High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Michael Bretti
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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Fri Feb 16, 2018 5:02 am

BUILD LOG UPDATE #01

Roughing Line Assembly

Now that all of the major calculations have been completed, besides some additional CAD work, organizing drawing sheets for the full system, and thermal modelling, the system can finally start being pieced together. While the calculations are very enlightening, actually building the system is where the real fun begins. It really is thrilling to see a high tech system slowly come together after months of planning, especially when it comes to life exactly as designed in CAD.

Below are all of the parts for the roughing line, laid out on a sheet of clean, coating free aluminum foil. All of the parts were handled with clean nitrile gloves and thoroughly wiped down with 99.9% pure anhydrous isopropyl alcohol. While this is not really needed for the low vacuum side, it never hurts to get into the habit of proper practices and procedures for handling high vacuum equipment.

20180210_185125.jpg

The foreline trap needed to be disassembled and cleaned prior to use. The trap material, some form of zeolite or alumina pellets, were already well contaminated and spent. They can be seen as small purple pellets in a plastic bag at the top of the picture below. The inside of the trap along with the stainless steel basket was thoroughly cleaned spotless with the alcohol. I did not refill the trap yet with new adsorbent since I would be qualifying the roughing line first and calibrating the thermocouple gauge on a different oil-free pumping station, so opening up the container and absorbing moisture during qualification and calibration runs is unnecessary for now.

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After all of the parts were cleaned, the roughing line was assembled. The side on the left with the 90 degree manual valve goes to the diffusion pump roughing inlet, while the side on the right with the foreline trap goes to the backing pump.

20180210_195510.jpg

Finally, the adapters for the roughing pump were assembled to the pump. The pump inlet is a 3/8" male flare fitting. Several adapters are needed to go from this to the KF25 to 1/2" female NPT adapter. I decided to keep the adapters equal to or larger than the original adapter on the pump to keep pumping speed high. A 3/8" female flare to 1/2" male flare adapter connects directly to the pump fitting, followed by a 1/2" female flare to 1/2" male NPT, which goes directly to the KF25 adapter. Teflon tape was used for the 1/2" male NPT, but not for any of the flare fittings. The pump also includes an additional small 1/4" male flare fitting off of the side of the main 3/8" flare, which I replaced the provided plastic cap with the appropriate 1/4" brass flare cap.

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With the foreline components assembled, the next phase would be to qualify the vacuum integrity of the line and calibrate the thermocouple gauge sensor. This has already been accomplished in the past couple of days, and I will go over the calibration procedure and data in the following posts.

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Richard Hull
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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Richard Hull » Fri Feb 16, 2018 9:12 am

Looks like my system on fusor IV. Short connections, pump to TC gauge, fore line valve and Diff pump. You bought well.

I wish others would start out this way. Pump down of the fore line should be near instantaneous. The diff pump will also be quick.

My system can hit fore line plus the diff pump to 20 torr in about 3 minutes. Once the fusor chamber valve is opened it takes about another 5 minutes to hit 15 microns throughout the entire vacuum system. I turn on the diff pump heater once the system crosses the 50 torr mark. The boiler takes over 15 minutes to start pumping. By that time, the system is at about 12 microns due to the mechanical pump.

Your stuff and assembly shows proper fore thought gained through study. Most newbies here have to blunder into these realizations.

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
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Michael Bretti
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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Fri Feb 16, 2018 9:30 pm

Richard Hull,

Thank you for your comments and sharing your experience with your system. I ran the final qualifying test today with the roughing system, and I am very pleased with the results, and even more excited to continue now. Having gone through many prior posts and seeing how many issues others have had pulling a vacuum on their system with KF and even CF hardware (which is designed to not leak even remotely close to fusor pressures), I was honestly worried that this would be a massive struggle. However, so far everything has progressed very smoothly without any issues yet and has been significantly better than I was expecting. I would say that doing thorough research ahead of time, working very slowly and methodically, planning each step, and breaking the whole thing down into smaller subsystems has already proven to be massively beneficial. Troubleshooting during qualifying runs took about a minute or two for the roughing side.

For those who are starting out, I would absolutely recommend to qualify each subsystem in order. Don't bother putting together your high vacuum side until you build the roughing side and qualify the vacuum integrity of the line, calibrate your roughing sensor, and establish your minimum forepressure. It's very tempting to rush ahead and start bolting nice and shiny high vacuum hardware together and try running a fusor as soon as you get all the parts, but there is really not much point until you can properly back the system first. This will eliminate issues with a major part of the whole system, and make troubleshooting easier as you progress. Even better if you can CAD your system out first, as it will help figure out the optimum orientation and allow you to plan, build, and change things without the need to spend money.

