INTRODUCTION
The history of antimatter begins with a young physicist named Paul A.M.Dirac (1902-1984) and the strange implications of a mathematical equation. This British physicist formulated a theory for the motion of the electrons in electric and magnetic fields. Such theories had been formulated before, but what was unique about Dirac’s was that his included the effects of Einstein’s Special Theory of Relativity. This theory was formulated by him in 1928.Mean while he wrote down an equation, which combined quantum theory and special relativity, to describe the behavior of the electron. Dirac’s equation won him a Nobel prize in I 933,but also posed another problem; just at the equation x2 = 4 can have two solutions (x 2, x = -2). So Dirac’s equation would have two solutions, one for an electron with positive energy, and one for an electron with negative energy. This led theory led to a surprising prediction that the electron must have an “antiparticle” having the same mass but a positive electric charge.
1n1932, Carl Anderson observed this new particle experimentally and it was named “positron”. This was the first known example of antimatter. In 1955, the anti proton was produced at the Berkeley Bevatron, and in 1995, scientists created the first anti hydrogen atom at the CERN research facility in Europe by combining the anti proton with a positron Dirac’s equation predicted that all of the fundamental particles in nature must have a corresponding “Antiparticle”. In each case, the masses of the particle and anti particle are identical and other properties are nearly identical. But in all cases, the mathematical signs of some property are reversed. Anti protons, for example have the same mass as a proton, but the opposite electric charge.
Since Dirac’s time, scores of these particle-antiparticle pairings have been observed. Even particles that have no electrical charge such as the neutron have anti particle.
ANTIMATTER PRODUCTION
Anti protons do not exist in nature and currently are produced only by energetic particle collision conducted at large accelerator facilities (e.g. Fermi National Accelerator Laboratory, Fermi Lab, in US or CERN in Geneva, Switzerland). This process typically involves accelerating protons to relativistic velocities (very near to speed of light) and slamming them into a metal (e.g. Tungsten) target. The high-energy protons are slowed or stopped by collisions with nuclei of the target; the kinetic energy of the rapidly moving protons is converted into matter in the form of various subatomic particles, some of which are anti protons. Finally, the anti protons are electro magnetically separated from the other particles, then they are captured and cooled (slowed) by a Radio-Frequency Quadrapole (RFQ) linear accelerator (operated as a decelerator) and then stored in a storage cell called as a Penning Trap.
Note that anti protons annihilate spontaneously when brought into contact with normal matter, thus they must be stored and handled carefully. Currently the highest anti proton production level is in the order of nano-grams per year.
ANTIMATTER STORAGE
As we know that the antiprotons annihilate spontaneously when brought into Contact with normal matter, thus, they must be contained by electromagnetic fields in high vacuums. This greatly complicates the collection, storage and handling of antimatter. Thus, just after the production of antiproton they are captured and cooled by a RFQ linear accelerator and then stored as gaseous plasma of negatively —charged antiprotons. The storage cell is called a Penning trap; it uses magnetic fields to trap charged particles.- These are under development by Los Alamos National Laboratory (LANL) and Pennsylvania State University (PSU) fore use in particle physics research experiments.
However, the storage density of an antiproton plasma in a penning trap is too low to be feasible for propulsion applications where all of the propulsive energy is derived from matter-antimatter annihilation. Thus, it is necessary to convert the anti protons into a high-density storage form such as solid antihydrogen. To do this, positive antielectrons are combined with negative anti protons to form antihydrogen atoms. This is done in a Paul Trap, which uses oscillating electric and magnetic fields to trap neutral particles (such as atoms). The atoms are then allowed to combine to form molecules possibly as clusters of ions and molecules; then the molecules are cooled to form a solid. Unfortunately, currently only the antiatom production step has been demonstrated. Still the remaining steps that is conversion of antiprotons to anti atoms to anti molecules to anti solid H2 has to be demonstrated; this represents one of the major feasibility issues associated with antimatter propulsion.
PORTABLE ANTIPROTON PENNING TRAP
The picture below shows a schematic and actual photo of the portable antiproton Penning Trap being developed by Pennsylvania State University (PSU). The Penning Trap was completed in 1996. It is designed to hold —1010 antiprotons. In late 1997, the Penning Trap will be filled with antiprotons at CERN (Geneva, Switzerland) and transported to the Air Force Phillips Laboratory SHIVA-STAR facility at Kirkland AFB, where a demonstration of antiproton-catalyzed micro-fission (but not fusion) is planned for 1997-98. An improved Penning Trap (with higher capacity) will be assembled in 1998, and used for a demonstration of antiproton-catalyzed micro-fission and fusion in 1999-2000.
