magnetic retention of a plasma

It is retained in a magnetic field for short periods in things like the Tokamak reactor.

WOW!

W -what

O -of the trillions

W -wasted on this folly

Do magnetic assembly tools in welding have a detrimental effect on the arc?

In a practical or pragmatic sense, I'd say no. Magnetic fixturing has no observable affect on the arc at all, in my experience.

Direction of magnetic field matters a great deal. Plasma is like electric current in gases. If magnetic field is in same direction to the of the current flow, then it will not interact due to cross product of two vectors which make it zero force in perpendicular direction (Fleming's Left hand Rule). However if magnetic field is perpendicular to the direction of plasma and it is interacting with plasma then charge particle will end up experiencing a circular motion and some centrifugal force also. This can change the path of the charge particles or position where charge particle will reach.

http://en.wikipedia.org/wiki/Fleming's_left_hand_rule

Hi Shyam:

I agree completely that there is the theoretical near certainty that magnetic fields will affect plasma. However, in a practical sense (thus the preface in my response re practical , pragmatic, and the reference to my own experience) magnetic fixturing seems to have no readily-observable effect on the arc. In the several hundreds of times in which I've used magnetic fixturing, I have never had the urge to reposition the fixturing to modify the shape of the arc.

Of course, when welding with a hand-guided arc, it would be difficult to discern when deviations are caused my muscle tremors and lack of eye-hand coordination vs their being caused by moving through the complex and subtle changes in flux strength and direction caused by magnetic fixturing. Typically, the fixturing is kept away from the weld zone to avoid heat damage to the fixturing, and the magnetic flux tends to be directed through the material to be welded and the welding table, leaving flux in the area of the weld itself rather weak. But perhaps in robotic welding, where the arc is directed along a much straighter line (assuming a straight intended weld) then the effects of magnetic fixturing could be seen more easily.

Dear Ken_fry

Perhaps when you use plasma torch, you know where to point it. You can do so even if plasma plume is shifted a bit. With fixed magnet, it will be stable and with slow varying magnetic field it will cause serious problem and with high frequency magnetic field the plume will look broad. As this all happens with large magnetic field, you may notice nothing with small magnets. I magnet is placed in the plume direction then it will have no effect even if it is a supper magnet.

You are a practical engineers and that is what matters in the end.

Right. The physics of magnetic confinement was "refined" in Tokamak "reactors", and was for plasma that they could never sustain, to verify their claims.

However, partial verification is present in the CRT (TV-Tube) technology, where a magnetic (cross-bipolar) ring, is directing the electron beam, very precisely onto specific pixel spots on the screen.

Charge particle rings of few km diameter generally used to store accelerated electrons and charge particles by magnetic field rotation and electrostatic field acceleration. If charge particles are bunched then their energy is precisely estimated and field applied is also precisely controlled. For DC rind such estimates become difficult due to large deviation in kinetic energy.

It does seem apparent that a suitable electro magnetic field, could be used to control the shape of an RF induced plasma, by winding a suitably shaped coil. This coil might have to be toroidal? with windings made of tube for cooling purposes?

So what effect does the "Curie factor have"

GF

Curie factor for what? Dose to Curie factor? Are you talking about plasma ions or something else like plasma temperature DC and AC? What is your real question?

Shyam.

My question relates to the containment of a high temperature plasma produced by micro wave excitation. The containment of a plasma of 6000 deg or more would require a fluid shrouding or cooled retainment system.

I believe the Curie factor has a neutralizing effect upon magnetic fields above a certain temperature. I just wondered what that would be?

GF

gfwhell asks: "I believe the Curie factor has a neutralizing effect upon magnetic fields above a certain temperature. I just wondered what that would be?"

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The curie point/temperature of a magnetic material is the temperature above which the magnetic domains within the material become completely randomized. When the domains are aligned, the material is magnetic.

Consequently, a permanent magnet heated above its curie point loses its magnetism. Reducing the temperature below the curie point (in the absence of an external magnetic field which would tend to re-align the domains) does nothing; the domains remain randomized.

The curie point has nothing to do with magnetic fields, per se. It is a material property.

I think you are talking about solid material. In plasma electrons and positively charged particle and negatively charged particles will experience magnetic field due to their very nature of having charge and motion. However, at high temperature the mean free path of collision leading of change in directions and kinetic energy of the particle and electrons which is temperature will have mixed effect. As we have an average direction for plasma charge particle so will be the effect of magnetic field but not the way we see in electrons in vacuum or charge particle in vacuum. Vacuum makes the path more defined and magnetic field will continue to guide the charged particles as we do in accelerators and we can reach GeV energy. Loss of energy is part of the mechanism.

Shyam,

I am interested in the retention of an ionised gas plasma within a micro wave enclosure where gas can pass through a perforated entrance and exit.

The perforations being smaller than the wavelength of the microwave.

In excitation of the gas, a rise in temperature will occur, a means of preventing the plasma coming in contact with surfaces would be an advantage. do you think a magnetic field would help?

