. And fact the current flowing through the gas helps ionize

it. I forgot to mention that gas has already been treated by a feeble discharge so it's already sort of ionized you prepare a pair and prepare them for what you're going to do to it next. And this then gives you a photograph of the outside edge of the expanding magnetic That's right and this can be misleading and so we go into the interior of the space with small probes to find out what the magnetic field is doing. And these are rather simple little tiny coils a millimeter or so in diameter just a loop of wire like you know just the generators actually did it. So in the magnetic field goes through Mexico and so then you have two ways going to check in and I want a photograph of the outside edge and then I actually well we have another several other tricks we can put objects in the way and watch the flow of gas around it we can see the wake and that sort of thing. But usually this disturbs what we're trying to measure. So the gas moves to at the same time as the gas moves very quickly along with.

Well you mentioned that these two cylinders eventually hit each other of course and since how far apart by the way are the two posts right there five inches four inches and the collision occurs in about two microseconds So that leaves us nearly six microseconds to watch what happens after the collision. Well what does happen. I'm glad you asked that. The whole point of the experiment is to see how the fields which are driving the shocks interact at the point where they too could sharks collide. It happens if you think a moment that the magnetic field lines are wrapped around the two rods in the same direction and that means that where they meet the fields are in opposite directions so they tend to cancel each other they tend to cancel each other out and one would have no problem and seeing what happens if there were no gas present then the fields would simply cancel each other out and the magnetic pattern of lot of lines of force would

be a figure 8. There would be a line running around both rods giving you a nice little figure 8 and some field outside of somebody'll outside of the figure 8 and some circular lines inside each of the two loops. You say the problem comes about because there's a gas present. What have problem comes about because the whole medium is a conductor. And what happens is that at the collision point the current rises very rapidly and. Distorts the figure eight completely flattened the middle point of the figure 8 into a flat sheet which runs at right angles to the line between the two right so that the the current that was carrying the return flow from these two rods is now used to guess when the cylinders hit each other there's a sheet of current carrying gas that is squeezed in the middle and has no place to go.

That's right. Actually the whole system is unstable to form the sheet it favors the sheet until it grows to a certain extent and has tracked quite a bit of magnetic field. Well let's go back just to how we got to figure 8 and then when they assume that the current builds up most strongly were right in the middle of the figure 8 with a y right cross right now what happened you said as a sheet was formed as a sheet of this perpendicular sheet between the two rods and this sheet lengthens quite rapidly but prevents any flux from one side from canceling out the flux on the other or the field. So not only is the current trapped air but the magnetic field instead of behaving nicely is also sort of stuck. It's stuck but then suddenly something happens. The sheet gets extremely thin at the very middle and then the. A process takes place in which the all the gas is expelled from the sheet very rapidly and then the

flux can relate. The point being that the process is held back by the conductivity of the high conduct evictee until a certain geometry is reached a certain shape a certain form and then almost an explosive change in the magnetic field takes place. Cece of the magnetic field a.. You cannot cross the sheet as long as it's staying there stable a conducting electricity even though it spins pinched very. Two very narrow dimensions. Yes but what does this change that and takes place and allows it to what happens a flux the magnetic field now can break through this barrier. Yes actually it gets technical here to try to describe exactly what happened but the process is related to the problem of the diffusion of magnetic field. Through a conducting medium and this takes time but

if the region over which it has to diffuse is thin enough it doesn't take much time and the point is that all the forces of the field and the current are arranged in such a way that it favors the production of a very very thin region where the where the diffusion can then suddenly take place very rapidly and then the field takes the figure 8 form again that we had to start analyzing starts over and then it starts over again so the process is what we call a relaxation Oscillation which is a big complicated way of saying it squeaks is very much the same thing. Now on the in a flare on the sun is this kind of process and many people believe this is rather like that. This week is a very low period or very low period. Oscillation it recurs at most every hour

or so in the laboratory. It occurs every two microseconds and this points out the difference between doing experiments in the laboratory and during and observing the same kind of phenomenon going on on the sun. Namely there are tremendous differences in time duration that go along a course with the tremendous differences in space. Sometimes these extreme differences in scale mean that there are actually different physical processes. This is always the worry of trying to duplicate or even even even in a most crude way anything like the glorious size things that happen in nature in the laboratory and squeeze them down into small size. However this problem I think is. The experiment is serving to show I think for the first time how a process like this might go on. And incidentally this is exactly the same process is thought to go on

