Thursday, February 14, 2008

Reactions

I do believe plasma physics has gone astray by the unfortunate use of the term instability to describe how a plasma reacts on itself. I think the term reaction would help to open up people's mindset. A plasma is not a stable thing. It reacts to everything including itself.

Plasma kinking is not an instability, plasma kinking is a reaction.

Sunday, February 3, 2008

LC POPS

A resonant circuit at the natural POPS frequency might be a way to generate POPS energy without an RF supply.

It should go in series with the DC supply and be just before the grid input connection to the reactor.

This would make any RF generated naturally synchronized with the internal oscillations of the reactor. Phasing could be adjusted some by detuning the tuned ckt.

A good low impedance capacitor from the HV input to the LC circuit to the ground side of the supply would be a very good idea.

This is beginning to look a lot like Tesla Coil country. In fact this Tesla Coil CAD program might be of some use for coil winding and calculating resonant frequencies. Or formulas if you want to check the prepackaged calculators or roll your own. A list of Tesla CAD programs. A self resonance coil calculator.

You can check them against this equation as long as the coil desired is at least as long as its diameter.

Fmhz = (29.85 x (H/D)1/5)/(N*D)

F= self resonant frequency in Mhz of an 'isolated' coil
H= coil height in meters
D= coil diameter in meters
N= total number of turns

===========

Update: 03 Feb 008 0714z

If the coils are made close to self resonance then a very small capacitor can be used to resonate the coil. That means the unit could be tuned over a small range by putting a sheet of dielectric between the capacitor plates.

The first thing to do is to get your fusor operating properly and then use a spectrum analyzer or 100KHz to 30 MHz radio receiver to find out what the natural frequency is.

A high frequency capacitive voltage divider with a diode detector hooked to the HV between the LC and the grid would be a good idea for tuning the coil. Tune for maximum RF.

The HV side could be from .5 to 2 pF (depending on frequency). With the low side capacitance on the order of 50 to 200pF (dependent on the high side capacitance). What you want is 100:1 divide ratio. Roughly. To start. If you use a 1N4148 diode as a detector. You should be able to go from about 100 VAC to 3000 VAC (diode limit is 3750 VAC - actually 1/2 of 75 PIV guaranteed*100). If the voltages you get are outside that range adjust your divider accordingly. The exact range is not too important as you are using it for tuning and not measurement. A voltage in the range of 5 to 10 V at peak output should give plenty of margin and yet give good tuning indication.

A lot is going to depend on lead dress. Keep everything as short as possible. The leads and everything that they contact is now part of your tuned circuit. As much as possible on the HV DC go for a single point ground.

What I'm thinking is that we have a Q multiplier here. If we use Q multiplication to raise the RF at the grid that should enhance the production of RF further raising the RF drive.

It will be interesting to see if it has any effect on fusion output. And what it does to losses.

Any way there is something like a 10% or 20 % chance we will do it this way. If it works you could control the feedback by adjusting the tuning.

Another way to tune it to start would be to use a fluorescent tube in the vicinity and tune for maximum brightness. That should be good enough to start.

Let me add one minor word of caution. We may not use this on the initial test devices until we are sure of the stability and frequency of POPS. For testing it would be more useful to have a power amplifier driven system with octave band output filters.

I have put a bit up at fusor.net about this and it seems there is an interested party. If he gets results I might change my attitude.

A while back some folks were fantasizing about how to use a Tesla coil to run a fusion machine. It looks like it might be the other way around.

Update:

Here is another design idea for how to do POPS that will be a little safer. The coil and tuning capacitor both have one end grounded. Again. A star ground for the HV will tend to reduce common mode voltages and currents.

Note that C includes coil self capacitance.

If the output of POPS is low a good rough indicator would be a NE-2 neon lamp [pdf]. Get the ones with leads. You can also raise the sensitivity by applying an AC voltage (mains power) to the lamp.

I built one of these 50 years ago when I was in the process of getting my my first Amateur license, K0NMR. They work pretty good.

Update: 04 Feb 008 0521z

Have a look at the wiki on Klystron Tubes. It uses the natural bunching of electrons to create microwave frequencies. Since we will be using ions which weigh 3600 times as much as an electron (D-D) the frequencies will be 60 times lower.

POPS oscillation is proportional to (Vwell/Mion)0.5/Rwell according to the POPS paper by Park. For a 30 KV supply voltage POPS should be around 6 MHz in a small Reactor. In any case it has a very high probability of being in the .1 to 30 MHz range.

