FAQ - Gas flow rates in a fusor

[Richard Hull]

Again the fusor volume is fixed and can never change. Assuming no absorption or desorption, the number of particles will not change. Increasing temperature will increase pressure not through a density change which is the number of particles per UNIT volume, but through a change of velocity or KE of the particles. Lowering the temp will reduce pressure not through a change in density, (no particle number change or volume change possible in a closed system), but through a reduction of their KE (velocity related only via temp). This is what is actually seen to occur on a capacitive manometer during normal fusor operation.

Richard Hull

[longstreet]

How many times must I restate that this is for CONSTANT FRIGGING PRESSURE? You have two holes in the chamber leaking gas in and out to maintain the pressure. If the chamber was completely sealed up then NO, the density wouldn't change. But that is exactly the opposite of what we're talking about.

[Richard Hull]

I think we are talking about something that probably is a given and unavoadable in the operation of a fusor. One seeks a constant flow rate. This is impacted by far more than the leak in rate of gas and pump out rate. One can never attain a perfect flow rate in an amateur system. The best one might hope for is some operational stasis regime that is quite literary balanced on the head of a pin and affected by more variables than are readily controlable over a period of time or operating voltages and currents.

This is why, from start to finish, even on a long sealed system, 1-3 hours of prep time is needed to get the average system up from cold to its MAX fusing condition.

In the best D-D systems, you might detect fusion in under 30 minutes, but putting the system on the head of that pin is an art form and takes a while. The trickiness of going from first detection to maximum output for a given system and voltage is a supreme balancing act.

No one has reported or operated a "turn-on and go" fusor system. This is mostly because they are always taking it apart, modifying grids, changing valves, pumps etc. Most failure to ease into fusion is a residual water and wall gas related problem which tends to make a lot of folks think it is a D2 feed issue. Contaminants and water are just burned out of the system or deposited in the walls or pumped out over a very long period. (montrhs if no bake out and infrequent fusor use.)

A lot of fusor operators never take this to heart and often look for the quick fix. A quick fix would be a heat taped bakeout for 24 hours with all pumps running and ion bombarding during this time. Few, if any ever do this, so, ALL have issues with chamber gas flows, odd ball pressure puffs and what appears to be balky valving in the gas lines, etc. When all of this is really failure to prep the system to a high standard of vacuum readiness and seal.

Fusors work OK whether in this state of "vacuumist grace" or not, but if not preped and held at a high state of vacuum purity, they are balky.

Few fusioneers have $1000 micrometer thimbled sapphire needled gas leaks to warrant an asbolute inlet gas leak rate. The norm is an e-bay needle valve. Likewise, only a couple of us have formal feedback looped $3000.00 gate valve systems coupled to capacitive manometers. Most often it is just a 2.75" conflat hand operated bellows valve. So, instead of a smooth gas pressure we see variations until we get the our two crude valved system balanced on the head of that pin.

Richard Hull

[AnGuy]

>No, I'm just talking about basic introductory thermodynamics here. Go look at a PV diagram. If you hold pressure as constant and increase the temperature then your volume increases.

This doesn't really apply at the sub-micron pressure levels because the majority of volume is vacuum. In order for the pressure to rise, the density of gas atom must be high enough that they press against each other (or at least frequently collide enough). Consider this. if you have a single hydrogen atom in a vacuum chamber and heat the hydrogen atom, does the pressure increase?

In the context of a fusor, usually a vacuum pump is keep running to remove any containments released by the bombardment of gas molecules as well as metal vapors released by the hot inner grid. By far the major increase of pressure is cause by the release of these containments than the rise in temperatures of D2 gas. Pressure in a fusor is largely caused the amount of gas atoms present, not the temperature.

[AnGuy]

>Few fusioneers have $1000 micrometer thimbled sapphire needled gas leaks to warrant an asbolute inlet gas leak rate. The norm is an e-bay needle valve. Likewise, only a couple of us have formal feedback looped $3000.00 gate valve systems coupled to capacitive manometers. Most often it is just a 2.75" conflat hand operated bellows valve. So, instead of a smooth gas pressure we see variations until we get the our two crude valved system balanced on the head of that pin.

Speaking of Leaks, has anyone looked at using calibrated helium leaks for feeding D2? I assume that the leak flow is too low to use realistically in a Fusor (usually around 4 to 5 X 10 -8)

[longstreet]

Show me the proof that this doesn't apply. Of course the idea gas laws are only approximations, but there is no reason for them to stop working at lower pressures. Yes, one atom theoretically has increased pressure at increased temperature because it's banging against the sides of the chambers faster. In fact any gas is mostly vacuum. You don't need them to collide together to have pressure. It's only statistics determining how likely they are to bang against the wall etc...


