EM Drive

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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tokamac
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Re: EM Drive

Postby tokamac » Fri Aug 15, 2014 9:41 am

From Woodward's list, a VERY interesting exchange about klystrons and fine tuning of RF frequency in a waveguide:

KLYSTRON:
A klystron has multiple resonant cavities with a coaxial electron beam.  The electron beam causes the cavities to ring-- thereby transferring energy from the beam to the electromagnetic wave propagating down the slow wave ( group velocity much less than c) transmission structure comprised of the series strings of resonant cavities.

In short,  tune all the cavities to about the same frequency and yo have a very high gain, very efficient narrowband amplifier.

Stagger tune the cavities and you get a broader band amplifier with less gain and lower power conversion efficiency.

In 1977 the Russians were running klystron efficiencies in the 60 to 80% range. Note that both electrical and power efficiencies were measured. Here is an english translation:
http://www.slac.stanford.edu/cgi-wrap/g ... s-0181.pdf

WAVEGUIDE IRISES:
Consider a conventional waveguide with a rectangular cross-section
Constricting the wide dimension with a very thin conductive sheet introduces reflections such as those that would be developed by inserting a shunt inductance at the iris location.

Constricting the narrow waveguide dimension is the same as introducing a shunt capacitance.

Constricting both dimensions is the same as introducing both a shunt capacitor and a shunt inductor.  Depending on the geometry of the remaining aperture , the effect is the same as introducing a shunt connected, series resonant circuit, or a shunt connected parallel resonant circuit.

When inserting multiple irises, the distance between the irises and the characteristic impedance of the guide between the irises inserts an equivalent series inductor or series capacitance.  

So irises or in the limiting case simple conductive protrusions such as a threaded screw allow one to construct the equivalent of lumped constant impedance matching and or filtering networks.

One old trick is to tune a radar antenna for minimum vswr by taking a ball pean hammer to the waveguide.  A couple dents in the right place and you are good to go. 



The internet and Google are a marvelous research tool:
http://www.radartutorial.eu/03.linetheory/tl16.en.html
Waveguides Impedance Matching

COAX TRANSMISSION LINE (NASA) vs WAVEGUIDE (SHAWYER/NWPU):
I suspect that the waveguide approach even with its attendant tuner, isolator & iris would cause less spurious thrust signals that a piece of coax which could be leaky and cause spurious thrusts due to EM interactions with local fixed pieces of apparatus. I wish you luck in getting all that 340 waveguide gear along with the frustrum thruster onto a balance!

The magnitude of the time rate of change of power (dP/dt) to the test article can be much greater using a magnetron and WR340 waveguide verses a narrow-band solid-state microwave source tied to an RG-142 coaxial cable.  (A fire hose compared to a straw.)  And since we all know by now that dP/dt is king in this business, it’s worth the time and money to investigate this angle of the problem

Yes waveguide can handle large high power levels.  WR340 waveguide power level vastly exceeds the power level rating of RG-142  however, if you do a waveguide to coax power rating comparison, to be fair the coax should have about the same cross-sectional area as the waveguide and should have the same dielectric- ie air or largely air.

In such a comparison coax still has a lower power rating than waveguide but only by a factor of 4 or so.  The limitation in both cases is the same, dielectric breakdown - arcing) and conductor overheating

However I think coax is still worth a consideration as i don't think you will be using a multi-megawatt source and the practical advantages are significant.  Still I agree with you waveguide is better with respect to insertion loss and power rating  but coax is much much cheaper, much much more flexible, much much lighter and much easier to work with.

There are coaxial cables much better than RG142 but still fairly ordinary that work well at 2 GHz.  Look for cable with air dielectric. 

