what to build ?

Choosing a configuration for an amateur fusion reactor…

Research paths to create a viable fusion device are multiple and involve a wide variety of technologies. For more than a year, I explored on the internet all the possible options, taking into account for each the theoretical physical complexity, the complexity of engineering, the existence of amateur projects already realized or in progress, the existence of open documentation sufficient to acquire expertise, foreseeable material difficulties of realization (mechanical complexity, cost of the materials and necessary materials) … For each of the subjects addressed hereafter, you will find without difficulty an abundant and open numerical literature.

 

1/ Trying a tore magnetic configuration

Without going back to the history of research in the field of fusion, it is clear that an amateur can first build only a small device, and that it takes him quite naturally to the solutions opens a closed confinement loop. But you still have to deal with the major obstacle of designing and building the electromagnets for confinement.

Two main types of machines use toroidal magnetic confinement: stellarators and tokamaks. For both, the plasma is limited to move along the magnetic field lines according to a torus shape. The difference is that stellarators apply a different technique to create the helical (twisted) field configuration needed to counteract plasma drift losses.

In tokamaks, helical confinement is produced by the superposition of a large toroidal magnetic field and a smaller poloidal magnetic field, mainly generated by a toroidal current in the plasma itself. In the stellarator, the entire magnetic field is produced by external electromagnets. The physics of fusion (ignition criteria, Q factors and so on) is basically the same in both cases. The same is true for plasma diagnostic systems and the methods of heating the plasma (with the exception of ohmic heating). The main difference lies in the means of containment, and the effects of it on the plasma.

2.1/ Tokamak

So I explored the path of tokamaks, machines with closed magnetic confinement. However, the means necessary to initiate a plasma and feed the different containment coils are probably beyond what is possible for an amateur. I did not find any trace of an amateur project. You must keep in mind, however, that this is the area where the biggest investments are made, with promising results but huge machines to get closer to a device producing clean energy. The size of the device is a key factor for tokamaks.

2.2/ Stellarator

I also studied in detail the stellarators (with the work of a single amateur – v. Queral – who is however employed by the Spanish national fusion laboratory which supports him with tools and software) to conclude that this type of configuration is undoubtedly very attractive, but comes up against two difficulties: on the one hand, having access to complex computer software that makes it possible to design the complex parts of this type of machine that must conform to the outer limits of a plasma whose shape must be modeled (in particular the coils and their supports as well as the vacuum chamber), and, once this is done, have the material and financial means to produce 3D parts (metal in conventional configurations) particularly complex and having to withstand strong mechanical constraints.

 To get rid of the first obstacle, my first idea was to contact v. Queral to know if it was possible to reproduce the configuration used for his machine UST2. However, it turns out that he does not own the data that allowed him to calculate the geometric shapes that make up his project (which is detailed in the sources of inspiration).

To overcome the second obstacle, I began to explore the possibility of acquiring machines for milling and 3D printing.

Nevertheless, the appeal of this configuration is strong, in particular because of the particular characteristics of this type of machine (which will be developed in detail in the section devoted to theoretical studies): 

  • The implementation of this type of machine is simpler (than tokamaks for example)
  • The peripherals of the machine are standard (vacuum pumping, diagnostics, power supplies)
  • Plasma can exist at any temperature,
  • Long pulses can be obtained, so that diagnostics are easier

From the amateur point of view, if we manage to find a solution to overcome complex calculations to determine and build the forms necessary for the vacuum chamber and the modular coils, the other peripherals necessary for the operation of the machine are feasible: 

  • The power supply part is simple and essentially based on one or more DC sources that can be car batteries.
  • The plasma heating part is based on the use of one or more magnetrons if the option of a cyclotron resonance heater is retained. The associated techniques are very well documented.
  • Diagnostic tools are known and proven
  • The vacuum pumping system is straight forward.

But a simple method to build the coils supports and a very complex vacuum chamber with very hard thermal and mechanical constraints is not so easy to find…

I work hard in this way during 6 months, but finally understand that there is no way to build a stellarator with only amateur means.

2/ IEC (Inertial Electrostatic Confinement)

2.1/ fusor

My very first intend was to join the community of fuser builders, which are inertial electrostatic containment fusion reactors. Simple enough to build, this type of machine is the subject of a large community among the world, with in particular a dedicated forum. However, most of the observed realizations make it possible to obtain a plasma, but few parameters can be modified to go further. In addition, the feasibility for larger machines does not seem to be proved, which limits for me the long-term interest of this path. In addition, the existence of hundreds of prototypes already made does not excite my desire to go to unexplored amateur areas of research. This is how I turned to magnetic containment machines, this approach seeming to be fully open to multiple research on different configurations.

2.2/ Dense Plasma Focus

I came across the site of LPP fusion and experiments on a machine implementing the Dense Plasma Focus. Its essential interest is that it requires few means other than those I already have (a pumping device, a vacuum chamber, a gas supply system) and that the most important equipment to design is a pulsed high voltage power supply based on a network of capacitors. Furthermore, the reactor core has a simple mechanical arrangement based on easy to machine metal parts. However, there are still major difficulties that relate in particular to the high voltage supply, the triggering of the simultaneous switching of the capacitors use of very expensive thyratrons), the insulation of the anode and cathodes, and the probable difficulty of inserting diagnostic tools. In financial terms, the need for a large number of specific and expensive capacitors ans specific triggering thyratrons that can withstand heavy currents is a problem. For the theoretical part, the subject has been extensively studied since the 1960s and seems rather well modeled with tools that are freely available and known scaling laws. It should be noted that the technology used is used for multiple experiments that not only focus on plasmas, but also on the generation of intense beams of ions and electrons, but also X-rays and neutrons. Because of this, this type of machine has been and is used in many universities and laboratories, and the architecture of the whole is rather well described, as well as the equations governing its operation. Clearly, achieving tangible results in this type of machine is conditioned by the cost and building of a DC electrical storage power, and the prospects for use for a sustained fusion reaction are not obvious.

2.3/ The polywell concept

I finally explored the concept of the polywell, with the detailed study of mark Suppes amateur work, very structured research on the subject of the University of Sydney and some other amateur attempts. For once, the realization of an amateur device seems feasible, but still faces the enormous difficulty of building a complex structure based on hollow metal rings that can contain solenoid coils while providing sufficient mechanical strength. Moreover, the latest work on this subject has shown that a viable implementation of the Polywell passes through a device for injecting a plasma (the plasma guns are not simple to design and achieve), by the presence of several electron guns in a vacuum chamber of restricted dimensions, and finally the use of materials and power supplies to generate intense magnetic fields in the coils (superconducting wire is overpriced for an amateur). My research on the Internet has shown that no amateur project has gone to term, quickly colliding with a prohibitive cost, for example to use superconducting coils.

Despite the great difficulties described, i m still considering the path of Magnetic Electrostatic Palsma Confinment devices as the most attractive way for an amateur project. But in order to go as far as possible in my dreams, i choose to build first all the system that allow to operate a vacuum chamber with all the required peripherals for gas feed, diagnostics, power supplies, cooling and vacuum, and this is a great challenge to face before deciding what kind of device installing in the vessel…