Torch Ships

External plasma drives

Torchship
Image from Steve Bowers
The plasma plume of a torchship shines brightly in deep space and can be observed from a considerable distance

Torch Drives: An Overview

Torch drive describes a class of thrusters for spacecraft propulsion in which a high energy yield detonation or pulse is initiated at a high rate external to the spacecraft. Magnetic fields are used to deflect the plasma produced by the pulse to generate thrust. By detonating the pulse outside the main structure of the spacecraft, the neutral radiation by-products of the pulse (neutrons, bremmstrahlung x-rays, gamma rays, and thermal radiation) can mostly escape into space, without creating a large thermal load aboard the spacecraft. This allows very high energy thrusters, which can combine both high thrust and high delta-V simultaneously.

Common design principles

A torch drive requires one or more current carrying loops surrounding the reaction region to produce the magnetic field that deflects the plasma and charged radiation from the drive pulses. Typically, these loops are made of superconductors, since any normal conductor would quickly melt or vaporize under the high currents needed to produce the field. The field coils are backed by a high tensile strength support to withstand the magnet-current back reaction from bursting the superconductor.

These field coils must be protected from the intense neutral radiation that the drive pulses produce. This is because a superconductor that becomes too hot will cease to superconduct. A common shielding design is a sheet of tungsten with a "V"-shaped cross section at a very narrow opening angle, resembling a knife-edge. The point of the V faces the radiation source, the field coil runs along the open top of the V. At very small angles of incidence, tungsten makes a good reflector of x-rays so that most x-rays are simply reflected at low angles away from the field coils and into space. Since tungsten atoms are much heavier than neutrons, a collision between a tungsten nucleus and a neutron results in the neutron rebounding with most of its original energy, delivering only 1% of its energy on average to the tungsten shield. This scatters the neutrons away from the field coils. The narrow opening angle means that the tungsten knife edge is essentially a sheet perpendicular to the incoming radiation, allowing a large radiator area compared to the cross section exposed to the radiation.

Rates of tungsten sublimation become problematic at temperatures above 3000 K, so the shield is typically placed far enough away from the drive pulses to keep its temperature at or below this value. At all times, the shield must be kept below 3695 K, the melting point of tungsten. At these temperatures, the tungsten knife-edge sheets are radiating at a blazing yellow-white color, with the intensity of an M-class star or an old style incandescent bulb filament. A torch drive in operation appears as a brilliant flare primarily from the thermal radiation of the tungsten shields - the drive pulses themselves emit relatively little visible light in comparison. With a 200:1 aspect ratio for the length of the knife blade to its width, a heat shield that absorbs 1% of the incident radiation and scatters the rest can withstand an incident intensity of 90 GW/m^2.

A plasma in a magnetic field will expand against the field while the energy density of the plasma is greater than the energy density of the field. In SI units, the energy density of the field in J/m^3 is given by B^2/(2*mu_0) where B is the magnitude of the magnetic field in tesla and mu_0 is the magnetic constant (mu_0 = 4 pi * 10^-7 N/A^2). If the field coils produce a uniform field, then a drive pulse with energy E in its plasma will expand against the field until it reaches a volume V such that the following relation approximately holds
V = 2 mu_0 E/B^2.
Assuming the radius is approximately spherical, the pulse's blast will expand into a fireball with a diameter d approximately the cube root of this volume. The time t it will take for the pulse to expand to a stop before it is deflected is approximately
t = d/V_ex
where V_ex is the velocity of the torch drive exhaust. If a second pulse is detonated before the first pulse has been fully deflected, it will add its energy to that of the first and require a larger volume to hold the combined fireball.

The size of the field coils is set by the requirement that the tungsten shield remain cool enough not to sublimate, and for the drive pulse fireballs not to contact the tungsten shield. Since the tungsten shield extends a considerable distance toward the detonation point (the distance from the field coil to the tip of the shield is typically 200 times or more the width of the field coil), the field coil must be set far enough back that the tip of the tungsten shield does not evaporate.

If there are multiple field coils producing the magnetic field, then the knife-blade heat shield of one coil will scatter neutrons and radiate thermal heat onto the heat shields of the other field coils, compromising their ability to shed heat. Consequently, many designs use only a single field coil despite the loss of efficiency. Those torch drives that use multiple field coils typically space them at distances significantly larger than the length of the knife-blade shield.
MFT shield
Image from Luke Campbell
Configuration of a shield designed to protect a superconducting coil against neutron radiation. The tungsten surface of the shield is inclined at an angle of 200-1 to the source of radiation, so that the neutrons will glance off.
As previously mentioned, the tungsten knife-blade heat shield that protects the field coils will glow very brightly. At 3000 K, it will radiate 4.6 MW/m^2 of heat as thermal radiation. As an example, consider a torch drive with 2 cm wide field coils, 4 meter long knife-blade heat shields that are exposed to 90 GW/m^2, and 10 MW of neutral radiation produced by the drive pulses. At this rated intensity, the tip of the heat shield can be as close as 9 meters to the detonation point. This produces a radiating disk with an outer radius of 13 meters and an inner radius of 9 meters, which will therefore radiate 2.6 GW of heat and 76 Glm of luminous power combined from its front and back. The apparent brightness will depend on the angle of the disk with respect to the observer, but unless the disk is edge-on, the unaided dark-adapted eye of a baseline human could detect the disk at a distance of approximately 1 Gm (gigameter) and would appear as an apparent magnitude 0 point of light at approximately 0.05 Gm. For comparison, the distance between Sol and old Earth is approximately 150 Gm. A dedicated 1 meter aperture early alarm scanning scope could detect the disk at approximately 600 Tm (terameter) with 1 kilosecond exposure time. For comparison, this is over 1% of the distance from Sol to its nearest stellar neighbor, Alpha Centauri.

