by Paul Gilster | Sep 20, 2023 | Advanced Rocketry |
I want to drop back to fusion propulsion at this point, as it bears upon the question of a Solar System-wide infrastructure that we looked at last time. We know that even chemical propulsion is sufficient to get to Mars, but clearly, reducing travel times is critical if for no other reason than crew health. That likely puts the nuclear thermal concept into play, as we have experience in the development of the technology as far back as NERVA (Nuclear Engine for Rocket Vehicle Application), and this fission-based method shows clear advantages over chemical means in terms of travel times.
It’s equally clear, though, that for missions deep into the Solar System and beyond, the high specific impulse (ISP) enabled by a theoretical direct fusion drive sets the standard we’d like to meet. In his presentation at the Interstellar Research Group’s Montreal symposium, Jerry Carson discussed the ongoing work at the University of Maryland on creating fusion conditions using deuterium/deuterium (D/D) and deuterium/tritium (D/T) fuel with centrifugal mirror confinement. D/T fusion will likely drive our first fusion engines, but its higher neutron flux will spotlight the advantages of helium-3 when the latter becomes widely available, as shielding the crew on a fusion-powered spacecraft will be a critical factor.
Image: The Centrifugal Mirror Fusion Experiment at the University of Maryland at Baltimore (principal investigator Carlos Romero-Talamás, University of Maryland, Baltimore County). The plan is to achieve fusion conditions (D/D) by 2025. Credit: UMD.
Let’s dig into the centrifugal mirror (CM) concept. The beauty of plasma is that it is electrically conductive, and hence manageable by magnetic and electric fields. Hall thrusters use plasma (though not fusion!), as do concepts like Ad Astra’s VASIMR (Variable Specific Impulse Magnetoplasma Rocket). In a centrifugal mirror, the notion is to confine, compress and heat the plasma as it is spun within a fusion chamber, as opposed to the perhaps more familiar compression methods of inertial fusion, or the magnetic field structures within tokamaks. Carson argues that the CM makes for a more compact reactor and greatly reduces radiation and momentum loss.
The Maryland work implements this effect using magnetic ‘mirrors’ to create the rapid spin that imposes radial and axial forces on the plasma, confining it into a ‘well’ where fusion can be attained. The fuel is bouncing back and forth along the lines of force between the two magnets, a method first explored in the 1950s, when research indicated that mirrors of this kind are leaky and cannot maintain the plasma long enough to ignite fusion. Carson said that it is the addition of an electric field via a central electrode in the UMD design that spins the ‘doughnut’ around its axis, so that the plasma is held in place both axially as well as radially. The basic diagram is below.
Image: Centrifugal mirror confinement of a high energy plasma. Credit: UMD.
The ongoing work at Maryland grows out of an experimental effort in the 2000s that has led to the current Centrifugal Mirror Fusion Experiment (CMFX). The latter is designed with terrestrial power generation in mind, so we are talking about adapting a power-generating technology into a spacecraft drive. To do that, we fire up a centrifugal mirror fusion reactor in tandem with warm plasma (likely a reservoir of hydrogen, though other gasses are possible), so that high-energy fusion products escape the reactor downstream and deposit their energy in the plasma, causing it to expand as it passes through a magnetic nozzle to produce thrust. The reactor also uses energy leaving the upstream mirror to continue its own operations.
A direct fusion drive of this kind could, Carson said, make the round trip to Mars in 3 months, and reach Saturn in less than three years, a sharp contrast to nuclear electric methods. Even nuclear thermal methods would take over a year to make the Mars mission. Looking further out, the Uranus Orbiter and Probe (UOP), which is being considered as a flagship mission for the upcoming Decadal Survey, would make for a 12 year journey using chemical propulsion and a gravitational assist at Jupiter, while DFD-CM in these specs could do a considerably larger mission to more distant Neptune in as little as 3 years. A second generation Interstellar Probe (50 years to the heliopause in the NASA concept) could reach 1000 AU in 30-35 years using DFD-CM.
