4 ms·

Okay firstly, how are they getting fusion? You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to

by twic 3y ago

Okay firstly, how are they getting fusion? You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to have that much kinetic energy it needs to be going at 5340 km/s [2]. "The exhaust velocity of a [pulsed plasma thruster] is of the order of tens of km/s" [3]. So what gives? I'm guessing that exhaust velocity figure is for heavy ions, but are protons really that much faster? Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that? [1] https://www.nature.com/articles/s42005-023-01135-x https://www.nature.com/articles/s42005-023-01135-x [2] https://www.omnicalculator.com/physics/relativistic-ke https://www.omnicalculator.com/physics/relativistic-ke [3] https://en.wikipedia.org/wiki/Pulsed_plasma_thruster https://en.wikipedia.org/wiki/Pulsed_plasma_thruster

Gravityloss 3y ago
Very odd. How could they then reach any fusion if they have a hundredth of the needed velocity (50 km/s instead of 5000 km/s)? If they still detected alpha particles... I get 1600 km/s from a naive calculation, still a factor of 30 off: https://www.wolframalpha.com/input?i=sqrt%282*150+kev+%2F+%28boron+atomic+mass%29%29 https://www.wolframalpha.com/input?i=sqrt%282*150+kev+%2F+%2...
ben_w 3y ago
Although I don't know the specifics of this thruster — the phrase "generates high speed protons through the ionization of water vapor" sounds like the reporter/press release writer didn't understand what the scientists told them — it's very easy to get protons up to 148 keV in a lot of different ways. There's a specific kind of fusion reactor which gets done as a highschool science-faire project every so often, similar voltage level: https://www.popularmechanics.com/science/energy/a34312754/12-year-old-builds-working-fusion-reactor-world-record/ https://www.popularmechanics.com/science/energy/a34312754/12...
- cryptonector 3y ago
  The story at the link doesn't say, but this is probably a Fansworth-Hirsch fusor.
jessriedel 3y ago
EDIT: Eh, maybe not. I see it's describes as the "exhaust plume" elsewhere...which really does suggest the fusion is taking place outside the motor. So I remain confused. ---- > Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that? I think (?) the idea is that it's being added to the exhaust stream before it's left the motor and in particular while it's still in the magnetic fields. This would indeed be like an afterburner, where the additional fuel is added to the "exhaust" of the turbine but while still inside the engine. (The fields accelerate both positive and negative charges in the same direction. It does seem like you'd need a positive-negative bias to the typical output velocities in order to use the Lorentz force here, but maybe I'm confused.) > By contrast, electromagnetic thruster ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.[1][2] https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster
jessriedel 3y ago
> You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to have that much kinetic energy it needs to be going at 5340 km/s [2]. "The exhaust velocity of a [pulsed plasma thruster] is of the order of tens of km/s" [3]. So what gives? I'm guessing that exhaust velocity figure is for heavy ions, but are protons really that much faster? This isn't necessarily a satisfying answer, but in their patent they mention "laser-assisted fusion": > In addition, a plurality of pulsed lasers are utilized to focus on focal regions of the exhaust region to catalyze boron fusion events...The pulsed lasers are modulated to create a superposition and constructive interference region where fusion is likely to incur in the reaction region. The plurality of pulsed lasers assists fusion ignition and/or further energizes the protons frame to assist fusion reaction. It is an insight of this invention that acceleration of particles using multiple small lasers would be helpful as only small hot regions of plasma are necessary for measurable fusion results. https://patents.google.com/patent/US20240018950A1/en https://patents.google.com/patent/US20240018950A1/en
simne 3y ago
148 keV is not too much. As electronics engineer I few times talked with colleagues about fixing X-ray devices, and there very typical to use voltages 60kV..120kV, which are directly converted to electrons kinetic energy, and than it converted to X-rays.
cryptonector 3y ago
https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Polywell https://en.wikipedia.org/wiki/Polywell and related You can very much do table-top fusion in your garage if you like. You just won't get net energy out of it -- far from it. But if you're interested in higher ISPs for deep space rocket engines, then the fact that this isn't energy efficient might not matter to you -- getting enough extra delta-v for your propellant and reactor/thruster might well justify the expense. The real problem is that the electric power needed to run such fusors has to come from somewhere, and in deep space the hardest thing to do is to cool that something, though here I suppose a fusion-enhanced thruster might be able to use propellant as coolant for a fission reactor that generates the electrical power for the fusor that add enough additional heating to the propellant to make this workable.
pfdietz 3y ago
The low exhaust velocity implies the plasma temperature is low. This means the loss of energy from protons to electrons will be very high, so high that even with a purely boron plasma, the fusion yield will be quite small. Attempts to make p-11B work typically have electron temperature above 100 keV and ion temperatures of maybe 300 keV. For warm or hot electrons, the energy loss of energetic protons to the electrons scales as T_e^(-3/2), so a colder plasma will cause ruinous energy loss from the protons. From this, I conclude any fusion heating is quite minor compared to the simple plasma heating from the protons slowing down.