The torch drive is a variety of rocket, with the pulse plasma forming the rocket exhaust and the magnetic field acting as the rocket nozzle to transmit the reaction force to the field coil. Thus, there must be some mechanism to transfer the thrust from the field coil to the main body of the spacecraft.

There are two main methods to do so. The rocket can be built to push the spacecraft ahead of it, requiring compression spars to transmit the force; or it can be built to pull the spacecraft behind it, requiring tensile cables to transmit the force. The cables or spars must also be protected from the neutral radiation produced by the drive pulses near the point of detonation.

Spacecraft with components, cargo, or crew which are sensitive to the ionizing effects of radiation may also need radiation shielding against the neutral radiation and, in the pulled configuration, from any high energy charged radiation that might be present in the exhaust. Magnetic shielding is usually used to deflect the charged radiation. Neutral radiation is absorbed with a shadow shield placed between the torch drive and the spacecraft. The shadow shield is typically made of a very high atomic weight metal to stop photons (usually lead or bismuth, although tungsten may be used if the shadow shield needs to withstand high temperatures) sandwiching a thick layer of borated polyethylene to block neutrons. Multiple layers may be needed as very high energy photons can produce neutrons from the shadow shield itself via photo-nuclear reactions, and neutrons emit gamma ray photons when captured by a nucleus. The exact design will depend on the nature of the neutral radiation, which in turn depends on the physical mechanism that causes the drive pulse detonation.
Most modosophont torch drives use some variant on inertial confinement for their drive pulses. High intensity beams strike a fuel pellet from all sides, heating its surface to a plasma. As the plasma blasts off, it pushes back on the remainder of the pellet, compressing it by one or more orders of magnitude.

Some method is then used to heat the interior of the pellet, which ignites a fusion reaction. The fusion detonation provides the drive pulse. The energy required for the driver beams is typically between 1% and 0.1% of the energy per pulse, depending on the sophistication of the heating mechanism. Handling the waste heat created by the beam generator can be a major engineering issue, and will require large radiators operating at significantly lower temperatures than the knife-blade heat shields. Common driver beams are lasers operating at visible, near ultraviolet, or vacuum ultraviolet wavelengths, or heavy ion beams. One consequence of this design is that the torch craft can re-direct its beams for other purposes at the expense of propulsive power, leading to a large population of heavily armed spacecraft. Those spacecraft designed to enter combat often allow the beams to be tuned for longer range - lasers into the x-ray band and particle beams switching to beams of cold relativistic neutral hydrogen plasma

Varieties

D-T inertial confinement fusion (ICF)

This is one of the earliest torch drive designs. The fuel and propellant are plastic-clad pellets of deuterium/tritium ice. The ignition method is rather primitive - either the shock heating of the primary driver beams ignites the fuel (direct drive) or a second, ultra-short laser pulse impinges on the fuel pellet at maximum compression (fast ignition). The D-T reaction is the easiest fusion reaction to ignite, allowing these crude ignition methods to work. However, 80% of the fusion energy is lost as neutron radiation. In addition, tritium is radioactive, with a 388 megasecond half life - consequently, D-T fuel cannot be stored for long periods of time.

Direct drive ICF is quite inefficient, driver beams can only produce pulses of about 10 times their beam energy. Due to the neutron energy losses and the high beam energy, pulse drives of this design usually need an additional power source for the driver beams (although some designs surrounded the reaction volume with a blanket of molten lithium to recover the neutron energy and breed additional tritium. Needless to say, these lithium-blanketed designs could only run at relatively low power to avoid melting the fusion chamber). Fast ignition D-T ICF can deliver drive pulses of around 100 times the driver beam energy. This allows some energy to be directly harvested from the drive pulse plasma to power the driver and igniter beams, while simultaneously allowing higher thrusts.
At 10% burnup and 85% nozzle efficiency, pure D-T fuel delivers an exhaust velocity of 0.9% c or 2,700 km/s. However, because of the platic cladding necessary for driving the implosion, typical exhaust velocities are around 0.5% c or 1,500 km/s. The exhaust velocity scales as the square root of the burnup, so early designs achieving 1% burnup had exhaust velocities as low as 500 km/s.