Another cautionary note is the power rating specification you quote for WG340 and RG-142 both are I assume accurate but do not apply to your application.  Both waveguide and coax and as far as i know all transmission line structures have maximum power rating that only apply when operating with a 1;1 VSWR.  For example if you have a waveguide or a coaxial line- load combination that has a 2:1 VSWR, the maximum power rating drops by at least a factor of four because of the presence of a standing wave on the line that creates two to one voltage amplitude peaks and two to one current ratio peaks.  Power handling limitations come from dielectric breakdown and conductor overheating (I^2*R)  A 2:1 vswr requires a factor or four reduction in power to stay within the dielectric break down limits and the conductor heating limits.

Necessity of 3 stub tuner: consider the 3 stub tuner as tuning (minimizing the VSWR) the section of waveguide leading to its junction with the cavity as if this junction looks like a lossy (& phase rotating) absorber to the waveguide. Re: necessity of iris at the junction of waveguide and cavity: consider the difference in geometry, ie impedance between waveguide and cavity. Apart from the simple necessity of impedance matching, you also want to maximize the one-way power flow into the cavity from the 'source' ie the waveguide. The iris acts essentially as an shunt-connected radiating element into the cavity, its efficiency given primarily by its geometry. We've never been satisfied with calculations on iris geometry versus waveguide & cavity geometry so always made variable-sized slit irises which could be manually manipulated. The one in the photos is 1 cm wide on a 1/8" thick brass plate. We've also used a simple nylon threaded rod one end of which is able to be screwed across a simple constant-area iris to vary the VSWR.

READINGS:
Here is a microwave waveguide filter and impedance matching text that might well be useful.

- Matthaei, George L.; Jones, E. L.; Young, Leo (1980). Microwave filters, impedance-matching networks, and coupling structures. Dedham, Mass: Artech House Books. ISBN 0-89006-099-1.

This is an absolute classic, a canonical text.  IIRC correctly it is a collection of papers and research reports from the surge of post WW2 radar work.

There is a chapter in this text that I think might well be particular relevant, a chapter discussing how to generate extremely high microwave power levels for testing of waveguide windows.  Windows in this context are insulating sheets crossing the waveguide aperture allowing pressurization - or evacuation- of the waveguide structure.

The trick was to make a closed loop of waveguide and feed that loop with a directional coupler.  The point is the waveguide loop was a resonant structure having a high Q and therefore having the equivalent of very high circulating currents.  This way a 10 kW test source could test waveguide windows at equivalent power levels of several megawatts.

Seems to me there may be some lessons to learn from that.  One thing that comes to mind is maybe the resonant cavity EM thrust device could better be treated as a two port thereby easing the impedance matching requirements.

You might want to pick up a copy of Skolnik (MIT) books:
- Introduction to Radar Systems, McGraw-Hill (2002)
- Radar Handbook (3rd ed.)
 
Mark Richards (Georgia Tech) is the other prolific author I would trust, but Skolnik's Radar Handbook is required for your level of work.

Betruger
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Re: EM Drive

Postby Betruger » Fri Aug 15, 2014 11:27 am

peter-b wrote:The EMDrive folks approached my friends in the propulsion group at SSC about doing some experiments in the SSC propulsion lab. The EMDrive people refused to allow their drive to be tested using SSC's extremely sensitive laser thrust balance, and as a result the group told them to go elsewhere.

I've spoken a few of the propulsion group about the latest results: the consensus is poor methodology and/or experimental error.
http://forum.nasaspaceflight.com/index.php?topic=29276.msg1242404#msg1242404
You can do anything you want with laws except make Americans obey them. | What I want to do is to look up S. . . . I call him the Schadenfreudean Man.

GIThruster
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Re: EM Drive

Postby GIThruster » Fri Aug 15, 2014 2:22 pm

Aero wrote:Is it possible to design one of these EM Thruster devices to have a wider resonant bandwidth?

Yes, but it is not nearly as efficient. Essentially what you have is a cavity where the plates are not parallel, so the wavelength between them is continuously variable. This iges an actual width instead of point of resonance, but most of the power is out of resonance at any given time.

Tom Ligon wrote:]Although possibly one could fine tune the frequency a little by adding a field control coil.

