#5 FAQ - Fusor orientation and design

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Richard Hull
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#5 FAQ - Fusor orientation and design

Post by Richard Hull »

The mechancial arrangement of the fusor and it positioning is key to ease of operation and maintenance.

A recent posting by Josh Perlow has sparked this FAQ. see his post at.......

viewtopic.php?f=6&t=2974#p18366

In that post, Josh noted that after working his first fusor, he would do things differently.
Nothing teaches like the doing and hindsight is always 20-20.

No matter how many iterations of a device or design are run, one can almost always improve on the next design, based on issues or problems detected in the current design. This post will be long, but should serve to address key concerns in the mechanical design of a functional fusor system.

Fusors are expensive toys and most can't afford to constantly redesign their system.

A good fusor design must consider the many access points and "hookups" to the fusor and the positioning of other ancillary or required gear needed to make it function. There are many considerations that must be "noodled" out to make a design.....

1. Functional
2. Serviceable
3. Modifiable
4. Safe
5. Exist in a given space
6. Keep costs resonable
7. Limit undue complication
8. Amenable to data collection

Let us examine these 8 points one by one.

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Fuctionality.

Fusors are expected to function. In considering a neutron producing or real fusing fusor, we will need to consider what is needed to function at the minimum level.

1. A vacuum chamber that will be the body of the fusor. and its mounting and electrical grounding system.
2. At least one inner grid assembly.
3. A gas line and its related connection to the fusor chamber and it related delivery system.
4. A vacuum rated insulator and stalk to bring in the needed negative polarity high voltage.
5. A view port and its related camera, mirror or telescope for viewing.
6. A pressure transducer port for a vacuum gauge.
7. A vacuum line port from which to exhaust the device.

We will touch on the above items without going into specific recommendations, but provide basic generalities.

The body of the fusor Is often two conflat flanges connecting two stainless steel, (SS), hemispheres to make a vacuum tight spherical chamber. Some fusors are not spherical due to economic constraints, but are conflated cylinders, tees, or crosses that are blanked off to make a servicable fusor chamber.

Ports for the needed vacuum, gas and electrical connections are welded or threaded into the chamber body at well thought out locations based on mounting schemes and ancillary equipment placement.

In all cases, the inner grid hangs, centered withing the vessel, suspended on a conductive stalk connected to the outside world by a vacuum rated insulator capable of withstanding the voltage tension to be placed on it.

The gas line is usually placed on the vessel at any convenient point.

A view port is placed such that the inner grid is centered within its field of view.

The vacuum line or port is often the largest hole or entry point in the device. Needless to say all of these points of entry, except the gas line and transducer ports, should always use a metal gasketed "conflat" vacuum tight port connection. The cheaper "KF" fittings will also work for most systems and are actually easier to remove and service, but have viton polymer gaskets, not favored by professional vacuumists.

These items are demanded to make a fusor minimally functional. Key to the functionality and versatility of the finished product is the positioning of each and every port.

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Servicability

The fusor can't float in mid air. It must be mounted. In its mounting, one must consider the ability to reach and have ready access to every port and connection on the device. Some ports are more vital to this servicability goal than others.

Key ports are the main vacuum line port, the high voltage insulator port and the view port. I call them the big three.

The view port is the least important of the big three if you are using a video camera. (This is highly recommended.) The salient point is that the port must face away from the operator or any spectators as it is a source of a beam of x-rays. (see also safety concerns) Position this port in any mounting scheme so that it is directing any beam out of it down at the floor or at the ceiling. It can also be oriented to "shine" through an outside wall into a yard where there is little or no nearby pedestrian traffic or loitering.

Always consider the space needed to mount the camera when working with this port position. In general, this port will need little service, but occasionally, the view window will need to be cleaned and the camera must be moved aside or dismounted with ready access to the port bolts.

The high voltage insulator terminal should be located so that casual observers will not have access to it or be able to come into contact with it. This the deadly end of the fusor. Position it carefully and with a lot of forethought to both safety and access to the power supply. You should double standoff the power cabling and, ideally, keep it short. Another key consideration here is that the inner grid is supported here and that removable of the grid via this port will avoid having to "crack" the entire chamber. Clearance should be left to allow the insulator, mounting stalk and grid to be removed with some ease.

The chamber's vacuum port is often hooked to the diffusion or "diff" pump or its associated gate valve. Most often, the diff pump is, in effect, the base mounting assembly for the fusion chamber, itself! In some rare cases, a very short, large diameter line connects the mounted chamber to a separately mounted "diff pump". Most of the mass of the fusor system and, thus, the most important to design supports for, is the diff pump/fusor combination.

