Some great benefits of impartial particle beam propulsion appear clear: Whereas a laser’s photon beams can trade momentum with the sail, impartial particle beams switch power and are significantly extra environment friendly. In reality, as we noticed within the first a part of this essay, that effectivity can strategy 100 %. A mission idea emerges, one which reaches a close-by star in a matter of many years. However what concerning the particle beam turbines themselves, and the onerous engineering points that demand answer? For that matter, how does the idea examine with Breakthrough Starshot? Learn on as James Benford, working in collaboration with Alan Mole, describes the salient points concerned in constructing an interstellar infrastructure.
By James Benford and Alan Mole
We talk about the idea for a 1 kg probe that may be despatched to a close-by star in about seventy years utilizing impartial beam propulsion and a magnetic sail. We describe key parts of impartial particle beam turbines, their engineering points, value construction and sensible realities. Comparability with the Starshot laser beam-driven idea provides roughly comparable prices.
Beam Generator Idea
Determine 1 Block diagram of early impartial particle beam generator . Drift-Tube Linac shouldn’t be proven.
Creation of the impartial particle beam begins with
1. Extraction of a adverse ion beam (unfavourable ion with hooked up electrons) from a plasma supply; it then drifts into the primary acceleration stage, the RFQ. The primary component of the accelerator will seem very similar to the geometry proven in determine 2. Right here ions are extracted from the plasma supply on the left by electrostatics and introduced by a converging magnetic subject to the linear accelerator.
Determine 2. Ion beam on left is propagated alongside converging magnetic subject to the linac.
2. The ion beam enters a radiofrequency quadrupole (RFQ) accelerator, a vane-like construction the place the appliance of radiofrequency energy produces a steady mild acceleration very similar to a surfer driving a wave. It additionally supplies robust electrostatic focusing to stop divergence progress. The construction bunches the particles in part area.
The RFQ fulfils on the similar time three totally different features:
- focusing of the particle beam by an electrical quadrupole area, notably worthwhile at low power the place area cost forces are robust and traditional magnetic quadrupoles are much less efficient;
- adiabatic bunching of the beam: ranging from the continual beam produced by the supply it creates with minimal beam loss the bunches on the primary RF frequency which are required for acceleration within the subsequent buildings;
- acceleration of the beam from the extraction power of the supply to the minimal required for injection into the next linac construction.
three. After the ions exit the RFQ at energies of some MeV, additional acceleration to extend the particle power is completed with a drift-tube linac (DTL), which consists of drift tubes separated by acceleration areas, as proven in Determine three. Particles arriving on the gaps on the correct part within the radiofrequency waves are given acceleration impulses. When the electrical subject of the wave reverses, the particles are shielded from being accelerated by passing via the drift tubes. The standard accelerating gradient is a number of MeV/m.
Determine three. Drift-Tube Linac, which consists of drift tubes separated by acceleration areas.
four. In an effort to keep low emittance and produce the microradian divergence we want, the beam is expanded significantly because it exits the accelerator. Beam dealing with parts should have minimal chromatic and spherical aberrations.
5. Beam pointing to be finished by bending magnets with giant apertures.
6. Lastly, the additional electrons are stripped from the beam, making a impartial particle beam. This may be achieved with by stripping the electrons in a fuel neutralization cell or by photodetachment with a laser beam. It might be potential to realize 100% neutralization by a mixture of strategies. So far this high-efficiency neutralization has not been demonstrated.
There are a number of attainable schemes for constructing the beam generator. Each electrostatic and electromagnetic accelerators have been developed to supply excessive energy beams. The probably strategy is to make use of linear accelerators. Up to now, the price of an electromagnetic accelerator is on the order of an individual yr per meter of accelerator (~1 man-year/m) however this could possibly be bigger for the extra refined applied sciences.
The facility system to drive such accelerators might come from nuclear energy (fission or fusion) or solar energy. Moreover, if it have been to be space-based, the heavy mass of the TW-level excessive common energy required would imply a considerably large system in orbit. Subsequently Mole’s suggestion, that the impartial beam be sited on Earth, has its points of interest. There’s additionally the query of the consequences of propagating within the environment, on each beam attenuation and on divergence.