If the cavity has a dielectric in it, it is seemingly possible you could have some small measure of tuning by putting a DC bias on it. This would cause it to extend or contract according to its piezo coefficient or electrostriction. 1/4 wave acoustic resonators are this way. Whether it will work to useful limits for EM is dubious but possible. I think you'd need to crunch numbers to know.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

Tom Ligon
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Re: EM Drive

Postby Tom Ligon » Fri Aug 15, 2014 3:09 pm

GIThruster wrote:
Aero wrote:Is it possible to design one of these EM Thruster devices to have a wider resonant bandwidth?

Yes, but it is not nearly as efficient. Essentially what you have is a cavity where the plates are not parallel, so the wavelength between them is continuously variable. This iges an actual width instead of point of resonance, but most of the power is out of resonance at any given time.
Tom Ligon wrote:]Although possibly one could fine tune the frequency a little by adding a field control coil.

If the cavity has a dielectric in it, it is seemingly possible you could have some small measure of tuning by putting a DC bias on it. This would cause it to extend or contract according to its piezo coefficient or electrostriction. 1/4 wave acoustic resonators are this way. Whether it will work to useful limits for EM is dubious but possible. I think you'd need to crunch numbers to know.


I was talking about the magnetic field applied to the cavity in a magnetron. That's nearly a vacuum except for a sniff of gas and the electrons. The magnetic field there should ideally 875 G for a 2.45 GHz magnetron, but it would be impossible to set that perfectly with a pair of donut ceramic magnets. The cavity dimensions should dominate what frequency the magnetron produces. But still, looking at the output distributed over frequency, tweaking the magnetic field ought to have an effect on the output. We never looked into it analytically, but indeed, switching on the magnets of PXL-1 (adjacent to the magnetron) did affect the spectrum. The overall frequency didn't change but the hash peaks moved around, and amplitude changed.

What we were doing was using 2.45 GHz microwaves to "light up" the 875 gauss topology in the machine, increasing electron production inside. But for a practical machine, that puts the electron production too deep in the potential well. You would really want higher frequency so you sourced electrons from near the magrid. Hence, the tunable TWT, that I never saw run.

PXL-1 was, in fact, an "inverse magnetron". I think all Polywells qualify, they're just not configured with a cavity optimized to make narrow RF output.

Aero
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Re: EM Drive

Postby Aero » Sun Aug 17, 2014 1:41 am

Well is it possible to drive more than one EM Thruster having different resonate frequencies with the same magnetron? And still get reasonable drive power to each thruster.
Aero

GIThruster
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Re: EM Drive

Postby GIThruster » Mon Aug 18, 2014 3:53 pm

Betruger wrote:
peter-b wrote:The EMDrive folks approached my friends in the propulsion group at SSC about doing some experiments in the SSC propulsion lab. The EMDrive people refused to allow their drive to be tested using SSC's extremely sensitive laser thrust balance, and as a result the group told them to go elsewhere.

I've spoken a few of the propulsion group about the latest results: the consensus is poor methodology and/or experimental error.
http://forum.nasaspaceflight.com/index.php?topic=29276.msg1242404#msg1242404

What is SSC?
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

ladajo
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Re: EM Drive

Postby ladajo » Mon Aug 18, 2014 4:07 pm

Stennis Space Center, Exit 2, I-10, MS

Lots of bugs and gators. Spent some time there.
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
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GIThruster
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Re: EM Drive

Postby GIThruster » Mon Aug 18, 2014 4:57 pm

Good for Stennis that they were willing to give it a try. I'm surprised anyone would say no to them. What was the supposed reason they said no?
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

birchoff
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Re: EM Drive

Postby birchoff » Tue Aug 19, 2014 1:33 am

Here is a talk Dr. White gave at the Mars Society Convention on August 8. He covers what I assume is the latest news on the Warp field tests, along with the information from the tests on the Cannae and EmDrive replica reported in the AIAA paper from July 30 2014.