The vacuum port is the number one port of the big three as it will "force" the mounting of the diff pump no matter what arrangment is made. There is only the rarest occasion that the fusor chamber will need to be completely removed from the diff pump. This would be to change the oil or to clean the system after an oil related accident in the handling of the vacuum system. The mounting of the diff/turbo pump and chamber will be the mechanical heart of any fusor system. The mechanical forepump will, in turn, have its position forced by the positioning of the diff pump-fusor mounting as the line from the forepump to the diff/turbo pump must be very short.

I would design the system based mostly from the last paragraph's three large mechanical components and then move out to the other two ports considerations.

The gas line is the least important port and its connection can be placed anywhere. This is usually a swagelock or VCR fitting welded into the body of the chamber or conflat blank-off. Gas lines are not restricted by length or other considerations. They can be routed over a long run and are easily formed and bent. They can be forced to conform to the fusor mounting regardless of position or orientation. Once a gas line is in place, it will never need to be detached or serviced unless the entire chamber is to be dismounted.

The pressure transducer gauge port need only be arranged in the mounting scheme such that it is out of the way of passers-by. I prefer to mount it far from the gas line and the vacuum ports so that the inrush of gas or the rarifaction of gas at the exhaust port do not "color" the real pressure in the chamber. ***Important*** I have found a distinct advange in putting a right angle SS elbow on this port, especially when using a sensitive capacitive gauge. This port is often best served with a short, welded KF 20 stub and KF 20 right angle elbow added on to it and the gauge mounted to the elbow. The elbow with also thwart gravity should any piece of debris fall into the port.

WARNING!!!! Bolts are used in all flanges....Lots of bolts! In your finished design, can all flanges, based on where you have located them, allow all bolts on all flanges to be inserted without going afoul of another flange or tube you have placed and welded near all other holes???? Plan and place all ports with a lot of for thought. Usually, it is the giant belly-band conflat flange that gobbles up bolts. Issues can quickly develop here in bolt collision with other stuff you have welded above and below it. Particularly suseptable to issues... if you order one of the two large joining flanges "pre-tapped". This means your bolt must go in in only one direction. Be very careful. I choose to get both flanges untapped with "through-holes" for bolts with nuts. In this manner if you go stupid in one direction, you may be saved in another. In addition, these are the least expensive type of flange.

Rotatable flanges might help here but are far more costly than fixed flanges.

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Modifiability

Leave room in your mechanical design for future additions, if this is to be a "research" fusor. From this standpoint, the larger the fusor body, the better. Large cylindirical, tee, and cross systems offer a lot of expandability not found in spherical systems. Also, consider future changes in supporting ancillary gear. New pumps, gauging, etc.

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Safety

This is almost totally related to the high voltage system, and its mechanical placement, already mentioned. However, there is the issue of rad safety part of which was covered during the viewport placement discussion. All that remains is the neutron and "through chamber" x-rays produced by operation above 35-40kv. Some sort of movable or fixed shielding is a must if you plan on personally operating close to the system or have vistors in the near vacinity of the fusor while running. This is far more critical if you plan on spending a lot of time running the device.

You are responsible for not only your own safety, but that of others around the fusor who are ignorant of many common safety concerns.

Run all high voltage lines well clear of you, the operator, or visitors or bystanders.
point the camera port so that it "shines" into the floor, the ceiling or out of a building wall. Shield for extreme high voltage operation to avoid neutron and "shine through" x-ray exposure. Leave some wiggle room and mount your fusor so that these safety issues are fully addressed.

Finally, it is always good to lock out your system in such a way that children, if you have them, are unable to harm themselves. You are on your own here. I have a key operated circuit relay on my high voltage system such that I must be in attendance during operation. Placing the gas bottle and its valve out of the reach of little ones is also important. Best of all is to put the system in a locked room that only you have the key for. This will insure overall safety with a single lock.

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Fusor space

The physical space taken up by a complete fusor setup need not be extreme. There is little reason for a fusor system to take up much more floor space than a full sized office desk or, if totally stacked in a vertical racking system, the size of a large refrigerator. A modified, surplus office desk would, in fact, make a great base and fusor station. Likewise, a section of unused work bench will also suffice. I would think 100 cubic feet would be more than enough to house a fusor system with no more than 18 square feet of floor space needed. (3X6 floor X 6 feet high). Needless to say, 1/2 this space will suffice for the agoraphobic while double this might satisfy the claustrophobic.