GF

When you apply static field along with RF, you can control the direction in which plasma can move. Hence, static electric field gives initial direction and kinetic energy and then external magnetic field can shape the plasma cross section. Now if ions are bunched out then they can move in controlled way for some time. Bunching can be done using AC electric field. I am not an expert on design of endless confinement of plasma, particularly when it has both positive and negative ions and free neutral gas. Perhaps in high vacuum things will work much better than in low vacuum.

If you look into active screen plasma research then you can notice that ion move thru active screen but main plasma zone remains confined.

a swirling gas flow can keep the plasma from your walls. I do not know what your set up is, but magnetic fields in a low density plasma may keep off the walls. A denser plasma will require some serious magnets. I do not know how temperature affects the use of magnets, though they are utilized in fusion reactors. I have read that neutral plasmas generated by microwaves weakly interact with magnetic fields--but general statements are usually wrong. If it is like a microwave thruster similar to M. Micci at Penn State, the discharge was initiated at low pressure and brought up to atmospheric utilizing a swirling gas flow in a cylinder. The microwaves passed through a piece of quartz, ionized the gas and passed out a very small aperture/nozzle at the base of the cylinder creating thrust as the gas heated. The nozzles seem to last pretty well though they were downstream of the main heat flux. Once a plasma is ignited in a properly designed resonant cavity, you should get very high coupling of the microwaves to the gas with low reflective power. In surface waves, these will travel down the outer perimeter radiating out--watch the radiation. Many torches utilize quartz cylinders traversing a waveguide where the discharge is initiated in a swirling gas flow to keep the plasma from the walls. Mit has several papers on the net. Generally at 2.45 ghz an atmospheric discharge will have a max electron temp of 11,000 K translating to a gas temp in Ar of around 5,500 K. There are also atmospheric discharges that have a relatively cool gas temperature, depending on what you are trying to accomplish these may be a better solution and you can utilize plasma facing materials that are resistant to sputtering with very specific surface finishing. good luck

Shyam observes: "I think you are talking about solid material."

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My post makes it very clear that I am talking about a solid material. The poster asked about Curie point. My reply addresses his question specifically. His question, again:

"I believe the Curie factor has a neutralizing effect upon magnetic fields above a certain temperature. I just wondered what that would be?"

You are right, you have written about solids but this initial post talks about plasma. There were some research about Curie factor for ions in plasma but I am not having any practical data on it and hence will like to point out to search for information. I am developing instruments to measure the ion types in plasma, ion density, ion kinetic energy, and effect of external magnetic field other than the magnetic field of the ion current. NASA research on sun plasma points to plasma ion currents and magnetic field, plasma flow and solar flair relationship. I am not sure if some clear documented data is available for plasma in lab. I am still hunting. Some 20 years ago I have seen a report from NIST about plasma chamber and plasma current of somewhat uniform density over larger area. Perhaps now their research must have progressed much farther. In my research lab few scientists were developing plasma coating machine and right now my interest is in understanding of the plasma ions density and kinetic energy in Nitriding Plasma chamber and Plasma Torch. Perhaps this Curie factor has become something of interest to me too in its relation to plasma temperature and plasma density. I think we are pointing to two different things here.

Shyam.

Thank you for your knowledgeable input to my question. As I understand it, the possibility exists to ionize gas to a high temperature by using microwaves at the convenient frequency of a domestic microwave oven. up to, and above the reformation stage, making it possible to diamond plate surfaces etc.

The ion being the much heavier particle would probably be subject to magnetic induction as well as centrifugal force. I understand it is possible to induce a vortex within a microwave cavity.

To me, this sounds like a worth while project for a thesis,

Regards

GF

ICP-MS gives you initial idea of generating RF plasma for injected gases in low pressure cavity and then the plasma moves in the static electric field. Usually cathode is the end point and anode is initial portion or body of the chamber and heavy positive ions move towards cathode. Sometime you get hollow cathode where most of the plasma gets confined due to greater static field formed. This is avoided by filling the cavities in Plasma Nitriding process for surface modification or hardening where Ions are implanted by diffusion. Higher velocities are not good and can cause sputtering or etching. However, continuous hitting on Iron or steel ions of Nitrogen, Oxygen, and Other gases or their combined ions heat up the material and then easily drift in. This temperature may be critical for proper drifting of ions in the material surfaces. Generally for Steel 350C to 500C temperature is used.

Ion implantation requires greater velocities as is done in semiconductor manufacturing process. Nitriding processes are bulk processes.

13.56MHz is typical RF source used in plasma generation and impedance is dynamically adjusted with respect to plasma changes. These power supplies are often used from 500W to 10kW range.

Plasma processes are designed for a purpose and same thing can do many deferent things. I think you were talking about active screen when you talked about filter. It looks like a filter but is actually a real cathode before a cathode used for coating or diffusion. High velocity ions deliver energy to active screen and then drifting ions reach the coating cathode. There is no such active screen in ion implantation process. You need to filter ions of energy and direct them where they should reach and this gives pointed zone for ion implantation.

You can also refer to ESCA and SIMS where sometime we etch out the material for depth profiling. This is done by ion beam. Accelerator may also give you more idea where one type of ion can be change to another type simply by collision process.

For Ph.D. you will have enough time to search lots of literature. I think this is an interesting field and I should have done Ph.D. in this area.

That was highly informative and concise description. Thanks.