Wednesday, June 7, 2017

Rotamaks: Yet Another Ignored Fusion Technology

Note: This article was written between March and May of 2017.  My goal here is to communicate as simply and as accurately as I can.  For this post, I did some fact checking with folks inside Dr. Cohens’ group at Princeton.  Still, it is not perfect.  But, nothing ever is.  If we want to see this field develop, the US needs to fully fund this research.  Make no mistake: we are leading on this.  Enjoy.


In the late nineties, Sam Cohen had a big problem.  On paper, his life looked great.  He was living in Germany and supervising a large portion of the US ITER effort.  It was a big job [3].  Sam drove a fancy car.  He had a nice salary.  But Sam was having a personal crisis.

ITER was flawed.  There were problems; big problems.  Livermore had estimated that the core would be 60 times more massive than a common fission core [2].  That is a problem – and that was just the core.  A five-fold obstacle is something an engineer can manage.  But sixty-fold setbacks?  Those problems kill the machines’ commercial chances. Sam knew this - and it dug at him. 

Sam Cohen was faced with a choice.  It was a choice many researchers have to face.  Stick with ITER - or go off on his own.  For people that want to make fusion work, this is an all too common situation.  They want to make fusion work – but if they question the tokamak approach, they get pushed out.

Dr. Cohen decided to follow a new path. The technology he settled on was the Field Reversed Configuration.  This was not a new idea.  Over 40 of these machines have been built over the past five decades [29, 54, 41].  But Sam was adding in a twist: a new rotating magnetic field.  Sam was building a Rotamak.

A Rotamak is a structure built out of plasma and maintained by a rotating magnetic field.  This past year, Sams’ team announced the world record for the longest stable FRC, ever created by mankind [20, 55 - 62].  This is what drew me to his lab at Princeton in the spring of 2017.  But to understand Sams’ story, we need to understand all the parts involved.

Part 1: Rotating Magnetic Field

A Light Electron, A Heavy Ion

Using a rotating magnetic field on a plasma was first suggested by Henry Blevin, in 1962 [50].  He was very clever.  Blevin was trying to exploit the differences the between ions and electrons.  Each are very different.  Ions are massive.  Deuterium is at least 3,600 times more massive than the electron [47, 48].  Electrons are light weight.  This difference yields very distinct behaviors.  Electrons will spin very fast.  They move in quick, tight orbits. Because of their charge difference, ions and electrons spin in opposing directions [51].  This is shown below.

Blevin found a way to move just the electrons, but not the ions.  He realized – that under the right conditions – he could whip the light electrons around while the heavy ions would stay unchanged.  This was like wind blowing on leaf covered rocks.  The leaves can be picked up by the breeze.  But the rocks would stay put.  They are too heavy.  The electrons are like leaves; the magnetic field can move them.  But, the ions will not move.  This is a great technique.  But this would only work under the right conditions.

Getting It Right

There are countless ways you can configure a plasma.  You can mess with the: electric fields, magnetic fields, induced fields, geometries, charge concentrations, density, etc.  There are a seemingly endless number of ways to set up a system.  Blevin wanted a system where he could just heat the electrons.  Such a scheme had to have three traits.  First, it had to be collisionless [27, 50].  He did not want the spinning electrons to hit the ions.  This would screw up his system.  This is counter intuitive.  Normally, fusion plasmas like collisions [52].  To make a plasma collisionless, you must heat it [49].  Hot stuff expands.  It creates more space for things to move around.  Ideally, you want the distance a particle can move without hitting something to be a 1,000 times higher than a characteristic [27].  A characteristic distance might be the length of the machine.  This math is complicated, but I have pulled some equations for this below [27, 49, 63, 64].  

The second requirement for this effect is that it happen in a cylindrical shape.  That makes sense.  The poles of the magnetic field need to be able to encircle the plasma.  The best way to understand this, is to see it in a picture.  Below is an example of plasma motion. Plasma motion is dictated by the Lorentz force [11].  Predicting motion of a single particle is tricky – it depends on the starting velocity and the presence of electric and magnetic fields.  This example is exceedingly simple, there is only the plasma and a rotating magnetic field.  Real life plasma is never so straightforward.

