We Have to Try
Mankind is facing a storm. From shrinking water, energy and resources - we have problems. Climate change does not help this. Neither does the population boom. To solve this, we must try some new ideas.
Fusion power would be a tool; like a hammer or a gun. Like them, this tool can have a big impact. Like them, it can be used to help or to hurt us. Our goal is to get this to mankind. But our other hope is that we use it wisely.
This post covers work from Convergent Scientific Incorporated. It has four parts: experimental work, modeling, talk highlights and a conclusion. Experiment details were imperfect. CSI trapped electrons for 20 seconds, using: model one. It is assumed, that this is a wire shaped into a diamond - 14 cm a side with 1,500 amps and held at +500 volts. This was within a ~0.6 pascal vacuum with four emitters at each corner. Model one was cooled with a chilling system. The emitter voltage was varied from -500 to -9,500 volts and a probe measured the resulting trapping. Results were questioned because the emitters remained on the whole test. The magnetic and electric fields are mapped using single wire and point charge models. The excel file is open to the public. The forces are plotted along the face path of the diamond. The resulting motion is described. The effect of sliding or rotating the emitters is explored. Talk highlights are given. These include: the impact of pressurizing, moving, shaping and forming structure within the plasma. The relevant plasma instabilities are mentioned as well as a discussion of structure. Finally a call for experiments is made, with a list of good reference material.
Part 1: Experimental Work:
The team is testing model 1. This is a single tungsten or rhodium wire bent into a diamond shape . Attached to it, is a cooling system, power supply and voltage source. This is placed inside a cylindrical vacuum chamber, about the size of a trash bin . Four electron emitters sit around model one . They may align with the device’s corners. There is also a Langmuir probe. The probe may be a simple wire, or a fancy tool with software. The probe is critical. It proves the concept. If everything works correctly, it should measure a negative voltage. The chamber is also connected to a pump and a gas supply. One possible chamber configuration is shown below.
Inside the chamber is model one. It is the most unique device in the chamber. It is shaped like a diamond, shown below.
This is a single wire. With only one pass, a lot of current will be needed. At full power, 1,500 amps flow through this wire; creating a 1,000 gauss field at the corners . This current, heats up the wire . The team tried to re-snake this many times – but the heat still built up . As you will see, heat is a common problem with model one. Hot wires create problems, like arching. Moreover, this problem grows as the device runs “long-term”. Tests could have been longer - if they could just keep the thing cold!
The first version of model one was a bent copper pipe. Coolant moved inside the pipe, while, current moved in its walls. This failed. The copper overheated. It exploded. The team wants you to know: do not use melting copper. They switched to a tungsten or rhodium wire. Both are very hard materials, with high melting points [12, 13]. Tungsten will be used for modeling . Unfortunately, a wire does not have a cavity for coolant. The team tried an outside ring of coolant - but this behaved poorly . Finally, Model one was merely arranged to touch a chilling system . Heat was conducted away. This is a mediocre solution. Better chillers would allow longer runs.
The chilling system is rather elaborate. The first cooling loop uses a Fluorinert. This is a liquid, often used to cool electronics. The fluid does not conduct; lowering its negative impacts on electric conduction . The fluid moves in a closed loop: from the pump, near the device, and into a heat exchanger. The exchanger moves heat into a second water and glycol loop. This flows into a giant open tank. A sketch and model of the cooling system is shown below . This coolant system can pull about six kilowatts of heat from model one . Estimates (using joule heating) show that this is probably more than they need.
CSI examined three ways to make electrons . The first is field emission. Electrons can spontaneously leave metals in a vacuum. This can happen at room temperature and may have happen inside CSI’s chamber . However this can easily avoided by engineering. The effect amplifies as the temperature rises. This is known thermionic emission. If you heat the wire, more electrons will leave. CSI purposely used four heated nichrome wires to do this. Nichrome is a common emitter . In addition, these wires can be part of a proper electron gun. This was CSI third method . The company altered an e-gun design from the Sydney team . A schematic and picture of their electron gun is shown below .
CSI ran experiments from January to late summer 2012 . Many tests were done. These included: several geometries, various emitters and even a fusor/polywell hybrid. Tests meant several steps. First, the vacuum chamber was prepared. The chamber was filled with helium, to check for leaks. Once sealed, nitrogen was pumped in. Next, they pumped down the chamber. It reached pressures between 1.3 and 0.04 Pascals . The next step is turning on the coolant system. This makes the chamber, low pressure and cool. Next, the voltages are applied. From here the test can start. The device and emitters are turned on. Runs typically lasted for 35 seconds . CSI states that for 20 of those seconds, it measured a steady, constant voltage drop.