Besides, I've already stated that I think contaminates are a larger factor several times. What I AM saying is I don't think you can immediatly dismiss the implications of change in temperature of the surrounding gas. Whether this be in density, ionization, whatever...

[Donald McKinley]

Is smooth regulated gas flow from the bottle a problem? How much $ is the preferred valve normally?

[Tom Dressel]

Getting back to richards first post on this thread, what was the voltage at thr 20 micron pressure that yielded the high neutron counts?

I have recently finished a gas delivery system that consists of a 30 CC Storage tank connected to a micrometer needle valve connected to an 8 inch long 0.005 inck ID capillary. This varable leak will increase the valved off chamber pressure 1 to 5 micrins per minute. I have also provided a low conductance line between the diff pump and chamber, a 1/4 OD SS tubing. I can now vary the chamber pressure from < 1XE-3 Torr to 1XE-1 Torr over a wide range of flow rates.

After some modification of my power supply I plan to make a series of practice-scrubbing runs using relatively inexpensive Helium. ( Toys Are Us).

Tom Dressel

[Richard Hull]

Toms questions are sage.

The voltage that yields the highest neutron counts in any fusor at any pressure is the highest voltage you can use and not burn the SOB to the ground!

At any pressure you strive constantly to take the voltage as high as possible...... 200mev or more.

Of course, you are stopped dead in your tracks by about 10 factors, but only two or three are of critical improtance. The gas pressure is one, the stability of that pressure is another and the current to the system another.

The gas pressure needs to be as high as it can be to warrant plenty of deuterons getting ionized and trying to fuse. I ussed to starve my fusor for fuel and got starved fusor results.

Once the pressure, whatever it is, is stable and flowing and the chamber itself is saturated by the smooth and continuous flow you reach a gas stasis.

Here, the voltage is turned up and with it the pressure usually drops slightly provided you can keep the current low enough to not dip into electron emission runaway. You sit and wait and the current goes down and the count goes up. You re-adjust the gas and then inch the voltage up a bit more keeping the current within a limit established only through familiarity with your system. you continuosly jockey all this and within a short 1-2 hour period, the fusor is really humming at about 500,000 neuts/second isotropic or more.

My voltage ceiling has never been found, but I have readily operated my system into the 32-33kv range at about 6-10ma after a couple of hours fidgiting and tweaking. This might be close to my operational limit, but I can't say as at this point I am usually pretty frazzled out and tired of being a meaty feedback loop for 4 delicately interrelated variables.

Richard Hull

[Frank Sanns]

I am not so sure that higher voltage is necessarily better. D-D Cross section increases up to a point that maximizes somewhere up in the CKevs range but other losses also can go up more than linearly. Somewhere in this scheme, effiency is maximized at some point. I have seen no evidence that the highest effiency occurs at the same point as the highest fusion rate.

Complicating this fact is the type of power supplies that some of us try to use. I have complicated ones with feedback loops and current and voltage limiting and I have simple HV transformers with a recifier and a variac power control. Which do I prefer for fusor work? Without a question, I will let my Glassmans on the shelf for the variac/HV transformer supply. It lets me control the POWER to the fusor and lets the current and voltage trade off with each other to keep the power constant. This makes it easier to operate the fusor and correlate neutron output vs power. It is MUCH easier especially at the higher voltages and lower pressures near extinction. The only exception to that is that I do wish I had a dual polarity supply that I could thow a switch and near instantly change from negative gid to positive to test the effect on neutron output. Over the last 2 years I have bid on many a dual glassman supply with no luck. There are only a handful of us that can do the experiment with operational fusors yet we can not get our hands on the elusive dual polarity supply. I will keep trying unil I decide to put a diode switching relay on my transformer setup and do it that way. Probaly a far less expensive venture.

Frank Sanns

[Richard Hull]

Of course, my 200mev was hyperbole. The reaction actually peaks about 1-2mev and then the oppenheimer phillps reaction occurs and fusion drops to near zero.

The upshot is that no amateur here will ever attain a voltage where fusion rolls off, but instead is only enhanced. One sees a part of the cross sectional curve near 200kev where an increase in voltage repays the effort in ever smaller increments. Still, it climbs out to 1mev or more. The best part of the curve starts to roll rapidly after 150kev. So, no amateur will likely ever hit this range either due to neutron fluence and x-ray issues cutting his life short with continuous operation at this level.

Within the above context, The amateur will only see more neutrons with increasing voltage applied to the fusor at any given current and pressure.