near the earth in the interaction of the well it gets into another story but the we have a magneto hydrodynamic problem around the Earth that's for sure because yours has a magnetic field yes one is shooting all these charged particles at us. Well it's actually blowing quite a husky wind at the Earth all the time and the interaction of this wind with the magnetic field of the earth produces a configuration of field at the back in the wake of the earth that is in the tail end of the disturbance down stream which is rather similar to this. So that this experiment is of interest in trying to understand how this process. But here again you're working in huge scales the tail of the earth has this magnetic tale of the earth goes out several hundred thousand miles up. Yes. But now the point I suppose is that if you want to make and have any

faith in the reality or the correspondence between small scale laboratory experiment and the big ones you have to place your faith like a pilot flying with his instruments on the equations of Maxwell and the equations of fluid mechanics that is the language that describes it have to believe no matter how things and how big things get the equations still work the same. Yeah that's right. Well let's get back to the sun for just a minute the ice I presume at these two rods in your experimental set up to carry the current are analogous to the sunspots on the sun. Is this correct or is it could be yes this could be but that is really the relationship is very loose. Yes it's true the current rods are sources of field in the sense that as the current increases lines of force seem to appear around the rod and then expand and move outward

and new lines appear. And there is the neutral point the correspondence really ceases there oil of course is the conducting medium. The sun is a very different arrangement. But it's possible to trace the analogy through quite quite well. And we now I think can understand the in terms of this something of the nature of some of the very larger flares on the sun which have a rather regular appearance. Well so far we can make up rules about what they should look like and see that these rules are buried. We talked about the formation of this as a neutral point when the two expanding shells of magnetic field collide. We talked about how this collision spreads out into a thin sheet of very concentrated current and how finally magnetic flux breaks through the sheet and starts the process over again a very rapid fashion. But

what happens to the gas when all this is going on their word so far we're talking just about currents and magnetic fields What about the actual motion of the gas particles. Oh yes I forgot to mention that one of the most spectacular things about a flare is the expulsion of gas in three different ways. There are shock waves produced which are observed halfway around the sun and traveling very fast when you can actually see what is going through the atmosphere. Yeah you can see disturbances moving Heisey very rapidly across the sun for long distances. Then there is a an expulsion of gas which are a few particles which are so fast that when they reach the Earth we call them cosmic rays. Very very high energy. And then there is another larger amount of gas which reaches the earth a day later or so. Which is a large amount of gas moving quite rapidly but not as rapidly. So one can see three different kinds of things

happening in a flare at least plus a lot of a lot of other phenomenon. What about the flames that ago are they associated with is also the prominences. Well the prominence is interestingly enough I think. Again the certain kinds of promises can be explained by the same mechanism. If we allow ourselves to interfere with it we've tried a stunt with this experiment and can interfere with the rapid penetration of flux and this relaxation isolation. We can interfere with this and have a steady flow. A steady flow of flux through the neutral point. And here I think the prominence is the analogy with the prominence is indicated. I think the problem is really quite a different thing that is the quiet one. In that case let's go back before we get into the promise let's go back again to the actual expulsion of these high sodium gas particles in the laboratory.

We have not really done quite adequate work on finding all of the particles that are accelerated. But the calculations show that we should expect to see again these three different types of particle acceleration. Where do they come out. Do they come out of the sides of the sheet is informally sorted out the top but we ask there is a large amount of slower moving gas which would correspond to what causes our magnetic storms. They are very visible in the photographs and we've watched them interfered with by putting in barriers. Those go out the ends of the sheet and then at the very moment of the fast flux relaying there is a VERY MUCH faster flow still out the edges are still out the edges. The ends of two ends of the sheet and then the third one would occur up and down along the sheet in the direction of the current flow. And this would only occur if we finally break down all the rules