After today's test, I was able to qualify that the roughing line and the roughing pump were able to achieve a minimum stable pressure of about 12.5 microns, which is much better than I was anticipating, as my initial goal was to shoot for 20 microns. The pressure shot down to the several tens of micron range very fast, and leveled out after about 5-10 minutes of pumping to its ultimate pressure. I vented the system and re-pumped it back several times, and I got the same numbers consistently. The foreline itself was qualified to even lower pressures prior during gauge calibration. One thing that I will need to improve however is minimizing vibrations of the roughing line due to it being so short and directly connected to the pump, which may damage the thermocouple gauge in the long run, but I have located the largest source of physical vibration in the line (the foreline trap) and have a plan for reducing system vibrations. I will post the calibration procedure and curves for my gauge sometime soon.
Last edited by Michael Bretti on Sat Feb 17, 2018 7:42 pm, edited 1 time in total.

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Fri Feb 16, 2018 10:58 pm

BUILD LOG UPDATE #02

Roughing Line Testing, Thermocouple Gauge Calibration, and Roughing Pump Qualification

With the roughing line complete, I was able to run tests on its vacuum integrity as well as calibrate the thermocouple gauge sensor. I performed these tests earlier in the week, and finished the last bit of testing today.

The roughing line and thermocouple gauge were qualified using a scroll-pump backed calibrated Varian helium leak checker pumping station. First, the roughing line was tested. The roughing isolation valve was closed, and the line connected directly to the system's KF25 inlet, right to the foreline trap, where the backing pump would normally be connected. The system was turned on and allowed to go through its start-up sequence. The line was then pumped down, and achieved an ultimate vacuum of 2 x 10^-3 Torr, or 2 microns, which is about where the pumping station is rated at. No adjustment or tightening of the components in the line was needed. The foreline trap was also not filled with fresh molecular sieve during this phase of testing since the scroll pump is a dry, oil-free pump, so contamination of the thermocouple gauge from back-streamed oil is not present.

Next, the thermocouple gauge was calibrated against the leak checker. The thermocouple gauge is a VGT-1504 from LDS Vacuum. The gauge was connected to a very simple 0.22VAC source (several step down transformers controlled by a variac), and the output was measured directly by a sensitive enough voltmeter that could read down into the sub-millivolt levels. The line was first pumped to its ultimate vacuum of 2 microns with the thermocouple gauge off. Next, the gauge filament was slowly brought up to full voltage and allowed to stabilize for about 5-10 minutes. Using the isolation valve at the end of the roughing line, I slowly introduced very steady and controlled leaks into the line. This took a bit of fine control and practice, very slightly tapping the handle to introduce ever increasing wisps of air into the system, allowing the leak checker and thermocouple gauge to stabilize at each reading. The gauge was calibrated against the leak checker from 2 microns up to 700 microns (7 x 10^-1 Torr), where the signal from the gauge cut out. Below is a PDF of the calibration and plots of the calibration curve across its measured range:


Note that this curve is valid only for this particular gauge, since equivalent gauges will still vary. However, since thermocouple gauges are not the most accurate to begin with, these numbers may provide a rough guideline for others using equivalent gauges, though the gauge should still be properly calibrated against a known measurement source for more correct accuracy.

Once the gauge was tested, the system was evacuated and pumped down several times to reconfirm the numbers, which seemed to track with good repeat-ability. Next was to qualify the whole line attached to the vacuum pump. The roughing line was connected to the pumping station at the isolation valve, and the roughing pump was connected to its proper location under the foreline trap. The roughing pump has a built in isolation valve, which was closed during this next test. The pump also was not filled with oil. With this test, I could qualify the ultimate vacuum of the entire line, the brass adapters, and the isolation valve of the roughing pump. At first, I pumped the system down and got a pressure of 160 microns. However, since I already qualified the whole vacuum line prior, I knew that the problem could only lie either in the brass fittings or the roughing pump isolation valve. Sure enough, as soon as I tightened the extra 1/4" flare cap port first, the pressure dropped down almost immediately to 16 microns. I then tightened each of the brass fittings further on the main line, which dropped the ultimate achievable vacuum to 6 microns. The thermocouple gauge was also turned on and read to check the calibration numbers against the new readings.