If the beam generator have been to be on Earth, it ought to all be on the highest altitude for sensible operations. The Atacama Desert, for instance, would supply very low humidity and half of sea degree strain. As well as, a solution to scale back beam losses within the environment can be to launch a hole-boring laser beam prematurely simply earlier than the impartial beam. This laser would warmth up a cylinder of environment to decrease the strain, permitting the impartial beam to propagate with much less loss. Such hole-boring workouts have been carried out in laser weapon research and does look like a viable method.
The ultimate impartial beam could be generated by many small beam drivers or a single giant beam driver. If a large number of driver units and their related energy provides are required, growing the development and upkeep expense of this portion. In fact, economies of scale will scale back the price of particular person segments of the Beamer by mass manufacturing of the system modules. Making such decisions is an train for future engineers and designers.
Impartial particle beam turbines up to now have been operated in pulsed mode of at most a microsecond with pulse energy gear at excessive voltage. Going to steady beams, which might be mandatory for the seconds of beam operation which might be required at least for helpful missions, would require rethinking the development and operation of the generator. The typical energy requirement is sort of excessive, and any sufficient value estimate must embrace substantial prime energy and pulsed energy (voltage multiplication) gear, the main value component within the system. In fact, it is going to vastly exceed the price of the Magsails, which is an financial benefit of beamed propulsion.
Nevertheless, this wants financial evaluation to see what the price optimum would truly be. Such evaluation would keep in mind the economies of scale of a giant system in addition to the price to launch into area versus some great benefits of beaming from Earth.
Beamer Value Estimates
The interstellar impartial particle beam system described here’s a substantial extrapolation past the current state-of-the-art. However, estimates may be product of each the capital and working prices.
The price of the Beamer is split between the price of the accelerator construction (RFQ and DTL) and the facility system that drives it. For a price estimate for the Mercury system, we assume that the current day accelerating gradient is maintained for this very high-power system. That gradient is ~ 2 MeV/m. For the mercury impartial particle beam the size of the 1.35 GeV accelerator can be 675 m.
There’s an in depth know-how base for drift-tube linacs; there are numerous in operation all over the world . We use as a mannequin the well-documented 200 MeV Brooklyn Nationwide Laboratory 200 MeV ion beam system, which was accomplished in 1978 at a price of $47M. It used 22 MW of radiofrequency energy and was 145m lengthy. In that period, the price of microwave gear was ~$1/W. The fee right now is ~$three/W, so the 22 MW would value 22 M$ then and 66 M$ immediately. Because the complete value of accelerator was $47 M$, the Accelerator construction would value 47 M$ -22 M$ = $25 M$. Thus at this degree the 2 value parts are roughly equal. The accelerator construction then prices $25 M$/145 m = $zero.17 M$ per meter in 1978. We multiply all prices by an element of three to account for inflation to get right now’s prices.
To estimate the capital value of the mercury in NPB described right here, we now have the next relations:
Caccl= zero.5 M$/m x 675 m = 350 M$
Cmicrowave= three$/W x 18 TW = 5.47 B$
Subsequently the dominant value aspect can be the microwave system driving the accelerator.
Nevertheless, high-volume manufacturing will drive prices down. Such economies of scale are accounted for by the training curve, the lower in unit value of hardware with growing manufacturing. That is expressed as the fee discount for every doubling of the variety of models, the training curve issue f. This issue sometimes varies with differing fractions of labor and automation, zero.7 < f < 1, the latter worth being complete automation.
It’s properly documented that microwave sources have an 85% studying curve, f = zero.85 based mostly on large-scale manufacturing of antennas, magnetrons, klystrons, and so on . Right now’s value is about $three/W for ~1 MW techniques. Word that this consists of not solely the microwave producing tube, but in addition the facility system to drive that steady energy. The 18 TW energy wanted would require 18 million such models. Subsequently the fee is ~1.1 B$. Including collectively the accelerator and microwave energy system, the price can be 1.45 B$.