of Hydra magnetics and have to come to grips more directly with the fact that it is a gas it's a plasma and it has particles. And this third one we have not yet observed. But the conditions are favorable for it. Does this not give you sort of lost me there but haven't you been behaving as if Or haven't you been talking as if this were a gas all along. Yes except I was treated. Thinking of it as a continuous fluid at the same time gas like only in the sense it is compressible. But gas really I mean when you get down to the actual particularly nature of the matter ideally where then you begin to get even more complicated. Then we yeah then we have to think about a thing which we call the mean free path which is the distance that an atom can move before it bumps into a neighbor. And if that length the distance that an atom travels before it hits another atom is large compared to the dimensions of something in the

experiment. Then the particle nature of the gas has to be considered. I see so new sheets really get quite thin quite panch. That's right. All of a sudden ultimately when it gets very thin then we really have to realize it's no longer pure fluid. That's right. But up until then the as the sheet between these two impact Dino shells of field gets pinched the material the gas material is carrying the current in between gets not only squeezed but violently pushed out the ends of this forming sheet. That's right and this is you feel similar to what's happening on the sun and which sends out this material in space of course the earth isn't so unique that material goes all over space not just toward us. That's right. I'm rather interested in just polishing a material that takes place. Oh yes the only thing I think we can be comfortable more comfortable about the sun is that we can never get there to really be sure that what we're doing is right around the earth. That's why it's sort of interesting the same process goes on. We might be able to get out there and someday

see whether this is the same thing. You can actually get into a situation which is like European cheap somewhere around here. That's right it was a it was some sort of space probe. Go through what you have actually learned have there already is some data that looks that way. Does a check very much as what you would expect well is just an adequate really. Our data isn't adequate and their data is a beginning the beginning to believe that maybe you'll be able to develop both. Both pieces to fit eventually That's right. Well now let's go back to something we sort of left hanging awhile ago the prominences which even though they may not from the point of view of pure physics be so spectacular since a gas is moving so fast from the point of view of beautiful objects these huge gouts of flame that come up from the sun are certainly spectacular these also related to what you've been doing. I think they are there are. There seems to be. A number of people beginning to think of these things this way and I think this is the

correct way to look at them. Of course there are large numbers of different kinds of flares like every promise and like everything else one learns how to classify everything into different kinds and one speaks of makes us think we hedge know prominence it makes us think that we have just got it. Yes I really doubt that just because they look alike and are different from those and those look alike and we do believe there are different processes going on and the ones which are sort of quiet are probably this this kind and these are interestingly enough not always quiet there quiet maybe for months at a time and then are very active and when they go through an active phase they have the same property the prominences. Yes not quite like a flare but certainly very spectacular and they send gas large amounts of gas at high velocity. But these are still the result of two or perhaps more magnetic fields clashing together inside or carrying with them a gas which is conducting current SOTA conducting current which makes magnetic fields

and is acted on by them is involved with this whole crash. That's right. Tell me does the one other point that we probably should have covered earlier does the moving magnetic field as it goes out from the two poles. Move as fast as a shockwave through the gases or relation between the rate at which the magnetic field expands to this impact and the rate at which the shock waves in the gas move. Yes as a relation is it's not easy to say and word. Actually the you might say the flux lines the lines of force can sort of catch up to the wave in front which is another way of saying that it can deliver energy to that wave in front so that the magnetic field is feeding the shock waves as its kind of feeding the shock wave. Yeah certainly energy is flowing into the shock from behind magnetic energy and the senses so that the shock wave

and the current sheet do they coincide. Well the shock in our experiment the shock wave is incidental to setting up the problem of this mixing of flux lines at this neutral point where the collision occurred and we haven't really worried too much about the actual shock primary thing and as the actual sheet of current Yes what happens after the shocks collide is what. What is interesting here. Well Alex thank you very much for joining us tonight and giving us a picture of what happens inside a glass jar in your laboratory. Fortunately many many millions of what miles away on the surface of the sun. This was about science with host Dr. Albert Hibbs and his guest Dr. Alex Bratton all join us for our next program when host Dr. Peter lesson will lead a discussion about engineering aspects of the cardiovascular system about science is produced by the California Institute of Technology in

cooperation with station KPP C. Pasadena California. The programs are made available to the station by a national educational radio. This is the national educational radio network.