The north and south magnetic poles rest above and below the plasma.  Single particles sit in the middle [12, 13].  Now the magnetic field rotates.  The ions are unaffected - they are too heavy to move [50].  But, the electron starts to move. As the magnetic field moves around it, the electron rotates.  This field whips up the electrons.  They move faster.  The field is transferring energy to the electrons, raising the temperature, speed and kinetic energy.  Over time, electrons flow inside the plasma in a loop.  This is a current inside a plasma.  That pulls the ions into a loop with the electrons.  A plasma loop has formed.  The third and final thing needed to get this effect is the right spinning seed.  Too fast and it will be powerful enough to move the ions – like a hurricane picking up small stones.  Too slow and it will not affect the electrons.  Effectively, this meant that the magnetic field must spin at the right frequency.  Blevin reasoned that frequency should be higher than the frequency of the spinning ions, but lower than that of the electrons.  This is plotted out below for deuterium plasma at different field strengths [47, 48, 52, 53, 50].  

If done correctly, the effect will heat the electrons without touching the ions.  That is awesome.  That makes things simple.  Blevin knew this; and he had already schemed about how you could use it. He knew that the technique would be most helpful in creating plasma loop [50].  Indeed, his 1962 paper closes with this sentence: “By combining the magnetic field of plasmas rotating about axes … a variety of self-excited dynamos can be imagined.”  Blevin was right.

Part 2: Plasma Structures

Plasma Loops

Plasmas' ability to self-organize comes from the fact that it conducts electricity.   Plasma is a conductive soup of moving charges.  All moving charge can make its’ own magnetic field [26].  You can exploit this, to build semi-stable structures out of plasma.  Many approaches to fusion partially rely on this effect (General Fusion and ITER for example).  But, there are only two that fully rely on structures in plasma.  These are: the spheromak and the field reversed configuration.  Both are basically smoke rings of charged plasma.  Loops.  Spinning donuts of charged stuff - held in free space – by magnetic fields.  This is shown below [65].  You can see that this spinning makes a magnetic field, which self contains the material itself.

Both the Spheromak and the Field Reversed Configuration are loops like this.  But, there is a slight difference between them.   It is a small difference.  The Spheromak has one extra field, running around the outside.  It is a toroidal B-field that can run clockwise or counterclockwise [56].  In fact, the structures are so close together that some devices can switch between making one, the other, or both [56]. These are compared below. 

Note the shapes shown.  These loops do not have holes in the center.  The material spins right up inside the center.  The shapes can vary.  They have been described as jelly donuts, tuna cans and Christmas tree ornaments.  When the shape warps, distorts or otherwise twists – it is a sign the structure is falling apart [85].  It means that deadly instabilities are coming from the edge or the center to kill it [41, 3].  Both structures are nested inside a larger plasma environment. Outside, there is a normal field running from top to bottom, with stuff swirling around it.  But on the inside, everything is flipped. Material spins in the opposing direction, making an inside field. This inside fields runs counter to its’ surroundings. These nested structures are reversed from one another.  This is why this structure was first called a: “Field Reversed Configuration.”


The Field Reversed Configuration was seen first.  This was by accident [30].  In in 1960, researchers in England were working on a fusion pinch.  The pinch was all the rage in the early days of fusion research.  Pinches happen when current races down a tube of plasma. This makes a sharp inward field which crushes the ions in the center [66]. As researchers explored this parameter space, they made a weird observation. For a brief moment, the plasma was oddly stable.   As they explored this further, they found the ions and electrons were forming their own structures [30].  In the 57 years since their discovery, over forty machines have been built to explore both FRCs and Spheromaks [41, 29]. Below is some typical details on these machines [3, 20, 55 – 62,].  Note the annual amount of money spent on this research.   


Why are plasma structures so great?  The question should really be rephrased: what are the advantages IF these approaches end up becoming a power plant?  It turns out that plasma structures like the Spheromak and the Field Reversed Configuration could have three distinct advantages.  First, the plasma inside the FRC is hotter than its’ surroundings [40].  This is critical: in fusion, heat helps [67].  More heat means that anytime two ions collide, they are more likely to fuse.  Secondly, the surrounding machine is simpler than a tokamak.  By making their own fields, plasma structures need smaller external magnets then a tokamak.  And unlike a tokamak, they do not require an inner column [56].  In a sense, these approaches shift the complexity away from the engineering - and into the plasma itself.  The third advantage is that these structures could be more efficient than a tokamak.  They could better hold on to both mass and energy.