Once everything from the roughing pump isolation valve to the roughing line isolation valve were qualified, I could finally test the ultimate vacuum of the roughing line using the actual roughing pump. The line was disconnected from the leak checker pumping station and tightly closed. The roughing pump was then filled with oil, and the trap was filled with fresh sieve material. The pump was turned on, and pulled down to its minimum pressure shown by the built in dial gauge immediately. The thermocouple gauge was turned on and allowed to stabilize for another 5 minutes, then readings were taken. After about 5-10 minutes of pumping, the pump was able to achieve an ultimate pressure of about 12.5 microns, which corresponds to an output voltage of this sensor at 12.1mV. Since my initial target was 20 microns, I was extremely pleased with the results. This ended up being much better than I was anticipating. The line was vented and re-pumped several times to verify the results.

Now that the roughing system is well qualified, I can set this aside and work on the next subsystem, which will be the cooling system. This will consist of three separate cooling loops - a large, air cooled heat exchanger and water based line for the diffusion pump, and two smaller loops chilled with an insulated peltier cooling array for the water cooled baffle and the titanium sublimation pump, using some sort of coolant such as propylene glycol. The target temperature of the diff loop is 20C or better, while the two smaller loops for the baffle and sublimation pump are 0C or lower. Cooling flow, pump speeds, temperature, and interlocking will all be automatically controlled and monitored. The goal is to integrate this and other systems into a semi-automated pumpdown sequence and develop a full user interface for all of the subsystems for very precise monitoring and control of the whole system

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Mon Mar 05, 2018 5:50 pm

DOCUMENTATION UPDATE AND NEW THERMAL ANALYSIS

Just wanted to post a quick update regarding the efforts on the high vacuum system build. The efforts presented here so far are not just for a single project, but an ongoing research effort over multiple projects that will span years. As a result, there will be an incredible amount of additional documentation to cover. As is, from just my walkthrough posts alone, I have generated the equivalent of about 30 pages of Word documentation on explaining the calculations alone, in addition to close to 100 pages of total calculations. This is not including CAD models, drawing sheets, and simulation data. This is also only for a single vacuum system, which has not yet touched power supply design, cooling systems, instrumentation, and control. There will be even more when I start getting into ion beam design and other vacuum setups. Because of this, I have been working on migrating and expanding these efforts for a while now beyond just the initial scope presented, and will be focusing on a full initiative for creating open-source engineering and documentation for high vacuum and ion beam systems, centered around a hobbyist-budget approach. I have been slowly organizing and setting up these resources on a new site, which can be found here:

http://appliedionsystems.com/

I will still be posting updates on the progress of builds here, but the posts will be much briefer than prior posts in this thread due to the large amount of data and resources associated with each project. In the long run, having a dedicated site for documentation and resources will make it much easier to manage all of this information. The focus is also more on high vacuum and ion beam system engineering rather than strictly fusor efforts, although the fusor will be covered as I get to it.

As an additional update directly related to prior posts in this thread, I have just completed the first thermal modeling simulations for the system. The first series of simulations goes into modeling the steady-state thermal characteristics of the Edwards EO4 diffusion pump. This is a simplified model to give me initial data to start designing the temperature-controlled cooling system for the diffusion pump. The vapor jet stack is omitted from the model for simplicity. However, the proper fill of 175mL of DC-705 oil was included into the model with the proper thermal characteristics of the oil. The simulations are also run using convection coefficients for the oil as well as the internal surfaces for rough vacuum conditions to compensate for the model being static instead of simulating the dynamic properties of the oil spraying against the walls, condensing, and contributing to oil cooling. Based on the comparison between the uncooled vs. cooled diffusion pump models, the discrepancy between average oil temperature from simulated to expected is accounted for. The full summary can be found here:

http://appliedionsystems.com/portfolio/ ... sion-pump/

The second simulation was to look at the steady-state thermal characteristics of the water-cooled baffle and adapter-plate subsystem to be used in conjunction with the diffusion pump. This looks at a comparison of the thermal analysis of an uncooled vs. cooled water baffle while the pump is operating with proper cooling to determine what will be needed for the temperature controlled chilled baffle cooling loop. Details of the study are presented here:

http://appliedionsystems.com/portfolio/ ... ed-baffle/

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Sat Apr 07, 2018 4:07 am

Another update on the status of the build progress so far. Since all of the vacuum calculations have been complete I have been focusing primarily on the thermal management system planning, and recently in the past two weeks I have been preparing to run simulations looking at the sputtering damage and ion implantation effects of a diffuse plasma on stainless steel surfaces during glow discharge cleaning, which I will post more information later in the advanced topics section.