We’re not quite through with the University of Maryland, because Carson’s colleague Tom Bone has been analyzing a unique way to take advantage of otherwise problematic bremsstrahlung radiation, which complicates foperations with various kinds of fusion fuels. I’ll run through that work in the next post. Turning this challenging radiation into usable energy is conceivably a possibility, but requires fuel other than the deuterium/tritium combination examined for the DFD-CM drive. Bone’s choice is intriguing, to say the least, but more about this next time.
by Paul Gilster | Sep 22, 2023 | Advanced Rocketry |
Let’s talk about fusion fuels in relation to the recent discussion of building a spacecraft engine. A direct fusion drive (DFD) system using magnetic mirror technologies is, as we saw last time, being investigated at the University of Maryland in its Centrifugal Mirror Fusion Experiment (CMFX), as an offshoot of the effort to produce fusion for terrestrial purposes. The initial concept being developed at CMFX is to introduce a radial electric field into the magnetic mirror system. This enhances centrifugal confinement of the plasma in a system using deuterium and tritium as fusion fuel.
Out of this we get power but not thrust. However, both UMD’s Jerry Carson and colleague Tom Bone told the Interstellar Research Group’s Montreal gathering that such a reactor coupled with a reservoir of warm plasma offers prospects for in-space propulsion. Alpha particles (these are helium nuclei produced in the fusion reaction) may stay in the reactor, further energizing the fuel, or they can move upstream, to be converted into electricity by a Standing Wave Direct Energy Converter (SWDEC). A third alternative: They may move downstream to mix with the warm plasma, producing thrust as the plasma expands within a magnetic nozzle.
Image: The fusion propulsion system as shown in Jerry Carson’s presentation at IRG Montreal. Thanks to Dr. Carson for passing along the slides.
We also know that fusion fuel options carry their own pluses and minuses. We can turn to deuterium/deuterium reactions (D/D) at the expense of neutron production, something we have to watch carefully if we are talking about powering up a manned spacecraft. The deuterium/tritium reaction (D/T) produces even more neutron flux, while deuterium/helium-3 (D/He3) loses most of the neutron output but demands helium-3 in abundances we only find off-planet. Tom Bone’s presentation at Montreal turned the discussion in a new direction. What about hydrogen and boron?
Here the nomenclature is p-11B, or proton-boron-11, where a hydrogen nucleus (p) collides with a boron-11 nucleus in a reaction that is aneutronic and produces three alpha particles. The downside is that this kind of fusion demands temperatures even higher than D/He3, a challenge to our current confinement and heating technologies. A second disadvantage is the production of bremsstrahlung radiation, which Bone told the Montreal audience was of the same magnitude as the charged particle production.
The German word ‘bremsen’ means ‘to brake,’ hence ‘bremsstrahlung’ means ‘braking radiation,’ a reference to the X-ray radiation produced by a charged particle when it is decelerated by its encounter with atomic nuclei. So p-11B becomes even more problematic as a fuel, given the fact that boron has five electrons, creating a fusion plasma that is a lively place indeed. Bone’s notion is to take this otherwise crippling drawback and turn it to our advantage by converting some of the bremsstrahlung radiation into usable electricity. To do this, it will be necessary to absorb the radiation to produce heat.
Bone’s work at UMD focuses on thermal energy conversion using what is called a thermionic energy converter (TEC), which can convert heat directly into electricity. He pointed out that TECs are a good choice for space applications because they offer low maintenance and low mass coupled with high levels of efficiency. TECs operate off the thermionic emission that occurs when an electron can escape a heated material, a process Bone likened to ‘boiling off’ the electron. An emitter and collector in the TEC thus absorb the heat from the bremsstrahlung radiation to produce electricity.
Image: A screenshot from Dr. Bone’s presentation in Montreal.
I don’t want to get any deeper in the weeds here and will send you to Bone’s presentation for the details on the possibilities in TEC design, including putting the TEC emitter and collector in tight proximity with the air pumped out between them (a ‘vacuum TEC’) and putting an ionized vapor between the two (a ‘vapor TEC’). But Bone is upfront about the preliminary nature of this work. The objective at this early stage is to create a basic analytical model for p-11b fuel in a propulsion system using TECs to convert radiation into electricity, with the accompanying calculations to balance power and efficiency and find the lowest bremsstrahlung production for a given power setting.
The scope of needed future work on this is large. What exactly is the best ratio of hydrogen to boron in this scenario, for one thing, and how can the electric and magnetic field levels needed to light this kind of fusion be reduced? “It’s not an easy engineering problem,” Bone added. “It’s certainly not a near-term challenge to solve.”