by twic 3y ago

Gravityloss 3y ago

Very odd. How could they then reach any fusion if they have a hundredth of the needed velocity (50 km/s instead of 5000 km/s)? If they still detected alpha particles... I get 1600 km/s from a naive calculation, still a factor of 30 off: https://www.wolframalpha.com/input?i=sqrt%282*150+kev+%2F+%28boron+atomic+mass%29%29 https://www.wolframalpha.com/input?i=sqrt%282*150+kev+%2F+%2...

ben_w 3y ago

Although I don't know the specifics of this thruster — the phrase "generates high speed protons through the ionization of water vapor" sounds like the reporter/press release writer didn't understand what the scientists told them — it's very easy to get protons up to 148 keV in a lot of different ways. There's a specific kind of fusion reactor which gets done as a highschool science-faire project every so often, similar voltage level: https://www.popularmechanics.com/science/energy/a34312754/12-year-old-builds-working-fusion-reactor-world-record/ https://www.popularmechanics.com/science/energy/a34312754/12...

cryptonector 3y ago
The story at the link doesn't say, but this is probably a Fansworth-Hirsch fusor.

jessriedel 3y ago

EDIT: Eh, maybe not. I see it's describes as the "exhaust plume" elsewhere...which really does suggest the fusion is taking place outside the motor. So I remain confused. ---- > Secondly, how are they turning the fusion into thrust? Causing fusion in the exhaust stream after it's left the motor will make it hotter, but how does that heat couple to the vehicle? In a pulsed plasma thruster, thrust is produced from the reaction against the acceleration of the plasma by the magnetic field. How does fusion affect that? I think (?) the idea is that it's being added to the exhaust stream before it's left the motor and in particular while it's still in the magnetic fields. This would indeed be like an afterburner, where the additional fuel is added to the "exhaust" of the turbine but while still inside the engine. (The fields accelerate both positive and negative charges in the same direction. It does seem like you'd need a positive-negative bias to the typical output velocities in order to use the Lorentz force here, but maybe I'm confused.) > By contrast, electromagnetic thruster ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.[1][2] https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster

jessriedel 3y ago

> You need a lot of energy to overcome the Coulomb barrier for boron - apparently at least 148 keV [1]. For a proton to have that much kinetic energy it needs to be going at 5340 km/s [2]. "The exhaust velocity of a [pulsed plasma thruster] is of the order of tens of km/s" [3]. So what gives? I'm guessing that exhaust velocity figure is for heavy ions, but are protons really that much faster? This isn't necessarily a satisfying answer, but in their patent they mention "laser-assisted fusion": > In addition, a plurality of pulsed lasers are utilized to focus on focal regions of the exhaust region to catalyze boron fusion events...The pulsed lasers are modulated to create a superposition and constructive interference region where fusion is likely to incur in the reaction region. The plurality of pulsed lasers assists fusion ignition and/or further energizes the protons frame to assist fusion reaction. It is an insight of this invention that acceleration of particles using multiple small lasers would be helpful as only small hot regions of plasma are necessary for measurable fusion results. https://patents.google.com/patent/US20240018950A1/en https://patents.google.com/patent/US20240018950A1/en

simne 3y ago

148 keV is not too much. As electronics engineer I few times talked with colleagues about fixing X-ray devices, and there very typical to use voltages 60kV..120kV, which are directly converted to electrons kinetic energy, and than it converted to X-rays.

cryptonector 3y ago

https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Polywell https://en.wikipedia.org/wiki/Polywell and related You can very much do table-top fusion in your garage if you like. You just won't get net energy out of it -- far from it. But if you're interested in higher ISPs for deep space rocket engines, then the fact that this isn't energy efficient might not matter to you -- getting enough extra delta-v for your propellant and reactor/thruster might well justify the expense. The real problem is that the electric power needed to run such fusors has to come from somewhere, and in deep space the hardest thing to do is to cool that something, though here I suppose a fusion-enhanced thruster might be able to use propellant as coolant for a fission reactor that generates the electrical power for the fusor that add enough additional heating to the propellant to make this workable.

pfdietz 3y ago

The low exhaust velocity implies the plasma temperature is low. This means the loss of energy from protons to electrons will be very high, so high that even with a purely boron plasma, the fusion yield will be quite small. Attempts to make p-11B work typically have electron temperature above 100 keV and ion temperatures of maybe 300 keV. For warm or hot electrons, the energy loss of energetic protons to the electrons scales as T_e^(-3/2), so a colder plasma will cause ruinous energy loss from the protons. From this, I conclude any fusion heating is quite minor compared to the simple plasma heating from the protons slowing down.