Note that like a klystron the POPS oscillator frequency changes with operating voltage. Suppose we got an impossibly high Q of 1,000 for the tuned circuit. that would mean we needed to hold the frequency within better than .1% (1 part in 1,000) to get the maximum effect of the tuned circuit. That means holding the voltage steady to better than .2%. Difficult. Not impossible. Of course with lower Qs wider excursions are possible. It means low ripple and low voltage servo variations. Servo variations of under .1 % imply open loop gains in the passband of over 1,000. Some fun.

Say we use 80 stacked 1,000 V @ 10 A supplies. The supplies would have to regulate to better than 1 V at full output and have less than 1 V ripple. At 30 KHz operating frequency that implies an output capacitance of 22 uF @ 1500 V rating. Able to carry 5A 30KHz AC without excessive dissipation.

Since this will effectively be an 80 phase supply due to the sequential firing of the stacked modules there will be some reduction of output ripple due to the stacking. That will come in handy at lower voltages where the allowable ripple becomes less.

The allowable bandwidth of the voltage control servo is in the 1 KHz to 3 KHz range due to the 30KHz operating frequency of the switching supplies.

It may also be possible to mechanically slew the tuned circuit frequency by .2% with a speaker capable of 10 KHz response connected to a small segment of the tuning capacitor. If that was the case, as long as the system was relatively stable in the 100 microsecond time frame the tuned circuit could be kept on frequency. A VSWR detector in the HV line could do that. What you would do is compare the phase of the RF current in the line with the phase of the RF voltage on the tuned circuit and use that to servo the speaker.

Here is how POPS might be done with Amplifiers. We might need to add in an automatic phase adjuster or a PLL to keep things properly tuned up. You can click on the dwg to make it larger.

Back of the envelope calculations say that for a 50 KV DC 50 Amp grid supply (2.5 MW) an RF Amplifier capable of 250 w to 1,000 w should do the trick if using an LC circuit is not practical.

I used the wrong envelope. If the p-p voltage required is 4% of the DC voltage that represents 2,000 V p-p. That would be roughly 1,500 VRMS. Assuming a the real component of the load is 1,000 ohms (same as the DC load) that gives about 25 KW. Doubling the p-p voltage would require about 100 KW.

At those kinds of powers it may be useful to run the RF generators from the HV DC supply.

Saturday, February 2, 2008

WB-100 Superconductor Magnet Cooling

I have been working on some of the cooling issues for WB-100 - the 100 MW test reactor using superconducting magnets.

The magnet will consist of a series of concentric pipes. The innermost will contain the superconductor and its coolant at 20K. Next will come a vacuum space and next will come LN coolant. In the vacuum space between the superconductor coolant and the LN coolant the walls will be silvered (or some such) to minimize the radiative heat flow between the superconductor coolant and the LN. Think thermos bottle.

Next space after LN coolant will be another vacuum space. It too will be silvered. Then H2O coolant at around 300K. Another silvered vacuum space. And finally H2O coolant at around 600K.

What we are going to have is a series of concentric vacuum bottles with LHe at 20K at the center and H2O at 600K at the outside. All this plumbed to allow enough flow to keep everything at the proper temperature.

Let me add that any electrons ejected from the surface of this contraption will carry away minimal energy. The alphas will be hitting with 2MeV+. The electrons (those that are not lost due to high energy) will be at 50KeV.

My current plan is to coat the outer surface of the coils with Boron which melts at 2349 K. The purpose is to prevent sputtering of the metallic pipes holding the coolant so the only material sputtered into the reactant space will be a reactant - B11. It has been suggested at Coulter Smithing that an outer sheath for the coil of Titanium might work well since any sputtered atoms would act as a getter. OTOH it might poison the reaction. Lots of unknowns here. We may just have to build one and see what happens.

If we use Boron, we will have to figure out how to balance Boron condensation on the outer magnet structure with Boron sputtering from the reaction.

Below is a picture of a cross section of the superconducting coil.

Update: 06 Feb 008 2046z

I was thinking. Since for a power reactor we will need to water cool the coils. Suppose we made the water jacket thick enough to thermalize neutrons. And then had a B10 layer to absorb them.

It should be possible to cut way down on coil damage and still run superconductors in a D-D machine.

MgB is interesting in that it becomes a better superconductor with some neutron damage

With MgB the resistivity goes up. Critical Field goes up. And critical temperature declines slightly.

The main problem seems to be defects caused by B10 absorbing neutrons.