The company ran three tests. In each test, the drive voltage was changed. This is the drop between the emitters and device. The device was always at a positive 500 volts; but the four emitters were set at lower voltages. Each time, the probe measured a negative voltage in the center. This meant that electrons were present. Gausses’ law gives a rough estimate of how many electrons (multiply by 5.5E7).
The company would not provide results for the third test (1,500 volts). These results prompt some questions. First, why were the emitters left on throughout the test? How does the company know it measured trapping – and not electrons merely streaming past? There are many good practices that need to be followed here, such as control tests and checking equipment. We must assume CSI abides by these rules. Despite these questions, the work is commendable – it took years to get this data.
Part 2: Modeling
The magnetic and electric fields need to be modeled. They create a Lorentz force which guides the electrons in. CSI gives some of specifications of model one. It lists the plasma volume as 1.4E-3 cubic meters . If this is the total volume, than model one is fourteen centimeters per side. We take the current to be 1,500 amps with a 1,000 gauss field at the corners. The electrons modelled as flying into the face of the diamond. The emitters are 30 centimeters from device center. This is the geometry to simulate.
The motion of a beam is wildly different than one electron. Electrons in a beam interact. They ricochet off one another, vary the surround fields and shift the forces. Hence, beams are modeled differently than one electron [19, 20, 21]. Also, the fields change once electrons fill the device center.
Changing The Geometry:
There was no time to look at the other path into model one. Here, the particles enter through the corners of the diamond. They reach the device sooner, and pass through the biggest magnetic field possible. This occurs at the tiny gap between the two wires. After this, they see a very sharp decline in the field. The sharper field should improve containment . Based on this knowledge, a rough sketch of the force plots is shown below. Between these two paths, there is a sense of the fields inside model one.
Part 3: Talk Highlights
The company’s first talk: physics of IEC devices will not clarify their experiments, like this post. The details listed above were from emails with CSI. The talks cover the issues they have found, after doing the tests. Lets’ assume the tests worked - and CSI can trap electrons. The next question is how to confine them. Here, the tokamak world can help. Plasma has been confined magnetically for many decades. There is plenty of literature that CSI can use.
The rule for magnetic confinement is this: as the machine gets bigger, the confinement gets worse. Nobody understands why this is (in all cases) . But, we have some tips to avoid problems. Devlin mentions four:
1. Pressurize It. The plasma is held in by a magnetic field. The higher the field, the better the hold. This is the approach Lockheed martin is taking .
2. Keep It Moving. It helps to keep plasma moving. Especially on the outside edge. The easiest way to do this is to spin it. You can do this by applying a revolving current. Once, a team stabilized a tiny amount of plasma for weeks this way . Both tokamaks and stellarators spin material for stability . People have also tried collapsing or converging it . You can also oscillate it in a wave, as LANL tried a few years ago .
3. Make Structure. Riders argues that if you merely had a hot blob of plasma (unstructured, thermalized, isentropic, uniform) that you cannot expect net power [29, 30, 31]. Hence, anything you do to move plasma away from a blob - helps. This means forcing any structure, through steep fields or steep density changes inside the cloud . It also means using clouds which are mainly positive or negative.
4. Shape it. We like big plasma volumes - with small skin areas . The best example is the sun. It is a big sphere. It has a tiny surface-area to volume ratio. Hence, the polywell improves as it gets bigger. It also improves if the cusps are pinched off; and the plasma balloons into a sphere .
Structure verses Instabilities:
1. Weibel instability. When a beam enters plasma, there is a chance that it can break up into filaments . This is the due to the fields that it generates during its motion. This has been studied extensively.
2. Diocotron instability. When two sheets of plasma move past one another, eddies form. This represents an energy loss mechanism.
CSI also hints at a structure within the cloud. Specifically: an edge and a core region. This is a bit controversial. Critics would argue the cloud lacks that level of detail. Supporters have opposed this. Now, we have some data. Khachans’ 2013 paper measured electron densities inside the cloud . The results hint at different density in center verses the edge. CSI want these densities to be vastly different, but, so far they have only found a 5 or 10 times difference .
Part 4: Conclusion
Modeling is for wimps:
No model is a substitute for data. This could be an excel worksheet, a matlab program or a vast FORTRAN code. This became clear - in 2012 - when NIF failed to get ignition. Vast teams of experts used models to predict success. Their models were flawed. For example, excel shows that CSI has met a condition for the mirror. Does this mean the device traps? Hell no. Only data can prove trapping. The model only helps you get a sense of the physics.
Theory is also for wimps:
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