Talk

Betruger
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Re: EM Drive

Postby Betruger » Tue Aug 19, 2014 3:06 am

GIThruster wrote:Good for Stennis that they were willing to give it a try. I'm surprised anyone would say no to them. What was the supposed reason they said no?

1 beer on the answer to that being effectively solely White's purview.
You can do anything you want with laws except make Americans obey them. | What I want to do is to look up S. . . . I call him the Schadenfreudean Man.

GIThruster
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Re: EM Drive

Postby GIThruster » Tue Aug 19, 2014 1:38 pm

I'm missing something. It wasn't White who turned down Stennis, was it? Usually the centers don't collaborate until one of them needs validation. I thought it was Cannae who refused SSC. Wish I had time to go back and read this thread better but I'm rushed today. Maybe later.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

Betruger
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Re: EM Drive

Postby Betruger » Tue Aug 19, 2014 1:48 pm

My bad. I thought Eagleworks was the only contact Cannae had with NASA. That is pretty weird. Them turning down Stennis.
You can do anything you want with laws except make Americans obey them. | What I want to do is to look up S. . . . I call him the Schadenfreudean Man.

Aero
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Re: EM Drive

Postby Aero » Tue Aug 19, 2014 4:08 pm

I'll bet there was a money issue in there somewhere.
Aero

AcesHigh
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Re: EM Drive

Postby AcesHigh » Mon Sep 08, 2014 5:52 pm

interesting post at NASA SpaceFlight Forums...

would love Paul March to answer this... and does anyone know IF this issue raised by this post, if valid, can also happen with Woodward Effect?

Rodal wrote:I have concluded that thermal transient effects are a likely explanation for the measured deflections and forces in NASA's torsion pendulum experiments of the Q drives. Explicitly, that they are the result of a shift in material location of the center of mass due to differential thermal expansion resulting from heating of the dielectric resonator which is positioned unsymmetrically. If this explanation is correct, Dr. White still should also be able to measure (slightly lower) forces when he places the Q drive in a torsion pendulum in a vacuum. However, if the Q drive were free in space (instead of supported from a pendulum), this transient, unsymmetric, thermal expansion would result only in a change in attitude (orientation).

I am posting here my letter to Dr. White in order to have a wider review of this explanation.

Dear Dr. White,

I have read with appreciable interest your paper (co-authored with D. Brady, P. March, J. Lawrence, and F. Davies) titled "Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum", presented at the 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 28-30, 2014 in Cleveland, OH.

I have thought about what may be responsible for the measured displacement (and force) in your reported torsion pendulum experiments. Air convection resulting from microwave heating of the air surrounding the Q-drives has been suggested by various people, as the air speed that can produce the measured force can be shown to be small. You will be able to check whether air convection is responsible when you perform experiments in a vacuum (which I understand from your report was not possible because of the aluminum electrolytic capacitors that need to be replaced by capacitors that can work in a vacuum environment).

However, I have wondered how air convection could be responsible for the reproducible and fairly consistent levels of measured force pulse, as well as the fact that the experimental pulses are so well defined (and that it took practically no time to achieve the measured forces and to go back to zero upon ending the microwave pulse), and that turning the Q-drive around by 180 degrees resulted in practically the same force in the opposite direction.

Based on my experience conducting experiments at the Massachusetts Institute of Technology Aeronautics and Astronautics Department (for my S.B., S.M. and Ph.D. degrees at MIT) and later on at industrial R&D laboratories, I have arrived at the conclusion that transient thermal effects in your experiments should be carefully considered.

Indeed, after much thought and some calculations my conclusion is that the measured forces can quite likely be the result of transient thermal effects that very slightly shift the location of the center of mass in the material body of the Q-drive, due to unsymmetric thermal expansion, resulting from internal heating of the dielectric resonator in the Q-drive.

The center of mass changes location in the material body, with respect to body-fixed, Lagrangian coordinates, as it expands unsymmetrically. If the body would be free (unrestrained) in space, this would result only in a change in attitude (orientation) of the body. If free in space, the spatial position of the center of mass will not change (with respect to an inertial frame of reference). However, because the tested Q-drives were restrained, suspended from a support point in a torsion pendulum, the unsymmetric thermal expansion will generate a small measurable rotation and (torquing) force because the center of mass changes location in the material body as it expands unsymmetrically.