As mentioned above, thermal modeling simulations have been completed for the two major thermal loads of the V4 system: the EO4 diffusion pump, and water cooled baffle. These simulations are used in the planning of the PID controlled closed loop cooling system. Since I have the manual for the diff pump, as well as the diff pump itself, I know just about all of the major parameters needed to model and simulate the thermal effects of running the pump and cooling. An extensive search through literature yielded the necessary thermal parameters needed to input into the thermal simulation software for various heat loads in the system, both for outside atmospheric conditions as well as vacuum conditions, and average values are used. The model was simulated without the vapor stack for simplicity, but the heater, internal structure, and proper fill of 175mL of DC-705 oil was modeled, with all of its thermal and physical characteristics. Since the thermal modeling software used can only simulate steady-state static models, the pump was modeled both without cooling and with proper cooling to determine the proper thermal gradients and effects of cooling. Fusion 360 was used for the modeling. Below are a couple of the resulting images:

Edwards EO4 Diffusion Pump Thermal Modeling - DC 705, No Cooling, Internal View, Legend.JPG
Uncooled Diffusion Pump

Edwards EO4 Diffusion Pump Thermal Modeling - DC 705, Water Cooling, Internal View, Legend.JPG
Diffusion Pump Cooled with 25C Water


All of the input parameters and full details can be found here: http://appliedionsystems.com/portfolio/ ... sion-pump/

A few interesting things to note:
1.) The heat shield, despite its simplicity, contributes to a large degree of thermal shielding, and illustrates its importance.
2.) The body of the diffusion pump, both in uncooled and cooled conditions, does not directly affect the oil temperature itself through thermal conduction through the bulk mass of the walls down to the oil pool. Based on the results, for the 850W power input of applied heating to the heater element results in an average temperature of the DC-705 of 291C, which is higher than the target operating temperature of 245C. The average temperature of the casing of the body, with the pump uncooled, rises to a steady state value of 118.5C. With the recommended max temperature of 25C of water cooling, the average temperature for the oil stays the same - however, the body drops to an average temperature of 50C. Due to the effects of the oil being cyclically evaporated, sprayed, and cooled along the surface of the casing, condensing back down into the oil pool and restarting the cycle, oil temperature is therefore controlled. The difference in temperature between the body casing of uncooled vs. cooled conditions and average oil temperature between simulation and expected nominal temperature therefore accounts for this effect.

The next thermal simulation looks at the effect of cooling on the baffle above the pump, accounting for thermal conduction loads while the pump is running. The simulation was run with the above diffusion pump input parameters, in a steady state cooled condition, looking at both the thermal gradients of the uncooled baffle and adapter plates, and cooling the baffle with a constant flow of 15C chilled water. Below are cross-sectional views of the baffle and planned aluminum adapter plates:

Water Cooled Baffle Thermal Modeling - Uncooled Cross-Sectional View.JPG
Uncooled Baffle Assembly

Water Cooled Baffle Thermal Modeling - Cooled 15C Cross-Sectional View.JPG
Baffle Assembly Cooled with 15C Water


The normalized results comparing the two scenarios is shown below:

Water Cooled Baffle Thermal Modeling - Uncooled vs Cooled 15C, Side by Side, Normalized Gradient.JPG

Full details of the simulation and all input parameters can be found here: http://appliedionsystems.com/portfolio/ ... ed-baffle/

Without any cooling on the baffle, with the diffusion pump running at nominal conditions, the average baffle temperature rises to 32.68C. With a flow of 15C water in the baffle cooling channels, the average temperature is reduced to 20.05C. The ideal goal is to run the baffle with coolant chilled to at least 0C, preferably lower, with 15C as the maximum coolant temperature. These simulations establish that cooling of the baffle with at least 15C water will allow for an average temperature of 20C, plenty enough for the baffle to be effective. The manual for the diff pump states that with 15C water cooling of the baffle and pump, and all metal gaskets, with the proper oil, the system can reach an ultimate pressure in the 10^-10 Torr region, without the need for cryo trapping. Even with o-rings in the system, which I am looking to add differential pumping along with the chilled baffle, ultimate pressures based on prior calculations in the 10^-7 Torr range and potentially lower are reasonable to expect.

As described before, there will be two separate cooling loops for the system: one for the pump, and one for the baffle. Each system will be a triple-loop, closed loop PID controlled chiller based off of Peltier cooled heat exchangers. Primary fan cooled heat exchangers will remove all heat from the loads before re-entering the coolant tank, which will be continuously chilled with recirculating coolant between the Peltier exchangers and the tank. The system will be automatically monitored and controlled during runs to give precise system control, and interlocked to the main and diffusion pump power as well, and will be part of the (hopefully) automated pumpdown sequence. The peltier chillers, pumps, chiller exchangers, and power supplies have been acquired. The tanks and main exchangers still need to be obtained. All cold areas will be thermally insulated with proper material and keeping lines as short as possible to minimize heat transfer from the cold sections to ambient air. The above simulations will be verified using a high-sensitivity biomedical thermal imaging camera that I acquired several years ago for free. More details to follow as the build progresses.