If B10 was used in shielding and B11 used to make the superconducting wire much longer lifetimes in neutron fluxes should be possible.

The the cross section difference is six orders of magnitude B10 to B11. With B10 @ 10,000 barns at .025 eV and B11 @ .01 barns.

Magnesium has a cross section of about .75 for 2.5 MeV neutrons

Mg is .063 Barns for Thermal Neutrons.

Which says that if we can get an operational life of the superconductors at 10 hours with ordinary Boron, a year should be possible with five to six nines pure B11.

Reduce the Flux another factor of 10 with water moderation and a B10 absorption layer and you are up to 10 years operation. Double that Boron 10 thickness and you are up to 100 years. Which should allow for various inaccuracies and production variations.

10 B has a Maxwellian thermal neutron flux cross section of almost 4,000 barns.

11 B is around .1 barn.

At room temperature Borax B(OH)3 is soluble at about 57 g/ liter. Which is about 9.3 g/ liter of B10.

Maximum properties of MgB occur at 2E18/cm^2 total neutron flux. Let us say 1E18 and have some safety margin.

Typical fission reactor neutron flux is 1E12/second. Let us say because of the lower energy per reaction a D-D reactor would have 50X that flux.

So that is 20,000 seconds at full power with natural boron. Say 4 1/2 hours. If we go to B11 superconductors assume a 1,000 time improvement. That is 4,500 hours. Say 6 months roughly. So we need a B10 shield that can reduce the neutron flux at the coils by a factor of 10. Giving a life of 5 to 7 years continuous operation.

Since MgB is cheap, replacing the coils every 5 to 10 years should not be a big burden. In addition preconditioned coils capable of sustaining 30 T might get a premium.

Update: 07 Feb 008 0414z

revised thicknesses
I think it is worthwhile to look at the B10 thickness required to absorb 1/10th of the incident thermal neutrons. I calculated it and came up with .005 cm. That is right 5 thousandths of a cm. To slow the neutrons from an average of 2 MeV to .025 eV (thermal energy) requires a thickness of water of about 2 1/4 inches (5.7 cm). About what I would expect to need on the basis of heat transfer and pumping considerations alone. It might be possible to include that B10 thickness (or even 3X that) in the construction of the 300K coolant channel. Just deposit it on the interior since there is no heat transfer consideration (except pumping losses from wall roughness) involved.

At a flux of 1E12 neutrons a second per sq cm., 1 sq cm will have a total flux of 3.16E20 in 10 years. To handle that number of disintegrations would require a thickness of .003 cm. Not too tough. Since the actual density required could be cut in half without seriously affecting the required volume of absorber, it might work out to fill an extra layer with boron powder. That way any break up of "structure" from radiation damage would have little effect on the absorbing properties compared to initial conditions. A layer .1 cm thick could be adequate if you recompressed it from time to time. Certainly a cm or two would be overkill.

I forgot that a D-D reactor with the same thermal power out as a fission nuke will produce about 50X as many neutrons. The 1E12 factor is based on a fission nuke. Still not a show stopper.

Update:

I have a show stopper. Each neutron absorbed produces 2.8 MeV. In a D-D reactor there is no way to carry the heat away without adding more water layers. At best a very thin layer might buy us some operational time for a test reactor. The advantage may go to using a B11 superconductor even with its lower Tc. That still only gets us months of operation. Probably good enough for experimental work.

BTW the neutron flux in a D-D reactor with a coil radius of 2 m at the coil radius is on the order of 3E14 neutrons a second at 100 MW fusion output.

Further Update:

With an intermediate layer filled with borated water to absorb 99% of the neutron energy, or 99.9%, you might get the flux down to where powdered boron could handle the rest. Great idea. At 9.3 g/l that is 9.3E-4 g/cc. Compared to 2 g/cc that would require about 10 cm - vs .005 cm for a factor of 10 reduction. Not going to work. So it still looks like MgB11 superconductors with B11 at 4 nines or better. That still only buys you a total of 1,000 hours - probably enough for initial experimental work at 100 MW.

If you could maintain a slurry of boron particles and still keep the whole contraption cool - it might work.

The trouble is that it almost doubles the neutron thermal load (1.75X). The neutrons lose 3.65 MeV thermalizing and then the B10 adds a 2.8MeV alpha. Which increases the total heat load by about 40% in what was already a marginal situation.

Friday, February 1, 2008

WB-6 Shopping List

Peter in the comments at Making the Well posted this nice parts list and operating procedure for WB-6. With his permission I'm reposting it here. A lot of the material has already been covered, but this is a nice recap.