Richard Hull

[Frank Sanns]

Did you say 200 TeV?

My post was more on effiency than total output. Just adding to your expert posts. No argument here.

Frank Sanns

[Richard Hull]

Frank, We need to watch our respective resorting to tongue in check word salads and hyperboles. (it was Gev) I fear some folks might have trouble following the more esoteric, extended scientific humor.

Back to more serious issues swimming around efficiency..........

Efficiency! Let's face it, we pretty much bull-head our way to neutrons in d-d fusion. Its cross section is pretty lousey and focusing the deuterons with guns would make the power consumption even higher. (most likely)

Certainly, at the amateur level, most fusion is going to be a power hog efficiency wise. Orders of magnitude efficiency gains are needed. What is amazing is that you can do real fusion hooked to your home wall outlet for about ~5 cents per reactor running hour! For the Amateur devices, the operating costs are low to non-existant (related to $3.00/gallon gasoline powering the average $32,000 SUV to go to the store to purchase a loaf of bread), but the fusor efficiencies are and will be, for a virtual infinitude of time, an abomination.

I mused over how many "breakthrough", 35% boosts in total input efficency of the fusor would be needed to achieve "no excess power, breakeven" operation. At about 8 such advances per decade we would need a string of 48 or more stunning 35% increases to hit no power gain break even or a Q of 1.

Fusion will need a massive singular stride based on some unknown and as yet un-imagined force or power to come home to papa. (total gravity intensity control, etc)

A bunch o' Euros and Dollars thrown at the ITER is a vain effort. Still, the possibility of an amateur bumping a fusor efficiency by 35% is more likely than the "supposedly" near break even fusion systems being sent that far forward. Of course they don't count all the beans in their operational reports to the public, either.

Richard Hull

[DaveC]

Richard -

I think what you're showing, very clearly is that fusion progress is NOT going to happen by accident, not is it likely to happen by simply bludgeoning with more current and voltage.

The way of progress, is to formulate some sort of theory - based on either prior experimentation or other known data - and the construct an experiment to test that theory. This takes a lot of time, based on my limited experience. Getting the data into one's head is one hurdle, then there is getting an "idea" to test, and then there's building the apparatus to test the idea.... and then.... there's doing it again and again until one gets it right. And often.. then and only then...discarding the "idea" as incorrect.

We should be very much wary of inadvertently getting into the monkeys and typewriters syndrome... expection chaos to breed order. Never happens.

I'm not sure what to suggest as a next step....quite possibly pursuing the higher pressure regime... and finding what it takes to operate there.... but my question is: at the higher Neuts/sec figures was the efficiency higher or lower? I tend to agree with Frank, that quite likely, higher efficiency comes from other causes, and could well occur at lower voltages and currents.

"Efficiency" data... would be a good benchmark for the present fusor designs. Effic. = (neuts/sec) / (kV x amps) For now just ignore all pump power and etc. Just use the Fusor input power. This would also allow all the speculations about inner shell designs, sizes and even polarities... to be settled.

Also... the Fusor scale factor could be checked... eventually... with a 2X diameter fusor... with eveything inside to the same 2X scale. This data should.... point out something...

The more I think of it... the more convinced I become that simply max Neut/sec is not enough of the story, unless..... it is true that the kV and amps are proportionally increased.

There's a couple things to do on the long Winter evenings ahead.

Dave Cooper

[Richard Hull]

Efficiency is little measured in the amateur efforts, as we tend to struggle to a point of static operation and hang there at any pressure or other parameter point. The real issue is that we have the cross sectional curve to deal with. A very small move up in voltage may double the number of neutrons near 30kv as the curve is steep and remains steep up to over 100kv. This is a factor that must not be confused with simple efficiency. One just gets a lot more fusion for only a little voltage increase in the amateur power supply end of the pool. Jon Rosenstiel is a prime example. I know of no one who has reached his voltages or his neutron output (fusions/sec). There is an obvious limit, of course based on the curves where you are getting little or nothing extra in return. We are no where near this level, of course.

I am pretty sure that this curve-voltage-accelerator path is not where the big boys are. They like bulk heating, just like the sun does, unfortunately they are looking at the maxwellian tail of the curve. (so is nature for that matter) There is a lot of energy under that curve where nothing happens.

They are hoping that, with ignition, that tail lifts into a major regime, ultimately, supplying the running energy that used to cost so much. With bulk heating you really have to have the right vessel and herein lay their rub. Magnetics are needed to keep the erossive plasma running around the center of the racetrack.