 To see how plasma structures stack up (efficiency-wise) to other fusion approaches - you need to grasp a few problems that effect fusion.  To understand this better, I have put together a quick chart of these effects and families of approaches.

On the left is a list of effects that can hurt each approach.  These are physical effects with plague fusion.  They kill machine efficiency by leaking mass and energy.  Mass loss is synonymous with energy loss - stolen particles steal heat out of system with their exit.  The first the loss is through the walls.  Metal conducts plasma.  If plasma touches it, it is gone.  I have seen three clever ways to beat this problem. All approaches try to put space between their plasma, and the walls.  They also try to use dipole magnetic fields to shield metal parts in plasma.  Finally people have built their walls out of Pyrex (glass), which conducts less [76, 77].  Nothing is perfect – in all cases conduction still finds a way to sneak in and steal mass away.  Ideally, a tokamak has a gap between its wall and plasma – but stuff still finds a way touch the walls [78].  Plasma structures handle this problem a bit better.  They are surrounded by more plasma.  Since these structures do not touch walls directly, it mitigates this problem.  Finally, cusp confinement approaches like Lockheed or the Polywell could go either way [77, 79].  Ideally, their plasmas hit a high pressure surface when it tries to escape [80].  This could be an awesome barrier, removing wall losses. At the same time, cusp systems need big metal magnets, very close to the plasma.  We do not know.  The human race has barely test a this kind of trap - so it is too early to judge cusp confinement fairly.  Magnetized and idealized cusp confined plasma are compared below [78, 79, 83, 84].

Efficiency – Scattering & Radiation

The next problem in this chart is leaking through a curved field.  In curved fields, the plasma wants to moves into the outside fields [78].  Plasma likes to move towards the broader, wider fields, on the fringes.  Eventually it is flung outwards – striking the walls.  Because they both rely on curved fields - both tokamaks and plasma structures will have this problem [78, 25].  By contrast, the ideal cusp confinement system will have fields that are bent inwards.  This pushes material into the center – lowering this loss problem.   This is such a big problem that we designed an entirely new machine to deal with this: the Stellarator.  A Stellarator is a twisted tokamak [81].  With each twist, it tries to push material back into the center.  Again, the goal here is to better hold in the plasma.  The last problem is energy lost as light.  All fusion plasmas bleed energy away as light.  This comes out as IR, UV, X-Rays and in the visible spectrum.  Energy loss as light becomes worse, in situations where the plasma mixes with an external magnetic field [82].  Particles are deflected by these fields and when they do they lose energy as light.  Because they rely on external magnetic fields, this problem should be more pronounced in tokamaks and spherical tokamaks.  By contrast, plasma structures and cusp confined plasma have spots where the external magnetic does not enter the inner plasma [78, 79, 83, 84].  This lowers the fields in the plasma; which lowers radiation losses.  It would never be zero – plasma has self-made fields that cannot be escaped.  These fields cause deflections, making light rays, which bleed energy away.  Lowering radiation is one of the most unexplored directions in fusion.  If we can lower these losses, we could have a more efficient power plant. 


Though they have many advantages, plasma structures do have a few problems.  First and foremost, they are tricky to make.  A loop of plasma can arise in many different ways [33 - 46].  In the figure below, I have drawn out the four methods to make a Field Reversed Configuration [56, 30].

The first method is pretty straightforward.  A beam of material is shot into some plasma.  As these beams of particles pass by, they “kick up” spinning loops of plasma.  Think of a boat passing by and leaving a wave behind it – that is a great analogy for what is happening.  These are Field Reversed Configurations.  Sometimes this is accidental.  For example, tokamaks use beams to heat their plasma.  This kind of loop in a tokamak would be unintended.  The second formation method is with a pinch [56].  Specifically: a        theta pinch.  A theta pinch is when a magnetic field runs down a plasma column and current spins around the outside [72].  That spinning current can pull plasma along - making a plasma loop [1].  Incidentally, pinch researchers found that if they “turn up” the spinning current, they can convert an FRC into a spheromak [56].  This is why one machine (a theta pinch) can toggle between making either structure.  The last two methods are: colliding spheromaks and the rotating magnetic field [37].  One of these methods is currently being pursued by the worlds’ largest fusion startup [86].