True enough, but it’s clear that we should be pushing into every aspect of fusion as we learn more about confining these reactions in an in-space engine. Experimenting with alternate fusion fuels has to be part of the process, work that will doubtless continue even as we push forward on the far more tractable issues of deuterium/tritium.
by Paul Gilster | Sep 6, 2023 | Advanced Rocketry |
Be aware that the Interstellar Research Group has made the videos shot at its Montreal symposium available. I find this a marvelous resource, and hope I never get jaded with the availability of such materials. I can remember hunting desperately for background on talks being given at astronomical conferences I could not attend, and this was just 20 years ago. Now the growing abundance of video makes it possible for those of us who couldn’t be in Montreal to virtually attend the sessions. Nice work by the IRG video team!
There is plentiful material here for the interstellar minded, and I will be drawing on this resource in days ahead. But let’s start with fusion, because it’s a word that all too easily evokes a particular reaction in those of us who have been writing about the field for some time. Fusion has always seemed to be the flower about to bloom, even as decades of research have passed and the target of practical power generation hovers in the future. In terms of propulsion, I’ve long felt that if we had so much trouble igniting fusion on Earth, how much longer would it be before we could translate our knowledge into the tight dimensions of a drive for an interstellar spacecraft?
But if you’ll take a look at Helicity Space’s presentation at the Montreal symposium, you’ll learn about a tightly focused company that approaches fusion from a different direction. Rather than starting with fusion reactors designed to produce power on Earth, Helicity’s entire focus is building a fusion drive for space. NASA’s Alan Stern, of New Horizons fame but also a veteran of decades of space exploration, is senior technical advisor here, and as he puts it, “our goal is not to power up New York City but to push spacecraft.” Stern appeared at Montreal to introduce a panel including Stephane Lintner, the company’s CEO and chief scientist Setthivoine You.
Let’s assume for a moment that at some point in the coming years – we can hope sooner rather than later – we do develop in-space propulsion using fusion. The easiest approach is to contrast working fusion with the methods available to us today. New Horizons was, shortly after launch, the fastest spacecraft ever built; it crossed the orbit of the Moon in a scant 9 hours, but it took a decade to reach Pluto/Charon. We can use gravity assists to sling a payload to the Kuiper Belt, perhaps a craft like JHU/APL’s Interstellar Probe concept, but here we’re dependent on planetary geometry, which isn’t always cooperative, and doesn’t provide the kind of boost that fusion could.
Image: Setthivoine You, co-founder and chief scientist at Helicity Space.
I’m a great advocate of sails, both solar sails and so-called ‘lightsails’ driven by beamed energy, but if reaching a distant target fast is the goal, we’re constrained by the need for a large laser installation and the likelihood, in the near future anyway, of pushing sails that are tiny. New Horizons would take two centuries to reach the Sun’s gravitational lens, a target of JPL’s SGL mission, which will use tight perihelion passes of multiple craft to get up to speed. We continue to talk about decades of travel time, even within the Solar System. As for interstellar, Breakthrough Starshot’s tiny craft, perhaps sent as a swarm to Proxima Centauri, might reach the target in 20 years if all goes well, but data return is a huge problem, and the mission can only be a flyby.
Helicity boldly sketches a future in which we might combine Interstellar Probe and the SGL mission into a single operation using a craft capable of taking a 5 ton payload to the gravitational lens (roughly 600 AU) in a little over a decade. Pluto becomes reachable in some of the Helicity concepts in about a year, with a craft carrying far heavier payloads than New Horizons and abundant power for operations and communications. All this in craft that could be carried into space by existing vehicles like Falcon Heavy or the SLS. The vision reminds us of The Expanse – imagine 4 months to Mars carrying 450 tons of payload as one step along the way.
All of this is a science fictional vision, but the work coming out of Helicity’s labs is solid. It involves so-called peristaltic magnetic compression to compress plasma and raise energy density. The method relies on specific stable behaviors within the plasma, a type of ‘Taylor State’ – these describe plasma within strong magnetic fields – discovered by You. These confine the plasma, while magnetic reconnection heating preheats it in the first place. Setthivoine You describes all this in the video, noting that what Helicity is developing is a method that we can consider magneto-inertial confinement, one producing a plasma jet stabilized through shear flows within the plasma itself. The firm’s computer simulations, in collaboration with Los Alamos National Laboratories, show four jets merging in a double-helix fashion inside the magnetic nozzle. You’s slides, available in the video, are instructive and dramatic.
