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Tokamak produces radioisotopes
Enthalpy:
I didn't find how to contain gaseous 15N as a target for 2.6MeV deuterons already stopped by 10µm nickel, so this gamma source needs a nonvolatile nitrogen compound.
The deuterons react with pretty much all light elements like B, C, O, Al, Si... to produce radionuclides that would still radiate after the useful 16N has decayed. My answer is to combine N with yttrium or heavier, as the 2.6MeV deuterons attain the nucleus rarely, a million times less often than nitrogen. Unless someone wants to shoot at HN3 or synthesize N8, of course.
The compounds I found are nitrides: YN, ZrN, NbN, TaxNy, WN, while HfN and others are less known, possibly nonexistant.
Among these, Ta3N5 provides the best nitrogen content and reaction probability for 15N: only /2.7 as compared with pure N. The yield drops from 19 to still excellent 7Ci/mA. It offers decent resistance to heat and water too. Some Nb in Ta is acceptable. The heavy Ta reacts extremely little with 2.6MeV deuterons.
The other compounds reduce the reaction probability by /4 roughly, so they can be considered if more stable for instance.
Marc Schaefer, aka Enthalpy
Enthalpy:
How big is a cyclotron for 4MeV deuterons used in the reaction with aluminium I proposed here on 14 Aug 2022?
1.78T lets deuterons orbit at 13.56MHz, so the acceleration power can use the 27.12MHz or 40.68MHz ISM frequencies. At 4.0MeV, the path radius is 0.23m with a slightly reduced gap there. 2×28.3kA achieve 1.78T in 40mm gap.
The flux returns between R=0.41m and R=0.51m, where 1/3 of the area is open. The cyclotron takes D=1.02m H=0.7m as sketched and weighs 4.5t.
Smaller is possible. Less γ activity enables less energetic deuterons. Cold coils, optionally superconducting, can produce a stronger induction, but then iron doesn't help.
Lukewarm copper coils can fill 90% of R=0.26m to R=0.40m and H=0.18m each. 430 turns of 0.3mm foil plus 0.025mm spacing resist 165mΩ, so 10.9V and 66A dissipate 715W per coil. Two gamer PC power supplies suffice.
Or use D=1mm aluminium wire at 20K. I imagine yarn wound loosely on the wire so 0.04mm spacing leaves helium through, with a polymerizing resin that impregnates the yarn. At 0.25T=2.5kOe, the magnetoresistance adds 0.7× the zero-field resistivity, but this induction drops linearly with the height and drops also where the return path is removed, so ×0.23 as a mean. 170×131 turns resist 0.87Ω so 1.27A and 1.10V dissipate 1.40W at 20K. 50+ plies of multilayer insulation wrapped around a coil leak 0.3W. The atmosphere presses on the separators, which may need more plies. If 30% as efficient as Carnot's limit, the singe cooler consumes 180W. Square aluminium wire would save 30W.
========== 2.6MeV deuterons
For the reaction with nitrogen, the cyclotron can be 0.813× as big, or D=0.82m H=0.56m. Consider a linear accelerator with RFQ.
========== Switch on faster
When the γ activity needs less beam current, starting with the full current achieves the sought activity faster.
Marc Schaefer, aka Enthalpy
Enthalpy:
[MRI apparatus use to be much shorter than depicted here on 31 July 2022. The conclusion remains.]
==========
Open Magnetic Resonance Imaging began with permanent magnets and 1/4T that blurrs the image. One present commercial success is Philips' Panorama High-Field Open that offers 1.6m opening between two pillars and 1.0T. The pillars seem too narrow for an iron return path, so the supposedly superconducting coils must create the induction unhelped. They look like horizontal Helmholtz coils
wikipedia
Smaller currents added nearer to the axis must even out the induction. More coils create induction gradients, produce and pick the RF fields.
20K aluminium coils seem globally cheaper than superconductors in that use. Cold sensor coils and preamplifiers are good for small signals too.
I compute with plain Helmholtz coils as the corrections need little current and power. R=1m and 1m spacing take 2×569kA×turn. Diagram appended. Data source as previously.
Aluminium wire is square 2mm×2mm here, finer can help varying fields. Band would limit the supply voltage. 15 900 turns fit in D=0.3m. A straight cable would create 0.76T at its surface, but I take 1.0T=10kOe, so the magnetoresistance would add 1.75× the 24pΩ×m zero-field resistivity with RRR=2100, and averaged over the radius it's 1.17×, leading to 52pΩ×m. Each 1.3Ω coil uses 35.8A and safe 46.5V, together 3.32kW at 20K.
Narrow (adhesive) tape or (resin impregnated) yarn wound around the wire insulates and defines the 100µm cooling channels. Interleaved 50µm windings guarantee the channel thickness. 0.13m3 helium at 1atm from 19K to 20K remove the heat. The mean speed is 1.5m/s in the channels, so 1.4mm2/s and 100µm define Re=150 < 2000 and the flow is laminar. Speed curvature 1.74Gm/s/m2 in the channels lets drop only 6kPa over 0.3m.
The heat insulation needs vacuum and reflective surfaces, multilayer insulation is optional. Polymer straps can hold the coils, but I didn't put figures. Forces are like 2MN while deformations are undesired, interesting.
The cooler consumes and dumps 180kW if 30% as efficient as Carnot's limit. This is much, but:
* Operation 3h/day and 250d/yr at 0.2€/kWh costs 0.11M€ electricity over 4yr, 0.27M€ over 10yr. That looks cheaper than superconductors.
* 15m3/s air absorb the heat and exit 10K warmer. At 3m/s it takes a D=2.5m fan blowing a h=0.63m exchanger on the roof.
* The big cooler can improve.Marc Schaefer, aka Enthalpy
Enthalpy:
Updates to my last message.
The magnetic forces are nearer to 1/2 MN rather.
0.13m3/s helium. The helium pressure drop over straight 0.3m would rather be 2kPa but the path zigzags. 4kPa wouldn't be pleasant, adding 520W heat to ohmic 1600W. A quadrupolar flow improves this. Or increase the 100µm spacing, it works cubed.
Helium data is from BNL, many thanks
bnl.gov
Enthalpy:
New update of the protons-to-neutrons conversion, starting from 40MeV.
98Mo is needed at the neutron target anyway. Y outperforms Ga and Ta. 100Mo(p,n+p)99Mo uses no intermediate neutrons. kg/m2 use the natural isotopic composition. d(d,n+p)d at 20MeV may be more efficient and convenient than p(d,n+p)p at 40MeV but I lack data.
Beam power is for 1.4×1014n/s as previously.
Nuclide Page kg/m2 Barn Conv mA kW
======================================================
2H 004 6.7 0.13 5.2% 0.43 17 <<< Cleaner!
89Y 411-417 21 3×0.30... 2.7% 0.83 33
238U 784 26 2.3×1.4 2.1% 1.1 43 Fission
98Mo 473 13 3×0.60... 1.5% 1.5 54
======================================================
100Mo 482 20 0.18 0.23% 10 400 99Mo directly
======================================================
2H needs a non-volatile compound or a container that waste few protons.
Marc Schaefer, aka Enthalpy
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