Similar issues (thermal distortion resulting in changes in orientation) were experienced, for example in spacecraft, most prominently with the Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft that got closer to the Sun: See: http://messenger.jhuapl.edu/the_mission ... u.2007.pdf

Interestingly, your paper points out the importance of the (Teflon) dielectric resonator concerning the experimentally measured forces:

<<The longer beam pipe is the RF drive antenna that in practice ends up being a ¼ wave resonance system in its own right and has a dielectric PTFE slug in the throat in both the slotted and null test article. It is this characteristic that became an item of further consideration after completion of the test campaign.>> (p.7)

<<There appears to be a clear dependency between thrust magnitude and the presence of some sort of dielectric RF resonator in the thrust chamber.>> (p.18)

<<We performed some very early evaluations without the dielectric resonator (TE012 mode at 2168 MHz, with power levels up to ~30 watts) and measured no significant net thrust.>>( p.18)

It is noteworthy that the test conducted without the dielectric resonator resulted in no significant measurable force.

My analysis of the reaction force that results from the greater thermal expansion of the drive adjacent to the dielectric resonator shows that the reaction force should be in the direction towards the end that has no dielectric resonator. That is, if the dielectric resonator is to the right of the center of mass of the drive, the reaction force will be to the left, and if the drive is positioned such that the dielectric resonator is located to the left of the center of mass of the drive, the reaction force will be to the right. This agrees with all your experimental results.

My analysis of the reaction force that results from the greater thermal expansion of the drive adjacent to the dielectric resonator also shows that if the drive were perfectly symmetric (for example having the dielectric resonator centered at the center of mass, or having identical dielectric resonators located at the same distance from the center in both directions), there would be no net thermal distortion forces as they would balance themselves out. In other words, if the Q-drive would have the dielectric resonator in the middle, or have two identical dielectric resonators positioned at equal distances from the center of mass, there would be no measurable forces.

Heat is generated inside the dielectric resonator due to the dielectric loss ("tan delta") material property of the resonator. This internal heat power is produced instantly as a result of the electromagnetic field but it takes a finite amount of time for the temperature to diffuse through the material and reach steady state in accordance with Fourier's equation of heat conduction, depending on the diffusivity of the material, and satisfying the thermal boundary conditions (convection and radiation if the experiment takes place in air, and just radiation if it takes place in a vacuum). Since the dielectric loss factor ("tan delta") is temperature dependent, the heat generated is also temperature-dependent, which introduces a nonlinearity in the solution of the differential equations for this problem. As the polymer ("Teflon" PTFE thermoplastic Fluoropolymer) dielectric temperature rises, it expands both in its radial and longitudinal direction. There is also a dynamic effect due to the inertial forces reacting to the sudden pulse, in addition to the torsional resisting force of the torsional pendulum.

The thermal expansion of the polymer dielectric resonator in the radial direction results in better contact and heat transmission to the copper structure of the drive. It can be readily shown that the effect of air convection in this experiment should be small in comparison with thermal conduction.

The reaction force produced by unsymmetric thermal expansion is proportional to the second derivative of temperature with respect to time.

To calculate how long it takes for the temperature distribution to reach steady state (and therefore for the second derivative of temperature with respect to time to become negligibly small) we may use the Fourier Number: the thermal diffusivity times the characteristic time divided by the square of the characteristic length. It is known that steady state is typically reached for a Fourier number exceeding unity, that is, for the characteristic time exceeding the ratio of the square of the characteristic length divided by the thermal diffusivity. The thermal diffusivity of Teflon is 0.124 (mm^2)/sec. I could not find the dimensions of the Teflon dielectric resonator in the report. I calculate that the time to reach thermal steady state exceeds 22 minutes if the characteristic length of Teflon is 0.5 inch (12.7 mm). If the characteristic length of Teflon is 0.2 inch (5 mm), the time to reach steady state will exceed approximately 4 minutes. If the characteristic length of Teflon is 1 inch (25.4 mm), the time to reach steady state will exceed 1 hour and 27 minutes. We know from the report that the microwave pulse was maintained for only 35 seconds during the testing (see Fig.12, p.9 in the report). Therefore, we know that the microwave pulse was maintained for an amount of time much shorter than the amount of time necessary for the temperature distribution to reach steady state in the Teflon dielectric resonator.