What emerges from these results is scalable performance, allowing a path forward that the firm hopes to test as early as 2026 in a demonstration flight in orbit, with the possibility of a functional spacecraft at TRL 9 within roughly a decade. The scalable design points to the possibility of compact, reusable spacecraft with the unique feature of continuous thrusting. Think of the Helicity system, Stern has suggested, as a kind of afterburner put onto an electric propulsion system, and one that can scale into a working fusion drive. The concept scales by adding more fusion jets, allowing higher and higher efficiency, until the engine eventually becomes self-sustaining – the energy required to sustain the thrust is less than what the fusion begins to generate.
Does Helicity’s technology have the inside track, or will a contender like the UK’s Pulsar Fusion get to in-space fusion first? The latter is working on a study with Princeton Satellite Systems to examine plasma characteristics in a fusion rocket engine. Helicity’s founders are appropriately cautious about their work and stress how much needs to be learned, but they are continuing to develop the concept in their prototypes. With its laboratory in Pasadena, the company is privately funded and maintains active collaborations with Caltech and Los Alamos as well as UMBC and Swarthmore. Grants from the Department of Energy and other sources have likewise contributed. Have a look at the Montreal video to get a sense of where this exciting research stands, and also a glimpse of the kind of future that in-space fusion propulsion could produce.
by Paul Gilster | Jan 6, 2023 | Missions |
1000 AU makes a fine target for our next push past the heliosphere, keeping in mind that good science is to be had all along the way. Thus if we took 100 years to get to 1000 AU (and at Voyager speeds it would be a lot longer than that), we would still be gathering solid data about the Kuiper Belt, the heliosphere itself and its interactions with the interstellar medium, the nature and disposition of interstellar dust, and the plasma environment any future interstellar craft will have to pass through.
We don’t have to get there fast to produce useful results, in other words, but it sure would help. The Thousand Astronomical Unit mission (TAU) was examined by NASA in the 1980s using nuclear electric propulsion technologies, one specification being the need to reach the target distance within 50 years. It’s interesting to me – and Kelvin Long discusses this in a new paper we’ll examine in the next few posts – that a large part of the science case for TAU was stellar parallax, for classical measurements at Earth – Sun distance allow only coarse-grained estimates of stellar distances. We’d like to increase the baseline of our space-based interferometer, and the way to do that is to reach beyond the system.
Gravitational lensing wasn’t on the mind of mission planners in the 1980s, although the concept was being examined as a long-range possibility by von Eshleman at Stanford as early as 1979, with intense follow-up scrutiny by Italian space scientist Claudio Maccone. Today reaching the 550 AU distance where gravitational lensing effects enable observation of exoplanets is much on the mind of Slava Turyshev and team at JPL, whose refined mission concept is aimed at the upcoming heliophysics decadal. We’ve examined this Solar Gravity Lens mission on various occasions in these pages, as well as JHU/APL’s Interstellar Probe design, whose long-range goal is 1000 AU.
What Kelvin Long does in his recently published paper is to examine a deep space probe he calls SunVoyager. Long (Interstellar Research Centre, Stellar Engines Ltd) sees three primary science objectives here, the first being observing the nearest stars and their planets both through transit methods as well as gravitational lensing. A second objective along the way is the flyby of a dwarf planet that has yet to be visited, while the third is possible imaging of interstellar objects like 2I/Borisov and ‘Oumuamua. Driven by fusion, the craft would reach 1000 AU in a scant four years.
Image: The Interstellar Research Centre’s Kelvin Long, here pictured on a visit to JPL.
This is a multi-layered mission, and I note that the concept involves the use of small ‘sub-probes’, evidently deployed along the route of flight, to make flybys of a dwarf planet or an interstellar object of interest, each of these (and ten are included in the mission) to have a maximum mass of 0.5 tons. That’s a lot of mass, about which more in a moment. Secondary objectives involve measurements of the charged particle and dust composition of the interstellar medium, astrometry (presumably in the service of exoplanet study) and, interestingly, SETI, here involving detection of possible power and propulsion emission signatures as opposed to beacons in deep space.
Bur back to those sub-probes, which by now may have rung a bell. Active for decades in the British Interplanetary Society, Long has edited its long-lived journal and is deeply conversant with the Daedalus starship concept that grew out of BIS work in the 1970s. Daedalus was a fusion starship with an initial mass of 54,000 tons using inertial confinement methods to ignite a deuterium/helium-3 mixture. SunVoyager comes nowhere near that size – nor would it travel more than a fraction of the Daedalus journey to Barnard’s Star, but you can see that Long is purposely exploring long-range prospects that may be enabled by our eventual solution of fusion propulsion.