==

I've been working on a shopping list of the specifications and requirements of WB-6.

Most of the data comes from:
Dr. Bussard's Final Lab Notes
Valencia Paper
Other bits are referenced.

If there are any mistakes or additions updates will be most welcome.

Polywell reactor specifications for a WB-6 equivalent reactor:

Vacuum chamber
• 2m diameter tank with a Faraday cage inside (WB-6 was 2m by 3.5 m) that can go down to 1*10^-9 Torr

Vacuum pump

• Able to pump (2m diameter chamber) down to less than 1*10^-9 Torr

Electron emitters

• Grounded

• Activated by fiber optically isolated Siemens switch

• Heating current of about 40 amps

• (Stainless steel?) poles to place them at a standoff distance approximately equal to the mean radius of the cusp face through which they are injected.
(to minimize electrostatic droop in the potential well at these corners)

• poles attached to the corners of the 'square' Faraday cage.

Microwave generator
• "microwaves at the ECR frequency corresponding to the magnetic field makes a death zone for neutral gas". (What is the ECR freq in WB-6?) Tom Ligon at the Fusor Forum

Magnetic field
• Preferably superconducting magnets (greatly reduces power requirements and magnet strength possible in a smaller space.)

• Otherwise 200 turns of approx 1000m of 0.15mm diameter copper magnet windings.

• Linked in series with up to 2000A of current running through them for just over 20ms.

• Make sure no tight bends in the windings.

Magnetic Grid Shell
• Stainless steel tubing welded and then polished. (Laser or electron beam welding should do the trick, so as not to damage the windings inside) M Simon in the comments

• Tightly conformal to the magnetic coils inside.

• Joined by small (approx 1cm long) tubing just outside the midplane of the magnetic field of the coils.

• Structural strength required to survive vacuum and force produced by six 0.2T magnets trying to separate from each other.

• No metal surface may penetrate the magnetic fields by more than 1*10^-4 of the total surface available to the recirculating electrons.

Structural support of Magnetic Grid
• Four support stands on the base toroid (or three or four on each with no (or slimmer) pipes joining the toroids.)

• (Stainless steel again?) encased and thus ˜hidden" from electrons by tapered ceramic supports.

• Has current carrying conductors inside helping to protect it from electrons by magnetic shielding.

• Make sure no tight bends in power supplies through the legs or the joins.

Gas supply
• Supplied by a (or several) tubes of a known tiny finite
o volume (less than 5cm^3)
o and pressure (300mili Torr too high. Must be small enough that the resulting gas pressure in the chamber is less than 3*10^-6 Torr).(This allows for the volume of gas in the reactor to be increased by tiny discrete intervals to ensure complete ionization and no flooding of the outer chamber with neutral gas.)

• Last section of tubing is glass to minimize electron losses.

• Gas input from tubes controlled by a fast acting (<1ms) solenoid valve

• Glass tube releases gas just inside the inner perimeter of the magnets. To one of the coil/coil spaced seam areas. The magnetic fields here are very strong and that reduces the likelihood of electron losses by electrons impacting the tube.

Sensors

• Sensitive Photomultiplier system
• Pressure sensors (sensitive down to 1*10^-9 Torr)

• Optical spectrometer

• Sensors for all currents and voltages on all supplies and lines and grounding cables.

• 3 neutron detectors at varying distances (of a type not affected by high voltage and able to give quick electronic output.)

• Cameras (They had two black and white ccd and 1 color camcorder) High speed color cameras operating at frame rates of much less than 0.1miliseconds would be best.

"The earliest Polywell, HEPS, was also verified to make a potential well, I believe by using four 94 GHz microwave beams across the chamber to map electron density.

The more recent machines have used at least Langmuir Probe methods (stick a wire in the thing and see what happens). And generating DD fusion is fairly convincing evidence, as well." Tom Ligon at Fusor.Net board.

Power supplies
• Car batteries for the electron emitters

• 240 RV batteries connected via an IGBT switch(able to safely produce at least 2000A)

• Twelve 225uF capacitors producing up to 15kV, 400kJ at 5A current (or 30kV at 2.5A) these can be discharged through the magnet windings.

• Fast acting pneumatic-driven copper block switch to connect capacitors.

• At least 1200W supply for microwave generator

Check Dr. Bussard's Final Lab Notes for operating procedure.

Update: 31 May 008 0337z

EMC2 has pulled the lab notes and they are no longer available on the www. You may be able to get a copy from EMC2.