The verical dimension is ruled by the stacking of the large mechanical pump topped by the diff pump system and then on top of this, is the fusor body and associated wiring and plumbing. The horizontal dimension is ruled by the power supplies, NIM detection and gas supply gear coupled to the width of the mechanical pump.

The key here is to always leave plenty of maneuver room around the mechanical setup. Nothing is more frustrating than trying to place and turn a wrench or hex key on a bolt that is in a totally inaccessable location or in a well placed, but cramped location. All of this must be considered in your mechanical layout, right down to where to weld on ports based on your planned mounting scheme.

Much of the above can be addressed with the entire system stacked vertically on a specially constructed, large, roll-around, angle iron assembly. This will allow, forever, full access to all sides of the mechanical construction and orientation for running in a manner that is rad safe and offer portability, (actually movability), within a school or dwelling.

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Costs - Control them

Most of us are not well heeled or have access to government funding. As such, we are usually wallet or discretionary fund limited. You might have to trade off a bit of design due to cost limitations. Here is where scounging abilities are worth at least double the cash value of what you have to spend. When you reach a point of economical contention or limitation, think around the difficulty, first. If this process reaches a hopeless economic blind alley, try to obtain what you need through any number of scrounging efforts. ( E-bay, dumpster diving, begging, borrowing, etc.)

This is an area that can limit or expand your mechanical design effort.

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Mechanical complications

Needless complications are to be avoided at all costs in the design process. By making a clean, neat and spacious system, you leave room for future expansion, (complication). Avoid "coming out of the chute" with overly complex designs.

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Data collection

Remember this is a fusor, The goodie or product of this system is neutrons. These can be used to activate materials which is a major use for the fusor if operated at the higher levels. The neutrons are all fast neutrons and must be slowed. This means a moderator must be used. In order to take advantage of the maximum flux from your fusor, the moderator needs to be "hard up" against the fusor chamber's shell. LEAVE ROOM FOR THIS ALL IMPORTANT ITEM!! This will absolutely dictate the mounting of every port, orientation of the fusor and, thereby, much of the ancillary gear.

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Summary

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Before you drill that first hole, weld that first port or mount any gear in place, you need to have a complete design on paper and not in your head. Once on paper, you need to operate the paper design in your head. Make believe your diff pump's heater goes out. What will you have to remove to get at the heater? How easy would it be to change your diff pump oil from 704 silicone to santovac? Suppose you need to clean your viewport. How much trouble will be involved in removing the camera or optical system?

Enjoy your effort. Still, with all the above, you may yet find yourself altering your design as you build. Once built, you will see a myriad of improvments that can easily be implemented and some that can't without a complete redesign.

In the meantime, look at what others have built especially those in their third and forth iteration or design. They have been there and made corrections over their earlier systems. Learn from them.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Chris Bradley
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Re: FAQ - Fusor orientation and design

Post by Chris Bradley »

A thorough and well thought out FAQ, as usual.

I would also venture to suggest that it is preferable to ensure the security of the space, such that an experiment is set up free of likely interference by others (especially kids - or, given the age of some of the active fusioneers, parents who think they know better!!) who might either cause injury to themselves or tamper so as to cause danger to you, that the area is not prone to condensing humidity, and also is not a dusty (particularly fibrous) place that could compromise vacuum seals during assembly.
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Richard Hull
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Re: FAQ - Fusor orientation and design

Post by Richard Hull »

There are many ideal situations possible and certainly design situations where you have unlimited cash or facilities or space. I was writing to the average fusioneer working out of a home or a basement or a garage; heated or unheat, dusty or damp.

Most folks rarely have even a satisfactory place much less a controlled work area.
Several fusioneers here live and do fusion in their apartments or rental houses. As such it is good to be very particular about that x-ray port, (view port), beam line!

Pets, especially cats, can climb into and hide in unsecured high voltage areas. Be very mindful of the HV setup if pets and children are about. Best to lock the room or limit access when operating your fusor.

I have attempted to include child proofing in my post via ammending the Safety section as a number of fusioneers might have kids around the house.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
David Nagy
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Re: FAQ - Fusor orientation and design

Post by David Nagy »

Is a pressure transducer necessary or could I use an analog vacuum gauge?
In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin; and this is a knowledge which they cannot lose.

J. Robert Oppenheimer
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Richard Hull
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Re: FAQ - Fusor orientation and design

Post by Richard Hull »

Again, post in the forum, itself and not in a FAQs.

Your vacuum gauge must be able to read to the milli-torr level, regardless of type.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Richard Hull
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Re: #5 FAQ - Fusor orientation and design

Post by Richard Hull »

Back to prominence Dec 2021
Heed this FAQ or design yourself into a corner.

RH
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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