Those fortunate enough to travel in Iceland will know SunVoyager as the name of a sculpture by the sea in central Reykjavik, one that Long describes as “an ode to the sun or a dream boat that represents the promise of undiscovered territory and a dream of hope, progress, and freedom.” As with Daedalus, the concept relies on breakthroughs in inertial confinement fusion (ICF), in this case via optical laser beam, and in an illustration of serendipity, the paper comes out close to the time when the US National Ignition Facility announced its breakthrough in achieving energy breakeven, meaning the experiment produced more energy from fusion than the laser energy used to drive it.
Image: The Sun Voyager (Sólfarið) is a large steel sculpture of a ship, located on the road Sæbraut, by the seaside of central Reykjavík. The work of sculptor Jón Gunnar Árnason, SunVoyager is one of the most visited sights in Iceland’s capitol, where people gather daily to gaze at the sun reflecting in the stainless steel of this remarkable monument. Credit: Guide to Iceland.
Long’s work involves a numerical design tool called HeliosX, described as “a system integrated programming design tool written in Fortran 95 for the purpose of calculating spacecraft mission profile and propulsion performance for inertial confinement fusion driven designs.” As a counterpart to this paper, Long writes up the background and use of HeliosX in the current issue of Acta Astronautica (citation below). The SunVoyager paper contemplates a mission launched decades from now. Long acknowledges the magnitude of the problems that remain to be solved with ICF for this to happen, notwithstanding the encouraging news from the NIF.
…a capsule of fusion fuel, typically hydrogen and helium isotopes, must be compressed to high density and high temperature, and this must be sustained for a minimum period of time. One of the methods to achieve this is by using high-powered laser beams to fire at a capsule in a spherical arrangement of individual beam lines. The lasers will mass ablate the surface of the capsule and through momentum exchange will cause the material to travel inward under spherical compression. This must be done smoothly however, and any significant perturbations from spherical symmetry during the implosion will lead to hydrodynamic instabilities that can reduce the implosion efficiency. Indeed, the interaction of a laser beam with a high-temperature plasma involves much complex physics, and this is the reason why programs on Earth have found it so difficult.
Working through our evolving deep space mission designs is a fascinating exercise, which is why I took the time years ago to painstakingly copy the original Daedalus report from an academic library – I kept the Xerox machine humming in those days. Daedalus, a two-stage vehicle, used electron beams fired at capsules of deuterium and helium-3, the resulting plasma directed by powerful magnetic fields. Long invokes as well NASA’s studies of a concept called Vista, which he has also written about in his book Deep Space Propulsion: A Roadmap to Interstellar Flight (Springer, 2011). This was a design proposal for taking a 100-ton payload to Mars in 50 days using a deuterium and tritium fuel capsule ignited by laser. Long explains:
The capsule design was to utilize an indirect drive method, and so a smoother implosion symmetry may give rise to a higher burn fraction of 0.476. This is where the capsule is contained within a radiation cavity called a Hohlraum and where the lasers heat up the internal surface layer of the cavity to create a radiation bath around the capsule; as opposed to direct laser impingement onto the capsule surface and the associated mass ablation through the direct drive approach.
Image: Few images of the Vista design are available. I’ve swiped this one from a presentation made by C. D. Orth to the NASA Advanced Propulsion Workshop in Fusion Propulsion in 2000, though it dates back all the way to the 1980s. Credit: NASA.
SunVoyager would, the author comments, likely use a similar capsule design, although the paper doesn’t address the details. Vista feeds into Long’s thinking in another way: You’ll notice the unusual shape of the spacecraft in the image above. Coming out of work by Rod Hyde and others in the 1980s, Vista was designed to deal with early ICF propulsion concepts that produced a large neutron and x-ray radiation flux, sufficient to prove lethal to the crew. The conical design was thus an attempt to minimize the exposure of the structure to this flux, with a useful gain in jet efficiency of the thrust chamber. SunVoyager is designed around a similar conical propulsion system. The author proceeds to make predictions for the performance of SunVoyager by using calculations growing out of the Vista design as modeled in the HeliosX software.