Colliding Spheromaks:

In the foothills of Los Angeles, there is company known as Tri Alpha Energy.  This is a fast moving startup.  The company’s goal is to try and change everything about the energy world.  Tri Alpha has raised over half a billion dollars to build a fusion power plant [73].  At the heart of their approach is an FRC made by colliding spheromaks [68].  The company is extending work in this field which stretches back to the mid-eighties [17, 24, 25].  It knows it must get the delicate process of making these plasma structures right.  They have to generate two spheromaks, move them together, merge them, stabilize them and then keep them spinning.  This fragile operation is shown below and modelled here [37].

Using metal rings, you setup fields which oppose each another. Then you release plasma. The field pivots. It switches from being made by the metal rings; to being made by the plasma donuts. These are the spheromaks.  They can then be moved by varying magnetic fields around them [29].  These fields guide the spheromaks into the center, where they merge, forming a new structure.  Next, the company tries to hold the structure by keeping it spinning.  This is done by firing particle beams along its’ edge [28].  This is like kicking the loop, to keep it spinning. The CEO of the company describes it as “keeping a top spinning, by continuously flicking it” [28].  Maintaining the spin is key.  If not, deadly instabilities can come in the center or along the edge to kill it [41, 3].  It is generally accepted that the FRC can only live for a hundred microseconds, or less, before collapsing [3, 20, 55 – 62].  But as of today, Tri Alpha is ahead of all of its’ competitors, in its’ ability, to hold a large, long lasting FRC stable [57, 68].

Part 3: The Rotamak
A Rotamak

The last way to make and sustain a plasma structure is with a rotating magnetic field.  This is known as a Rotamak.  A rotamak is a combination of the ideas discussed in parts I & II.  A rotamak is a field reversed configuration made using a rotating field.  The rotating field can actually help either a Spheromak or an FRC, but of these, the FRC is better assisted [1].  The value of adding these effects together is clear.  It simplifies most of the machine three ways.  The heating, generation and stabilization of the plasma structure can be done with one technique. Creating a plasma structure with a rotating magnetic field is an easy way to start the machine.  By spinning the field, we can get a current moving in the plasma.  This drags the ions along – making a loop of ions.  That technique is far simple than the collision mechanism described above.  Secondly, a RMF offers a simple way to heat the plasma.  Such a method is non-invasive and non-disruptive.  Lastly, the rotamak can keep the plasma stable.  Last year, Sam Cohens’ team announced they had held a small FRC sixty times longer than Tri Alpha.  This is a tiny group at Princeton against a sizeable startup.  Sams’ research could have moved much faster, had the US government properly funded their work.

Rotamak History

The person who deserves the most credit for the Rotamak was the Professor Ieuan Jones.  He got a doctorate in plasma physics from Aberystwyth University, in Wales, in the fifties [15].  For more than two decades, he was a professor at Flinders.  During all those years, Dr. Jones developed the idea, publishing paper after paper [10].  He built a machine known as the Flinders Rotamak.  I have included a picture of the machine a little further down.  A picture of Ieuan and Sam is shown below [15, 16, 18, 19].

Ieuans’ work did not go unnoticed.  In 1986, the US Department of Energy took an interest [20].  They commissioned a company named STI to build such a machine.  Construction took four long years.  Once finished, the Large S Experiment (the LSX) was installed at Los Alamos [17].  It was an exciting time.  But – idiotically – the government nixed the program just one short year later.  Spending four years, for just one year of testing - is yet another example of stupid funding choices in fusion.