The electrical energy to drive this massive system can’t probably come from the electrical grid of Earth. Subsequently a big value component would be the system that shops the 162 TJ of power. (Notice that the beam energy begins at zero and rises with time (t2) to 18 TW on the finish.) From Parkin’s estimates of the Starshot power storage system based mostly on Li-ion batteries, we take the storage value to be $50 per kilowatt-hour, which is $13,900 $/TJ. Consequently the price for the power retailer is ($13,900 $/TJ) 162 TJ = 2.25 B$. So the power shops value is corresponding to that of the accelerator.
The full capital value is
Caccl= 350 M$
Cmicrowave = 1.1 B$
Cstore= 2.25 B$
Complete accelerator capital value is three.7 B$.
The working value to launch a single Magsail is in fact far smaller. It’s merely the price of the spacecraft and the power to launch it. We’ll assume that the price of the spacecraft shall be on the order of $10 million. The price of the electrical energy on the present price of $.10 per kilowatt-hour is $four.5 million.
Complete working value for a single launch is ~15M$.
Comparability with Starshot
The impartial particle beam strategy is conceptually just like photon beams such because the laser-driven Starshot undertaking. An obstacle of reflecting photons from the sail shall be that they carry away a lot of the power as a result of they trade solely momentum with the sail. Impartial particle beams switch power, which is rather more environment friendly. The reflecting particles might in precept be left on shifting in area after reflection and thus the environment friendly power effectivity can strategy 100%.
The Starshot system, a laser beam-driven 1 gram sail with the objective of reaching zero.2c, has been quantified in an in depth system mannequin by Kevin Parkin . Since each the excessive acceleration impartial particle beam described right here and Starshot are each beam-driven high-velocity methods, we make the next comparability between their key parameters and price parts:
Bodily parameters and price parts of beam-driven probes
|Mercury Impartial Particle Beam System||Starshot|
|Sail mass||1 kg||1 g|
|Velocity||zero.06 c||zero.2 c|
|Beamer capital value||1.45 B$||four.9 B$|
|Power retailer value||2.25 B$||three.four B$|
|Complete capital value||three.7 B$||eight.three B$|
|Power value/launch||four.5 M$||7 M$|
|Kinetic power||1.6 1014 J||1.eight 1012 J|
|Kinetic power/ capital value||43.2 kJ/$||zero.2 kJ/$|
Right here we’ve got summed the accelerator and microwave energy system prices for the impartial Beamer and the laser and optics value for Starshot. A serious caveat is that Parkin’s estimates have real looking efficiencies of the techniques of Starshot, however our prices assume unrealistically excessive efficiencies.
Though they differ intimately, the 2 ideas give the identical order of magnitude value. Nevertheless, the kinetic power within the NPB-driven probe is 90 occasions that of the Starshot probe. This exhibits the drawback of reflecting photons from the sail: they carry away a lot of the power as a result of they trade solely momentum with the sail. Impartial particle beams switch power, which is far more environment friendly. The kinetic power/capital value ratio is 200 occasions larger within the NPB case.
It’s instructive that the high-energy requirement of interstellar probes drives the existence of a stand–alone storage system, which is a serious aspect within the complete value of each techniques. The similarity of prices for these slightly totally different beam- pushed methods provides us some confidence that these tough estimates on this paper are credible.
Impartial Particle Beam Realities
Sensible realities are all the time dangerous information. Efficiency of most methods degrades to under their design factors due to inefficiencies of processes. Observe that the beam techniques described listed here are completely environment friendly, as decided from equation 5. That’s, the beam displays from the sailcraft with good effectivity, in order to cease lifeless, transferring all of the power to the spacecraft. The realities of impartial particle beams within the current day are considerably poorer.
To see the place the issues lie, we think about a daring experiment referred to as BEAR, carried out 30 years in the past [1, 5]. A impartial particle beam generator was truly deployed and operated in area and its efficiency was measured.