In the tradition of Daedalus and Vista, SunVoyager explores ICF propulsion in the context of current understanding of fusion. I want to talk more about this concept next week, noting for now that a fast mission to 1000 AU –SunVoyager would reach that distance in less than four years – would take us into an entirely new level of outer system exploration, although the timing of such a mission remains hostage to our ability to conquer ICF and generate the needed energies to actualize it in comparatively small spacecraft systems. This doesn’t even get into the matter of producing the required fuel, another issue that will parallel those 1970s Daedalus papers and push us to the limits of the possible.
The paper is Long, “Sunvoyager: Interstellar Precursor Probe Mission Concept Driven by Inertial Confinement Fusion Propulsion,” Journal of Spacecraft and Rockets 2 January 2023 (full text). The paper on HeliosX is Long, “Development of the HeliosX Mission Analysis Code for Advanced ICF Space Propulsion,” Acta Astronautica, Vol. 202, Jan. 2023, pp. 157–173 (abstract). See also Hyde, “Laser-fusion rocket for interplanetary propulsion,” International Astronautical Federation conference, Budapest, Hungary, 10 Oct 1983 (abstract).
by Paul Gilster | Mar 22, 2022 | Antimatter |
Although I’ve often seen Arthur Conan Doyle’s Sherlock Holmes cited in various ways, I hadn’t chased down the source of this famous quote: “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.” Gerald Jackson’s new paper identifies the story as Doyle’s “The Adventure of the Blanched Soldier,” which somehow escaped my attention when I read through the Sherlock Holmes corpus a couple of years back. I’m a great admirer of Doyle and love both Holmes and much of his other work, so it’s good to get this citation straight.
As I recall, Spock quotes Holmes to this effect in one of the Star Trek movies; this site’s resident movie buffs will know which one, but I’ve forgotten. In any case, a Star Trek reference comes into useful play here because what Jackson (Hbar Technologies, LLC) is writing about is antimatter, a futuristic thing indeed, but also in Jackson’s thinking a real candidate for a propulsion system that involves using small amounts of antimatter to initiate fission in depleted uranium. The latter is a by-product of the enrichment of natural uranium to make nuclear fuel.
Both thrust and electrical power emerge from this, and in Jackson’s hands, we are looking at a mission architecture that can not only travel to another star – the paper focuses on Proxima Centauri as well as Epsilon Eridani – but also decelerate. Jackson has been studying the matter for decades now, and has presented antimatter-based propulsion concepts for interstellar flight at, among other venues, symposia of the Tennessee Valley Interstellar Workshop (now the Interstellar Research Group). In the new paper, he looks at a 10-kilogram scale spacecraft with the capability of deceleration as well as a continuing source of internal power for the science mission.
Image: Depiction of the deceleration of interstellar spacecraft utilizing antimatter concept. Credit: Gerald Jackson.
On the matter of the impossible, the quote proves useful. Jackson applies it to the propulsion concepts we normally think of in terms of making an interstellar crossing. This is worth quoting:
Applying this Holmes Method to space propulsion concepts for exoplanet exploration, in this paper the term “impossible” is re-interpreted arbitrarily to mean any technology that requires: 1) new physics that has not been experimentally validated; 2) mission durations in excess of one thousand years; and 3) material properties that are not currently demonstrated or likely to be achievable during this century. For example, “warp drives” can currently be classified as impossible by criterion #1, and chemical rockets are impossible due to criterion #2. Breakthrough Starshot may very well be impossible according to criterion #3 simply because of the needed material properties of the accelerating sail that must survive a gigawatt laser beam for 30 minutes. Though traditional nuclear thermal rockets fail due to criterion #2, specific fusion-based propulsion systems might be feasible if breakeven nuclear fusion is ever achieved.
Can antimatter supply the lack? The kind of mission Jackson has been analyzing uses antimatter to initiate fission, so we could consider this a hybrid design, one with its roots in the ‘antimatter sail’ Jackson and Steve Howe have described in earlier technical papers. For the background on this earlier work, you can start by looking at Antimatter and the Sail, one of a number of articles here on Centauri Dreams that has explored the idea.
In this paper, we move the antimatter sail concept to a deceleration method, with the launch propulsion being handed off to other technologies. The sail’s antimatter-induced fission is not used only to decelerate, though. It also provides a crucial source of power for the decades-long science mission at target.