University of Washington:

In 1992, the neglected machine was salvaged by two men: Dr. John Slough and Dr. Alan Hoffman [20].  Both were professors at the Redmond Plasma Physics Lab, at the University of Washington.  They moved the abandoned device to Seattle, in hopes to restart the effort.  Getting the funding was tough.  They had to rejigger both the machine and the effort to focus on tokamaks [74].  Stupid policies by the Department of Energy demanded that efforts show relevance to the ITER.  This went on for four years, until August of 1996.  After this, the team finally got the support they needed; they received $4,018,000 in funding for four more years of work.  They built the TCS machine (translation confinement and sustainment).  This machine used a rotating magnetic field to study plasma structures [7]. It worked well.  It showed a great deal of promise.  Unfortunately, the effort was killed by the Department of Energy, in 2003.  A stupid move.  Only a few years later, the entire Redmond Plasma Physics Lab was shut down.  After 2009, the facility was closed due to lack of funding [21].  Today, you cannot visit the place, the building houses a motorcycle factory [69].  Dr. John Slough was determined to continue this kind of work.  He has developed two startups (Helion and MSNW) which are trying to turn this approach into commercial products [22, 23]. 

Sam Cohen:

It is at this point that Sam Cohen re-enters our story.  At about the same time the Redmond lab was being shut down, Sam started making plans to build a Rotamak at Princeton [3, 4].  His idea was simple: stabilize, generate and heat the plasma in a FRC, with a rotating magnetic field.  He did this, by making a column of plasma [27].  The plasma bounced back and forth inside a Pyrex tube, reflected at each end [3].  Technically he used a magnet mirror to do this.  Around the pyrex would be a rotating magnetic field. The north and south poles spin around the plasma forming, heating and stabilizing the structure.  This mechanism shown below [1, 13, 14, 27, 57].

At the start, the magnetic field is straight on - from left to right [27].  The plasma corkscrews through this field.  The particles spin; moving along the field lines, like cars on the highway, bouncing back and forth at each end.  The ions spin on a wide path while the electrons spin in a much tighter orbit [13, 47, 57].  It is in this environment, that the rotating field is activated.  When it is turned on, the field lines bend.  They warp.  The magnetic fields add together. They re-form, into a curved field; the field lines are pulled up.  When this happens, the particles move upwards, in tow.  The next step is to start spinning the RMF.  Spinning causes the field lines to bow outwards like shells.  In the picture above, they looks like concentric shells.  At this point the electrons are effected.  Their speed rises, whipped up by the fast field [1].  The electrons move in a loop; tugged along by the RMF. Altogether, all plasma motion gets tighter.  The electrons’ speed up and their temperature rises.  Finally the plasma snaps: turning into a full-on FRC [3].  Now, the spinning plasma makes its’ own field.  A structure forms: a donut of spinning plasma.  Once formed, the rotating field keeps the structure hot, spinning and stable [1, 3, 55].


Like so many fusion approaches, the rotamak has been hurt by a lack of US government funding.  The US is not leading in fusion research.  This is dangerous and I have been repeating this for 9 years.  The US dominance in military, economic and political matters is underpinned by our scientific advancement. If we fail to hold the edge, we will fail this country. Our lack of funding for fusion has already killed off the ecosystem of researchers needed to develop it.  If humanity invented net power today, the US military would be caught completely off-guard.  It would be a disaster.  They would have no scientific talent to turn too to jump-start a program.  It would take years, and would leave our country dangerously vulnerable.  Net power is the next milestone in this field.  This could come at any time and from any direction.  Do not assume it is decades away.  Today, there are several companies who see a path to a commercial power plant [75].  Today, China has invested tens of millions into an advanced fusion-fission center, which that years ahead of the Americans [70].  What is our government doing?  Where is the leadership?  This must change.


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Monday, May 8, 2017

"Let There Be Light" Review


On March 10th a new documentary premiered at the South By Southwest Festival [1].  The name of the film was “Let There Be Light.”  This was a labor of love by the company Eye Steel Films [2].  The Canadian firm has been working since 2013 to develop a quality documentary on nuclear fusion [3].  The co-director, Mila Aung-Thwin was tired of the same old refrain about fusion.  “You always see the same story: the sun, a dreamy researcher, a tokamak and thirty years away.  That story has been told many times”[4].  This movie is different [5].  It zips through several fusion efforts: ITER, General Fusion, Focus Fusion and the Stellorator.  It also interviews both researchers and administrators on the state of their field.  Most viewers come in fresh – they know nothing about fusion.  For them, this is an eye-opener.  The public had mostly not heard of the giant ITER project or any of the fusion startups trying to compete with it.  For them - this is a whole new world.     