When the microwave power is turned off (Fig.12, p.9 of the report shows this happening 35 sec after it was turned on), the heat generating power suddenly becomes zero, and hence the second derivative of the temperature with respect to time (responsible for the reaction force) becomes negative when the microwave power is turned off, resulting in a force in the opposite direction as to when the microwave power was on.

Since copper's Young modulus is about 300 times stiffer than Teflon's, and assuming that the Teflon, particularly as it expands radially, is in frictional contact with the surrounding copper, it makes sense to assume that the expansion of the Teflon dielectric resonator is restrained by the much stiffer copper. Under that assumption, we can calculate the differential thermal expansion of the copper surrounding the Teflon as the product of the coefficient of thermal expansion of copper (16.6*10^(−6) 1/degC) times the longitudinal length of the Teflon resonator, times the "delta T": the temperature difference between the copper surrounding the Teflon and the rest of the structure.

If the longitudinal length of the Teflon resonator is 1 inch (25.4 mm), the delta T necessary to produce a differential thermal expansion of 4 micrometers is only 9.5 deg C (17 deg F). So it is quite possible to produce the measured deflections with a delta T in temperature of a few deg C. If the Teflon is unrestrained by the copper, the required delta T is 8 times smaller (since the coefficient of thermal expansion of Teflon is 135* 10^(−6) 1/degC, eight times greater than the coefficient of thermal expansion of copper).

I hope that these considerations, convince you (as has been my experience in testing at MIT and in industrial R&D) that thermal transient effects are important and therefore that it merits strong consideration that the measured deflections and forces in your torsion pendulum experiments of the Q drives are the result of a shift in material location of the center of mass due to differential thermal expansion resulting from heating of the dielectric resonator which is positioned unsymmetrically, as explained above.

Best regards,

Dr. Jose' J. Rodal
jrodal@alum.mit.edu

paulmarch
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Re: EM Drive

Postby paulmarch » Tue Sep 09, 2014 12:56 pm

[quote="AcesHigh"]interesting post at NASA SpaceFlight Forums...

Would love Paul March to answer this... and does anyone know IF this issue raised by this post, if valid, can also happen with Woodward Effect?

[quote="Rodal"]
I have concluded that thermal transient effects are a likely explanation for the measured deflections and forces in NASA's torsion pendulum experiments of the Q drives. Explicitly, that they are the result of a shift in material location of the center of mass due to differential thermal expansion resulting from heating of the dielectric resonator which is positioned unsymmetrically. If this explanation is correct, Dr. White still should also be able to measure (slightly lower) forces when he places the Q drive in a torsion pendulum in a vacuum. However, if the Q drive were free in space (instead of supported from a pendulum), this transient, unsymmetric, thermal expansion would result only in a change in attitude (orientation).


AcesHigh:

You can inform Dr. Rodal that most of the observed forces in the Eagleworks Lab frustum devices were prompt with the same rise and fall times as our electrostatically derived calibration forces and therefore are not thermal in origins. That's not to say we didn't see thermal effects, especially with input RF power levels greater than ~35W, but the thermal effects with these large copper plus dielectric test articles, (2.5 to 5.0kg), always take tens of seconds to develop and are easily distinguished from the prompt E&M or more interesting force inputs since they always exhibit exponential rise and fall times.

BTW, the copper frustum's temperature never rose more than 1.0 degree F. when using the above average power levels and test articles.

Best,
Paul March
Friendswood, TX


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