Michael Bretti
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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Tue May 08, 2018 2:49 am

After a couple of months of additional project documentation, CAD, thermal simulations, and just recently, advanced plasma simulations using TRIM, I have had the opportunity to work on some physical assembly. I started to assemble the main chamber and pipeline of the V4 system. An overview with a brief write-up as well as resources for where parts were obtained can be found here:

http://appliedionsystems.com/high-vacuu ... -assembly/

Below are some pictures of the initial assembly. Everything was assembled on a clean aluminum foil surface, with ports cleaned with 99.9% isopropyl alcohol and Kimwipes, and everything handled with clean nitrile gloves:

2,75in conflat blanks.jpg

High Vacuum System V4 Build Pic 1.jpg

High Vacuum System V4 Build Pic 2.jpg

High Vacuum System V4 Build Pic 3.jpg

High Vacuum System V4 Build Pic 4.jpg

High Vacuum System V4 Build Pic 6.jpg

High Vacuum System V4 Build Pic 5.jpg

High Vacuum System V4 Build Pic 7.jpg

The only deviation from the original CAD model so far was the decision to have the manual gate-valve be angled towards the back as opposed to the front. A few of the ports will be kept covered with the aluminum foil while I decide whether or not I have the resources and funding available to build and install an anticipated Faraday cup and high-power pulsed electron gun. These will be installed in the two upper ports on the main 5-way cross chamber for my first experiments (I won't need a gas handling supply for this setup, and have several experiments planned for the intense pulsed e-beam and associated x-ray production.) Chances are however I will just blank these ports off and focus on baking, conditioning, pumpdown, and the automated control system programming for now. I will wait to install the HPT-100 wide range vacuum transducer on the lower 4-way cross port until the full system is mounted to the pump and support structure to prevent damage to the sensor.

Next on the list will be to get the adapter plates machined, bolt the chamber to the diff pump, design the housing, and build/mount the chamber and pump to the housing. In parallel with these efforts, I will be working on finalizing the design and starting the build for the cooling system. I already have most of the parts for the diff pump chiller. Efforts will also be made to start writing code for automated pumpdown control and remote monitoring. I am also considering working on developing automated control and monitoring via wireless through a phone app. However, remote computer control and monitoring will be the primary focus for this subsystem for now.

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Dennis P Brown » Tue May 08, 2018 4:29 pm

Outstanding work to date; I really look forward to your neutron detection system build - or will you just buy a turn key system?

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Michael Bretti » Tue May 08, 2018 5:34 pm

Dennis P Brown,

Thank you for your compliment. While this system is certainly not the fanciest setup here, my goal is to utilize a simple and compact topology to create a fully engineered and modular test platform with precise control over every parameter within reason. It takes much longer and requires a large investment in time, planning, and research, but the end results should be well worth the effort. I would rather have precise experimental control than large neutron yields.

I am also particularly looking forward to when I get to neutron detection and instrumentation. At this point I would say that I would be looking to build my own detection system as opposed to purchasing a full turn key solution. I already have a bunch of ideas I would like to implement for multi-detector arrays, as well as potentially playing with neutron collimation. I will be taking the same engineering design and approach towards that build as well, though I don't foresee it happening for a quite a while unless I can secure some additional sources of external funding to speed up the process. Since I will also be working primarily with fast-pulsed systems (high peak power pulsing in the nanosecond time scale), detection will provide some interesting challenges to tackle as well.

My main focus will be pulsed beam-on-target, but I will also dabble with pulsed fusors. In a textbook I have on ion beam systems, there is a design for an ion gun that only requires a kW of input power and generates D2 beams up to several tens of mA continuous, which could certainly yield some high counts, or be excellent for target pre-loading. My current system should allow for two ion gun inputs with a single target. Pulsing a fusor at voltages in excess of 50-100kV should also be relatively straightforward in terms of the pulser design (as pulsed power is one of my areas of expertise in engineering), and should be able to generate peak currents in the range of amps to 10s of amps for short durations, (tens to hundreds of nanoseconds), with only hundreds of watts of input (one of the great benefits of energy compression in pulsed power).

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Re: High Vacuum Engineering Design, Analysis, and Build of a Small-Scale Multipurpose System

Post by Richard Hull » Tue May 08, 2018 5:50 pm

I am sure you are aware of the pitfalls related to pulsed fusion detection. I call this putt-putt boat fusion. Those multi-ampere pulses are massive noise generators making electronic neutron detection nightmarish. Activation analysis may have to be the sole arbiter in such systems.

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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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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