I traveled to Toronto to see the Canadian premier, in person.  It was exciting.  Personally, the most shocking part of the movie was: the ITER project.  As a tokamak outsider, ITER has always looked like an impenetrable, opaque effort.  What is happening?  Who works on this?  Why is it so expensive?  The movie dives right into these questions.  We meet Dr. Mark Henderson, who heads up the heating systems on the machine.  He is portrayed a good guy.  He is shown fighting for a technology he believes will change mankind, while mired in ugly bureaucracy.  We also meet an Italian named Allesandro Bonito-Oliva [5].  He is portrayed as a hard charging manager; driving his team forward.  But, the real character in this movie is the ITER bureaucracy.  It takes on a life of its’ own.  A web of tense, agitated bureaucrats pushing and pulling with each other.  Delicate communication.  Politics.  Critics both inside and outside the organization, jostling for space.  It was uncomfortable to watch.

General Fusion

There is a stark contrast between ITER and General Fusion.  The startup is really embodied by its’ founder: Dr. Michel Laberge.  People have stereotypes about physicists. In the vein of Doc Brown from Back To The Future; they are supposed to loveable, offbeat and brilliant.  Dr. Laberge is portrayed in this way.  People like him.  He argues for fusion - and makes a good case.  Also, it is not an accident, that he gets the last word in the movie.  The film is starting to cement Leberge as a minor celebrity in this field.  Great.  The public needs someone to root for.

Focus Fusion:

The other person we meet is Eric Lerner.  During this chunk, the film is a true documentary: it presents things as they are, with no opinion given.  Mr. Lerner is eccentric.  Having met him in person, I can attest to this.  He has been fighting for legitimacy for decades and despite the fact that he has almost no support, he has persisted.  The film gives Lerner a great platform to call for a broader crash program.  That said, I cannot comment on his approach.


This week, in my hometown, the lake flooded.  In fact, flooding destroyed thousands of homes and businesses across our entire state [6].  The region had seen its’ third wettest April since record keeping began - in 1900 [6].  Excessive rain, due to a changing climate, has inundated the land, rivers and lakes.  This destroyed homes and disrupted lives.  We know that climate change comes from the burning of fossil fuels.  We also know that fossil fuels will run out.  Fusion – with the huge amount energy it can produce - could change all of this.  This movie tells that story.  You should see it.  It does the field justice.  It is a good film.  For outsiders, it tells a new story.  For insiders, it offers insight into a diverse set of efforts.  But, the overall message is the same: the world is not paying attention.  If we want to realize fusion, we must change this. 

Works Cited:

1. Lowe, Justin. "'Let There Be Light': Film Review | SXSW 2017." The Hollywood Reporter. N.p., 10 Mar. 2017. Web. 08 May 2017.

2. Aung-Thwin, Mila. "LET THERE BE LIGHT" EyeSteelFilm. EyeSteelFilm, n.d. Web. 08 May 2017.

3. "Movie Review and Question." E-mail interview. 8 May 2017. Web.

4. The Canadian Premier of Let There Be Light. By Mila Aung-Thwin. Canada, Toronto. 28 Apr. 2017. Q&A.

5. Let There Be Light. Dir. Mila Aung-Thwin and Van Royko. Perf. Mark Henderson, Eric Lerner. Eye Steel Films, 10 Mar. 2017. Web. 8 May 2017.

6. Veronica Volk, Angelica A. Morrison, Dave Rosenthal. "Latest News on Montreal, Lake Ontario Flooding." WBFO. N.p., n.d. Web. 08 May 2017.

Thursday, April 6, 2017

Silicon Valley Gets In The Fusion Game

Silicon Valley Gets In The Fusion Game


On April first 2017, a Silicon Valley startup named Apollo Fusion announced its’ intention to create a fusion-fission hybrid power plant [3].  The company consists of two key people: a plasma thruster expert and an audacious internet entrepreneur.  Both are pictured below [2,4].  The entrepreneur is Mike Cassidy.