The actual antiproton storage compartment is kept at liquid helium temperatures so as to keep the antiprotons ‘cool” (i.e., so that they won’t have enough kinetic energy to escape the confining magnetic fields provided by the Trap’s permanent magnets). Thus, the Trap design provides for a large outer insulating liquid nitrogen and inner liquid helium volume to permit trips of several days without cryogen refill.
Finally, note that there is minimal hazard from transporting this small amount of antimatter: 1010 proton-antiproton annihilations, with an annihilation energy content of I .8x 1016 Joules per kg (0f proton plus antiproton total mass), would only release 0.6 Joules (0.14 calories), or the energy required to heat one drop (1/20 ml) of water 2.8°C.
ANTI-MATTER PROPULSION
Matter Anti-matter propulsion offers the highest possible physical energy density of any known reaction substance. The ideal energy density (E = mc2) of 9 x 1016 J/Kg is order of magnitude greater than chemical (lx 107 J/Kg), fission (8 x 1013 J/Kg) or even fusion (3 x 1014 i/Kg) reactions. Additionally, the matter antimatter annihilation proceeds spontaneously, therefore not requiring massive or complicated reactor systems. These properties make antimatter very attractive for propulsive ambitious space missions. This section describes antimatter propulsion concepts in which matter antimatter annihilation provides all of the propulsive energy.
Once produced and stored, antimatter can annihilate with normal matter to produce energy for propulsion. The annihilation produces tremendous energy in the form of energetic, unstable, charged and neutral sub atomic particles (mostly pions,p). Note that for a propulsion application, proton antiproton annihilation is preferred over electron positron annihilation because the products of proton antiproton annihilation are charged particles that can be confined directed magnetically so as to transfer their energy to propulsive “working fluid” like normal H2. By contrast, electron-positron annihilation produces only high-energy gamma rays, which do not “couple” their energy efficiently to a working fluid. Thus, in the annihilation of proton (p+) and the antiproton (p-), the products include neutral and charged pions (p0, p+, p-). In this case, the charged ions can be trapped and directed by magnetic fields to produce thrust. However, pions do possess mass, so not all of the proton antiproton mass is converted into energy. This results in an energy density of the proton antiproton reaction of only 1.8 x l0l6J/Kg.
To implement an antimatter rocket engine, the three main components required are antimatter storage system, feed system and thruster. In this fig. the antimatter is stored in the form of solid pellets of anti hydrogen. A high-density form of antimatter is required because storage as gaseous plasma in a Penning Trap is limited to about 1010 particles per cubic centimeter; the volume of 10mg of antimatter would be equivalent to 40 space shuttle cargo bays.
However storage as a solid requires low temperature to prevent sublimation of the pellets. Gaseous antihydrogen could not be contained; only the solid (or liquid) is diamagnetic and can be levitated by a magnetic field. Also, very high- quality vacuum in the storage chamber is required to prevent residual normal matter gas annihilating on the solid antihydrogen pellets. For eg. , in the image, both a vacuum pump and a series of air lock doors are required to prevent gas from the thruster entering the storage chamber. Finally normal hydrogen is used as the propellant working fluid; an excess of hydrogen is used such that the annihilation energy between a small amount of antihydrogen and normal hydrogen heats a large mass of normal hydrogen. This annihilation is accomplished inside the thruster.
ANTIMATTER THRUSTER CONCEPTS
There are four basic antimatter thruster concepts to harness matter antimatter annihilation energy for propulsion. They are the solid-core, gas-core, plasma-core and beam-core thrusters.
The solid-core thruster is similar in concept to nuclear rocket. Antiprotons annihilate inside a solid core heat exchanger made of tungsten or graphite. The annihilation heats the core, which in turn heats hydrogen propellant flowing through the core. The heated 142 then expands through a conventional nozzle to produce thrust. This device is very efficient and produces high thrust, but the specific impulse is limited to less than 1000 lbf-s/lbm due to material constraints. In the gas-core device, antimatter is annihilated directly in the H2 propellant to be exhausted. Magnetic fields are used to contain only the energetic charged pions (p+, p-) which spiral into the H2 gas to heat it. The heated 1-12 is then expanded through a conventional rocket engine. The device is less effective or less efficient than the solid-core concept but could possibly achieve higher specific impulse in the range up to 2500 lbf-s/lbm.