If we leave the launch and long cruise of the mission up to other technologies, we might see the kind of laser-beaming methods we’ve looked at in other contexts as part of this mission. But if Breakthrough Starshot can develop a model for a fast flyby of a nearby star (moving at a remarkable 20 percent of lightspeed) via a laser array, various problems emerge, especially in data acquisition and return. On the former, the issue is that a flyby mission at these velocities allows precious little time at target. Successful deceleration would allow in situ observations from a stable exoplanet orbit.
That’s a breathtaking idea, given how much energy we’re thinking about using to propel a beamed-sail flyby, but Jackson believes it’s a feasible mission objective. He gives a nod to other proposed deceleration methods, which have included using a ‘magnetic sail’ (magsail) to brake against a star’s stellar wind. The problem is that the interstellar medium is too tenuous to slow a craft moving at a substantial percentage of lightspeed for orbital insertion upon arrival – Jackson considers the notion in the ‘impossible’ camp, whereas antimatter may come in under the wire as merely ‘improbable.’ That difference in degree, he believes, is well worth exploring.
The antimatter concept described generates a high specific impulse thrust, with the author noting that approximately 98 percent of antiprotons that stop within uranium induce fission. It turns out that antiproton annihilation on the nucleus of any uranium isotope – and that includes non-fissile U238 – induces fission. In Jackson’s design, about ten percent of the annihilation energy released is channeled into thrust.
Jackson analyzes an architecture in which the uranium “propagates as a singly-charged atomic ion beam confined to an electrostatic trap.” The trap can be likened in its effects to what magnetic storage rings do when they confine particle beams, providing a stable confinement for charged particles. Antiprotons are sent in the same direction as the uranium ions, reaching the same velocity in the central region, where the matter/antimatter annihilation occurs. Because the uranium is in the form of a sparse cloud, the energetic fission ‘daughters’ escape with little energy loss.
Here is Jackson’s depiction of an electrostatic annihilation trap. In this design, both the positively charged uranium ions and the negatively charged antiprotons are confined.
Image: This is Figure 1 from the paper. Caption: Axial and radial confinement electrodes (top) and two-species electrostatic potential well (bottom) of a lightweight charged-particle trap that mixes U238 with antiprotons.
A workable design? The author argues that it is, saying:
Longitudinal confinement is created by forming an axial electrostatic potential well with a set of end electrodes indicated in figure 1. To accomplish the goal of having oppositely charged antiprotons and uranium ions traveling together for the majority of their motion back and forth (left/right in the figure) across the trap, this electrostatic potential has a double-well architecture. This type of two-species axial confinement has been experimentally demonstrated [53].
The movement of antiprotons and uranium ions within the trap is complex:
The antiprotons oscillate along the trap axis across a smaller distance, reflected by a negative potential “hill”. In this reflection region the positively charged uranium ions are accelerated to a higher kinetic energy. Beyond the antiproton reflection region a larger positive potential hill is established that subsequently reflects the uranium ions. Because the two particle species must have equal velocity in the central region of the trap, and the fact that the antiprotons have a charge density of -1/nucleon and the uranium ions have a charge density of +1/(238 nucleons), the voltage gradient required to reflect the uranium ions is roughly 238 times greater than that required to reflect the antiprotons.
The design must reckon with the fact that the fission daughters escape the trap in all directions, which is compensated for through a focusing system in the form of an electrostatic nozzle that produces a collimated exhaust beam. The author is working with a prototype electrostatic trap coupled to an electrostatic nozzle to explore the effects of lower-energy electrons produced by the uranium-antiproton annihilation events as well as the electrostatic charge distribution within the fission daughters.
Decelerating at Proxima Centauri in this scheme involves a propulsive burn lasting ten years as the craft sheds kinetic energy on the long arc into the planetary system. Under these calculations, a 200 year mission to Proxima requires 35 grams of total antiproton mass. Upping this to a 56-year mission moving at 0.1 c demands 590 grams.
Addendum: I wrote ’35 kilograms’ in the above paragraph before I caught the error. Thanks, Alex Tolley, for pointing this out!
Current antimatter production remains in the nanogram range. What to do? In work for NASA’s Innovative Advanced Concepts office, Jackson has argued that despite minuscule current production, antimatter can be vastly ramped up. He believes that production of 20 grams of antimatter per year is a feasible goal. More on this issue, to which Jackson has been devoting his life for many years now, in the next post.
The paper is Jackson, “Deceleration of Exoplanet Missions Utilizing Scarce Antimatter,” in press at Acta Astronautica (2022). Abstract.