Mr. Cassidy is a big deal.  His name alone, makes this company one to watch.  Most recently, Cassidy served as an executive director, at Google.  He has lead and sold off multiple companies since starting in business in early nineties [5]. In a word: he is a classic Silicon Valley entrepreneur.  The man is also well connected: Tech Crunch has reported that Larry Page and Sergey Brin of Google are both “super enthusiastic” about this startup [1].  The other person involved is Dr. Ben Longmier.  Dr. Longmier is currently an adjunct professor of aerospace engineering at the University of Michigan [4].  He has worked extensively, in plasma thrusters. 

Until April fourth 2017, I had never heard of either of these guys.  They both work in different fields from traditional fusion.  Whatever, we need them.  The quest for fusion power needs all the help it can get.

I was able to get a hold of Apollo Fusions’ patent application [7].  It was filed on September first of 2015.  I would guess that Dr. Longmier spearheaded this effort - and brought Mike Cassidy in later. The companies’ approach is based on plasma thruster research.  Their concept reminds me most of a virtual cathode made inside a penning trap.  That idea was originally suggested by Dan Barnes, of the Los Alamos National Labs, back in 2000 [23].  In any case, the gist of it is: get a lot of electrons together and have them attract ions towards one spot.  Get them moving at high enough speeds to create fusion.  That sounds a lot like a polywell.

Dr. Longmier is lucky to work in plasma thruster research.  Indeed, they may be better understood that traditional fusion devices.  Thrusters have gotten the funding, while our fusion field has languished. Your basic thruster relies on the Hall Effect.  This is a well-researched topic, with it’s’ roots back into the sixties [7].  Multiple research teams have been involved in this effort, over the past five decades and Hall Thrusters have been used to circle the moon [6]. 

A Basic Thruster:

How do plasma thrusters work?  They are normally tested in low pressure vacuum environment (1.3E-3 to 1.3E-6 Pascal).  In the past, these thrusters sat on large satellites, but there has been a recent push into smaller, cube stats.  A cube stat is a hunk of metal, 10 centimeters to a side.  They represent the next generation of satellites.  This change is reflected in Dr. Longmiers’ research.  On paper, he is part of the University of Michigans’ Plasmadynamics and Electric Propulsion lab.  That lab was founded in 1992 and since then, has hosted 15 or 20 big thruster experiments [9]. They were all housed inside a vast, 200 cubic meter, tank.  These represent the old kind of thrusters.  Dr. Longmier has pushed into smaller sizes [24].  He has focusing on cube stats.  The basic mechanism of a Hall thruster is shown below [8, 6].

To build a hall thruster, the first thing you need to do is set up your fields.  At the center of the thruster is a ringed cavity.  This is filled with both a magnetic and electric field.  These fields run perpendicular, to one another.  The magnetic field comes out from the center, while the electric field points out from the bottom of the cavity.  The Lorentz force dictates electrons inside this cavity will mostly stay there.  Once set up, you need to light your thruster.  To do that, you must inject a fuel.  Xenon is the most common fuel to burn.  This is because it is a big fat atom that is easy to ionize.  The Xenon atoms interact with the electrons and ionize.  Once positive, they fly away from the positive electric field.  When they leave this cavity they accelerate, pushing the satellite forward.  As they fly, they suck in electrons which neutralize the beam.  This makes a broad, neutral jet, pushing the satellite forward.   


The biggest differences between this kind of work and traditional fusion research - is the scale of everything.  For example, the magnetic fields used inside a hall thruster will only reach a few hundred guass [10].  Meanwhile, a polywell will need a field that is 80 times higher [11].  These are the kind of fundamental differences, which change everything about how you do your testing.  For example, a Hall thruster could be run for several minutes, powered by batteries or the wall outlet. In stark contrast, the Lockheed fusion reactor must run off capacitors.  Hence, Lockheed can only fire their machine for a few microseconds [12-14].  Another difference is the plasma temperature.  Thruster plasma is tens of electronvolts, aka very cold.  A typical fusor does fusion at 30,000 eletronvolts or hotter [15, 16].  That difference is stark, and it effects everything about how the plasma behaves. Finally, the fuels are very different.  Xenon is easy to use.  It is a noble gas, which works well in a vacuum system.  Vacuum experts will tell you, you need a noble gas to push out the air in a Vacuum [26]. A fusion fuel like tritium is very different.  When we worked with it at the Laboratory for Laser Energetics, it was a cost, regulatory and safety hassle.  If Apollo Fusion wants to burn tritium, they will need a great deal more technical staff and costs.   