The plasma-core thruster, which is similar to earlier one but operates by annihilating larger amounts of antimatter in H2 to produce hot plasma. The plasma is confined in a magnetic bottle configuration, which also contains the energetic charged pions, which heat the plasma. To produce thrust, the heated plasma is then exhausted through one end of the magnetic bottle. Since this device uses magnetic fields for plasma confinement, it is not limited in temperature by material melting points. It can therefore achieve much higher specific impulse in the range of 5000 to 100,000 Ibf-sllbm at useful thrust levels.
Lastly, the beam-core thruster employs a diverging magnetic field just upstream of the annihilation point between the antimatter and low density H2. The magnetic field is then directly focuses the energetic charged pions as the as the exhausted propellants. Thus the charged pions are traveling close to the speed of light, the specific impulse of the device could possibly range as high as l0 lbfs/Ibm, but at very low thrust levels.
ANTIMATTER ROCKET FOR INTERSTELLAR MISSIONS
The neutral pion rapidly decays into high-energy gamma rays (g), which are effectively useless for propulsion p°—2g The charged pions, on the other hand, have a longer life time and travel on the order of 21 m before decaying into charged muons (i’, f) and neutrons (n); the charged muons travel an additional 1.85km before decaying into electrons (e or positrons (e and neutrinos.
Thus the magnetic nozzle of the vehicle might be as short as 21 m or as long as several kilometers in length to effectively or efficiently capture and direct the pions and muons to produce thrust.
INERTIAL CONFINEMENT FUSION (ICF) PROPULSION
Inertial confinement fusion (ICF) requires high-power lasers or particle beams to compress and heat a pellet of fusion fuel to fusion ignition conditions. In operation, the pellet of fusion fuel (typically deuterium-tritium, D-T) is placed at the locus of several high-power laser beams or particle beams. The lasers or particle beams simultaneously compress and heat the pellet. Compression of the pellet is accomplished by an equal and opposite reaction to the outward explosion of the surface pellet material. Heating of the pellet results from both the compression and the inputted laser energy (or particle-beam kinetic energy). The pellets’ own inertia is theoretically sufficient to confine the plasma long enough so that a useful fusion reaction can be sustained; hence this fusion reaction is inertially confined.
Unfortunately, from a spacecraft perspective, lasers and particle beam ICF implosion ‘drivers are heavy, electric-power intensive systems. In an attempt to avoid these drawbacks, several alternative concepts have been proposed. One simple solution is to take the lasers off of the vehicle and place them in a remote location (e.g., Earth orbit) and beam the laser energy to the vehicle. Several chemical drivers have also been considered. For example, high-energy chemical explosives or high energy density matter (FDM) metastable species (e.g., metaslable helium) could be applied to the surface of the fusion fuel pellet and triggered to produce an implosion. Also, macroscopic kinematic dnvers (basically high-speed “hammers’) have been modeled. Finally, the most exotic approach is a variation on the Interstellar Ramjet; in this concept, fusion fuel pellets are fired (from Earth orbit using a mass driver or rail gun) Out ahead of the vehicle. At sufficiently high speeds, the relative velocity of impact between the vehicle and the pellet is sufficient to cause ignition.
VISTA SPACECRAFT
The inertial confinement fusion (ICF) reaction can ‘e used to provide useful thrust for space travel. This has been proposed in .5 concept called VISTA (Vehicle for Interplanetary Space Transport Applications). (A closely related concept uses a small amount of antimatter to trigger a micro-fission/fusion reaction.) In the VISTA ICF propulsion concept. a fraction of the fusion reaction energy produced is converted to electric power and u5ed to operate the laser (or particle beam) pellet implosion driver modules. A super conducting ring magnet at the base of the cone produces a magnetic nozzle, which directs the flow of the fusion plasma debris to produce thrust. The fusion pulse occurs at the apex of a 500 half-angle cone. The unique hollow-cone configuration of the vehicle is chosen so that a ring-shaped radiation shield 15-rn from the apex protects the rest of the vehicle in a conical radiation shadow.
The VISTA ICF spacecraft The red “tubes” are the driver lasers; the white rectangular “boxes’ between the lasers are the power processors. Mirrors used to focus the laser light onto the fusion pellet are on the standoffs (the mirrors are just visible in the picture). The VISTA hydrogen propellant tank is the ring-shaped bulge at the top of the vehicle (base of cone). Above this are cylindrical habitat modules and conical aero shell (Apollo-shaped) landers. Finally, note that the tethered astronaut (shown in the vehicle at Mars picture) is grossly out of scale; the vehicle is on the order of 100 m tall and 170 m in diameter (at the base of the cone)