There is no end of hope on obtaining successful, power ready fusion in most all quarters where it is seen to be done, from the filmy, elusive, cold fusion arena, to the IC and IEC areas, to the annointed and fully supported bulk heating efforts. The problem with fusion is it is so damned easy to do in the lab. The carrot hangs out there and has driven men forward for years.

I find it stunningly ironic that nature doesn't do fission in a normal, readily observable fashion. As a matter of fact, she hides it well and natural fission in, say, U238 or Th232 buzzes along at such a slow rate it was never detected until someone poked it with a sharp stick (neutrons). From the date of this first single poke, man had a bomb destroying cities in 7 years and energy from it flowing out of outlets in 25 years.

On the other side of the coin, nature is downright rubbing our noses in fusion as she powers a universe with it. Sadly, with billions of the money and 50 plus years we are still picking at it like the proverbial scab. Not one fusion watt has powered anything from an outlet. The H bomb is a purely fission powered device. It seems like fusion has no middle regime. We can make a bomb based on it, provided we fission power it, but can't get it to produce just short of a bomb with full control.

I have come to the sad conclusion that confinement is the issue and we can't confine by any of the common methods in our current physics kit as we have no gravity control in it. IC and IEC use the term "inertial" and inertia is electrical in nature at the core.

We can only get things moving kinematically by accelerating them and to do this we need to convert locked down potential energy and all of this is electrical in nature, save for fission which will light off an H bomb.

It is gravity control or nuclear density control or some unappreciated internal nuclear process ala cold fusion that will probably win the fusion battle.

The current efforts are rather shabby by any standard. We lack experience at "assmbly". We are hunter gatherers used to burning and reduction of higher systems for their hidden potential energies.

The idea of taking two things of no real potential energy and spending a massive amount of energy to join them and in the process get back extra energy is foriegn to us, especially when they just refuse to join in a quantity that pays us back the energy spent. Yes, it is tempting, especially, as the universe is obviously powered by this method.

I fear all the magnetic bottles and special ceramic lined torus racetracks and all the firing of matter particles at targets or collisional schemes are sad temptresses forever letting us do fusion well short of the mark.

We need new tools regarding matter confinement. Gravity control and pinpoint amplification of it would be nice.

In the mean time, efficiency in the 10% -35% range at 6-8 orders of magnitude below breakeven is a good way to while away the time. Certainly, it could make some pretty impressive amateur devices when judged by current standards.

Richard Hull

P.S. one thing is for sure..................Anyone going to this FAQ on gas flow rates and following it out to the end will sure get a wild ride!

[Tom Dressel]

Richard, my impression from your original post on this thread was, that you got more "neutrons per volt" at 25 microns and high flow rates than at 8 to 10 microns and low flow rates. My question is: at the high flow rates (pressures), is the voltage threshold to start fusion lower, or the same as, at the lower flow rates ( pressures)? If increasing the flow rate of D2 decreases the fusion threshold voltage, what in your opinionis the MINIMUM voltage required for fusion, at say 25 microns.

Tom Dressel

[Richard Hull]

I have NO measured data on pressure vs. voltage beyond the fact that at the higher pressures I got more neutrons than at the lower pressures. Notice I did not mention FLOW RATE! I am not sure the flow rate was any higher than before and have no way to quantify that.

Pressure is pressure. And, by theory, and the Lawson criteria, more fuel in the reactor means more fusion at any given voltage.

We seem to have determined that a FLOW of gas assists in fusion for some, as yet, unquantified reason. All fusioneers report this latter condition.

It is important that one realize that you can have many different flow rates at any given, fixed pressure. I am not sure this means beans.

I am sure that I got less fusion for a specific voltage at lower maintained pressures. Again, all of this is in tune with theory and the Lawson criterion.

Only those with a calibrated mass flow controller in their gas line, plus, a calibrated capacitive manometer can answer the varying flow rate issue at fixed pressures. Flow rate and pressure are determined by both the setting of the gas line flow valve and the vacuum throttle valve in a flowing gas fusor.

These two items, coupled with voltage are the three big adjustments in the fusor, assuming you are shooting for a target or constant current.

Many are the subtlties in neutron producing, functioning, fusors and one can't preset anything in amateur construction. It ultimately comes down to an operator learning his or her device and the delicacy of its controls. Once learned, you can go to anyone else's fusor and pick up their unit's idosynchrosies. Both Brian Mc Dermot operated my fusor well at the recent HEAS gathering, once they adapted to the subtlies of fusor IV. Both Frank Sanns and Carl Willis took a turn at it back in 2004.

Fusors are NOT smooth sailing until you locate a patch of quiet water (find the sweet spot in all the variables in the area you want to operate)

Richard Hull

Richard Hull