All this being said, Dr. Longmier has a history of pushing the bounds of technology.  He was part of a team that developed a 200 kilowatt rocket engine at the Ad Astra Rocket Company, in Houston [18].  As I understand it, this is an order of magnitude better than other rocket thrusters.  This company is widely known in the space community, because of its’ CEO, Dr. Franklin Chang Díaz.  Reaching such a high thrust means that many other plasma parameters and field strengths should rise in kind.  But here is the key question: would plasmas in rockets get so good that they could start to look like fusion plasmas? 

The Patent:

Patents are always vague.  At first glance, this concept looks like: an electron jet, inside some bigger fields.  At its’ core, is an electron beam.  This beam runs right through the center of the machine.  The idea is to get the electrons in this beam to bunch together. This, in turn, will attract the ions for fusion reactions. The patent does not mention how they plan to make this electron beam.  But, they will need some solution here.  If the electrons are not shoved forward with enough kick, they will be scattered by the other fields inside this machine.  Fortunately, there are several off the-shelf electron beams that they can buy [19].  Once shot forward, the electrons will encounter several other fields simultaneously. 

Normally, I would model these fields, as I have in the past for the polywell and Lockheeds’ CFR [20, 21]. A model is very straightforward.  You use excel, newtons’ equations of motions and the Lorentz force to map out how the electrons move around.  If you want to get fancy, you use MATLAB or COMSOL.  Regrettably, this patent gives us no hint as to the field strengths or dimensions inside this machine.  The picture that they do give, is reproduced below [7].

This is a very busy picture.  The first thing you notice the large green field on the outside. This is supposed to be a balanced magnetic field.  Ideally, this opposes the electron beam [7].  The patent promises that this magnetic should slow the electron leakage to the ring in the center.  The next thing that you notice is that big red ring.  That is just one big, positive voltage ring. A positive lump metal, for which the design is not finalized.  The patent, in fact, covers several ring designs.  The patent claims that if you pulse a voltaage on this ring, you can increase the plasma density in the electron beam by 700 fold.  Where is this claim coming from?  It be must from one of Dr. Longmiers’ publications. 

The center of that ring, is where the magic happens.  This increase in beam density is support to pull ions in towards the center. If everything works as planned, the ions will slam together and fuse. This will generate neutrons, which will fly away and hit the walls.  The patent then talks about using FLiBe, in the walls, to absorb the neutrons.  FLibe is a popular molten salt which has been pushed the thorium community [22].  This would represent a fusion-fission hybrid approach.


Obviously, this patent and Apollos’ website will not reveal how far along this research is.  Since the patent was filed in 2015, it is safe to say that Longmier has been looking at this problem for some years.  That said, he has a full time job, so he does not have infinite time to develop the idea.  This concept is similar to many other fusion devices – it looks like a tuning problem.  You have several parameters that you can adjust.  These include, but are not limited to: the fuel injection rate, the shape of the electrode, the vacuum pressure (1E-5 to 1E-10 torr), both the field strengths and their positions.  You can also change the beam characteristics and how everything is placed in the machine.  As you adjust these parameters, you toggle between affects that you love and affects that you hate.  One bad effect might be electrons leaving the beam and going somewhere undesirable.  Another effect could arise from the transient nature of the voltage that is pulsed.  Plasma can escape when fields change. Finally, there is the issue of squashing the material together.  We know there are limits to how dense you can squash a plasma [25].  Ultimately, the goal would be to “tune” this design to increase the fusion rate and dial down everything else.  An experienced researcher will know how to do that.  Using models and dimensionless numbers, they will try to map out these modes of operation, for this machine.  But once that is all done, Apollo will need to figure out how to scale it up.  They will also need to find a way to make regulators (like the US Nuclear Regulatory Commission) happy.  In any case, it looks like Apollo Fusion has a long road ahead of it.


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