D-3He Inertial Confinement Fusion

Advanced fast ignition systems can be used to detonate pellets of deuterium/helium-3 ice. Although this requires a relatively high beam energy to pulse energy ratio (thus decreasing the total drive power), it allows operation using only readily available fuel that can be extracted from any gas giant or star without radioactive isotopes or exotic forms of matter. Approximately 1/20 of the pulse energy is lost as neutrons from D-D fusion side reactions, and 1/5 of the pulse energy escapes as bremsstrahlung x-rays. The remaining 3/4 of the pulse energy is available for propulsion.

Because D-3He fusion is more difficult to ignite, even with fast ignition the drive pulse energy is only around 10 times the beam energy. Typically, this energy is extracted from the plasma, resulting in only about half of the total pulse energy remaining for propulsion.

At 10% burnup and 85% nozzle efficiency, pure D-3He provides exhaust velocities of 2.5% c or 7,800 km/s, although the cladding reduces this to about 4,300 km/s. All other designs listed below that make use of D-3He fusion achieve similar exhaust velocities.

Antimatter initiated microfusion (AIM)

The fuel for aAIM torch drives is a pellet of plastic cladded deuterium/helium-3 ice with an inner core of a heavy element, commonly uranium or lead. The driver beams impinge on the pellet, and at the moment of maximum compression a beam of neutral anti-hydrogen is shot into the pellet at low-relativistic velocities.

The antiprotons penetrate the low atomic weight cladding and fuel and slow to a stop in the dense core. On encountering a nucleus, they annihilate - a process that produces one or two pions and smashes the nucleus into fragments. The hot nuclear fragments rapidly dump their energy into the fuel mixture, heating it to the point of ignition. This method allows ignition with drive pulse energies on the order of 100 to 10,000 times the beam energy, depending on the amount of antimatter used. Consequently, for a given beam power the thrust power can be much higher than with un-boosted D-3He ICF, and more energy is available for propulsion since less needs to be siphoned off to power the driver beams. Like all D-3He fusion, 1/4 of the pulse energy is lost as neutrons and bremsstrahlung x-rays.

Conversion initiated microfusion (CIM)

Once magtron monopoles become commonly available, they can be used to ignite fusion in the D-3He fuel. The fuel pellet is a plastic-cladded ball of deuterium/helium-3 ice with a core of iron infused with magtrons and anti-magtrons. When the shock wave from the driver beams ionize the core electrons of the iron, the monopoles can begin to convert nucleons in the iron nucleus into pions and leptons. From there, the reaction proceeds much as happens in AIM, with the iron nuclei fragmenting when struck by the conversion pions into hot nuclear fragments, which in turn heat the deuterium/helium-3 fuel mixture. Since the iron core can be heavily loaded with magtrons, the CIM driver beams can produce drive pulses with about 10,000 times the energy of the driver beam pulses. Once the monopoles are liberated from their iron matrix, the magtrons can annihilate with the antimagtrons. This adds additional radiation in the form of very high energy annihilation gamma rays to usual neutron and x-ray radiation of D-3He fusion. The amount of annihilation, however, is relatively small.

Antimatter-thermal

In antimatter-thermal drives, a pellet of a heavy metal such as uranium or lead encloses a quantity of antimatter suspended in vacuum. This is shot into the magnetic nozzle and imploded with the driver beams. Since the driver beams do not need to provide any significant compression, but rather must merely collapse the antimatter containment, the driver beam pulse energy is essentially negligible. The power available to torch drives of this design depends only on the design of the field coils. Due to the high cost of antimatter, this is not commonly used for long duration thrusting. Instead, AIM drives may have a store of antimatter-thermal pellets for limited duration burns at high thrust.

Conversion-thermal

A conversion-thermal torch drive uses pellets of iron infused with magtron and anti-magtron monopoles. The driver beams ionize the iron and allow the monopoles to begin baryon conversion. As the hot nuclear fragments heat the iron further, the amount of inner-core ionization increases and with it the rate of conversion. This requires a relatively small driver beam pulse energy. As with antimatter-thermal drives, the magtron-iron pellets are fairly expensive and as such conversion-thermal is often used for a short-term high acceleration burn.

Magmatter mesh drive

Originally an S3+ transapient design which has become available in many locations as a gift drive, a typical Magmatter mesh drive (sometimes called a 'pac-man' drive) uses a 1 mm diameter mantle with a mesh tight enough to interact with every nucleus of matter than passes through it, resulting in a mass of about 80,000,000 kg. An additional magmatter shield protects the tip of a tungsten "ovipositor." The tip shield also masses 80,000,000 kg. The two magmatter components counter-balance each other as they oscillate back and forth at 20 MHz. At the distance of closest approach, the mantle is open and the ovipositor places a 1 mm diameter graphite fuel pellet into the mantle. As the mantle and shield separate, magmatter wires draw the mantle shut and flatten it, passing the magmatter mesh completely through the pellet.

Each normal matter nucleus has one or more nucleons undergo conversion, resulting in a plasma of hot nuclear fragments and pion radiation. Each pulse liberates approximately 8.3 GJ and produces plasma with an exhaust velocity of 0.3 c, and delivers an impulse of 50 kg m/s. At the 20 MHz repetition rate, this results in a total drive power of 100 PW and a thrust of 1 GN.
 
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Development Notes
Text by Luke Campbell

Initially published on 10 June 2011.