On July 13, 1989 the Beam Experiment Aboard Rocket (BEAR) linear accelerator was efficiently launched and operated in area by Los Alamos Nationwide Laborotory. The rocket trajectory was sub-orbital, reaching altitude of 220 km. The flight demonstrated that a impartial hydrogen beam might be efficiently propagated in an exoatmospheric setting. The cross-section of the rocket is proven in determine four.
Determine four. Beam Experiment Aboard Rocket (BEAR) .
The accelerator, which was the results of an in depth collaboration between Los Alamos Nationwide Laboratory and industrial companions, was designed to supply a 10 rnA, 1 MeV impartial hydrogen beam in 50 microsecond pulses at 5 Hz. The most important elements have been a 30 kev H- injector a 1 MeV radio frequency quadrupole, two 425 MHz RF amplifiers, a fuel cell neutralizer, beam optics, vacuum system and controls. The beam extracted was 1 cm in diameter with a beam divergence of 1 mradian. There was no sudden conduct reminiscent of beam instability in area.
The design was strongly constrained by the necessity for a light- weight rugged system that might survive the trials of launch and function autonomously. The payload was parachuted again to Earth. Following the flight the accelerator was recovered and efficiently operated once more within the laboratory.
From the paper and report describing this experiment we see substantial inefficiencies, which ought to information our future expectations.
The enter energy to the accelerator was 620 kW for 60 µs, a 7.2 J power enter. The beam as extracted was 27 mA at 1 MeV for 50 µs, which provides 1.35 J. The effectivity subsequently is 19%, so roughly four/5 of the power provided was misplaced within the beamline proven in determine 1. The most important loss was within the neutralizer which was a xenon fuel injected into the beamline. The effectivity of the neutralizer was modified by various the quantity of fuel injected. They obtained 50% impartial hydrogen and 25% every of adverse and constructive hydrogen. Subsequently the neutralization course of was solely 50% environment friendly in producing a impartial beam. This accounts for a lot of the loss. The opposite losses might be accounted for by inefficiencies within the optics of the low-energy beam area and the high-energy beam area.
Within the 30 years because the flight, little work on particle beams has occurred at excessive energy ranges, due to the termination of the Strategic Protection Initiative. Probably substantial enhancements might be made within the effectivity of NPB’s, given substantial analysis funding. Subsequently the idea on this paper, with its hundred % effectivity of power switch from the electrical system to the sail, is an higher sure on the efficiency. Consequently the parameters in Desk 1 and the capital and working value estimates given listed here are decrease bounds on what would truly happen.
The fee mannequin introduced right here is missing in sensible efficiencies. The subsequent degree of study ought to handle this lack.
We will forsee a improvement path: a System begins with decrease velocity, decrease mass Magsails for quicker missions within the inside photo voltaic system. Because the system grows, the impartial beam System grows and know-how improves. Economies of scale result in quicker missions with bigger payloads. As interplanetary commerce begins to develop, making commerce function effectively, outcompeting the lengthy transit occasions of rockets between the planets and asteroids, the System evolves . Nordley and Crowl describe such a improvement state of affairs . We conclude that this idea is a promising technique for interstellar journey.
1. P. G. Oshey, T. A. Butler, M. T. Lynch, Okay. F. McKenna, M. B. Pongratz, T. J. Zaugg, “A Linear Accelerator In Area-The Beam Experiment Aboard Rocket”, Proceedings of the Linear Accelerator Convention 1990.
2. H. B. Knowles, “Thirty-5 Years of Drift-Tube Linac Expertise” Los Alamos Scientific Laboratory Report, LA-10138-MS, 1984. See additionally reference four, pg. 81.
three. J. Benford, J. A. Swegle and E. Schamiloglu, Excessive Energy Microwaves, Third Version, pg. 77, Taylor and Francis, Boca Raton, FL, (2015).
four. Okay. L. G. Parkin, “The Breakthrough Starshot System Mannequin”, Acta Astronautica 152, 370-384, 2018.
5. G. J. Nutz, “Beam Experiments Aboard a Rocket (BEAR) Venture Abstract’, LA-11737, 1990.
6. J, Benford, “Beam-Pushed Sails and Divergence of Impartial Particle Beams” JBIS 70, pg. 449-452, 2017.
7. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.