Patent Number: 059303131
Section: description

DETAILED DESCRIPTION With reference to the drawing figures, wherein like numbers indicate like parts throughout the several views, FIG. 1 shows an apparatus according to the invention. A laser 12 produces an output 14 which traverses members 16 and 18 to reach chamber 20 containing target 28. Chamber 20 contains a gas 22 which laser output 14 ionizes to form a plasma channel 24. Member 16 is a particle accelerator, which generates a pulse of positive ions 26 directed through drift passage 18 into chamber 20 and plasma channel 24. Member 18 is optional, and could be any conventional means for maintaining an ion beam focused, for example a magnetic lens. FIG. 1 shows ion beam 26 inside chamber 20, where it has propagated since ejection by accelerator 16, heading in the direction of target 28. Target 28 could be any of a number of materials, ranging from a tool or structural metal which beam 26 will harden by impact, to a fuel pellet for nuclear fusion. In general, it is necessary to maintain a vacuum in accelerator 16, and also in the drift region 18. To accomplish this, chamber 20 may be separated from the drift region 18 by a thin foil through which the beam passes. Alternatively, there may be an aperture between 18 and 20, in which case the vacuum could be maintained by differential pumping in region 18. For certain applications, notably heavy ion fusion, it is necessary to strip the beam into a high charge state at the entrance to chamber 20. This will occur if the beam passes through a foil or gas puff. FIG. 2 illustrates the pinching of ion beam 26. Beam 26 travels through plasma channel 24 at a mildly-relativistic velocity indicated by arrow 30. Because velocity 30 is well below the speed of light, the electric field generated by the ions in beam 26 propagates ahead of beam 26. Being ahead of the beam, this electric field pulls free electrons 36 from the plasma axially towards head 37 of beam 26. This establishes a flow of electrons along the same axis (30) along which beam 26 propagates, but in the opposite direction. Since the electron charge is opposite to the ion charge, this electron current flows in the same direction as the ion beam current. Because of Lenz's law, the current which is initially established by the electron flow is maintained during the passage of the ion beam. As beam 26 traverses plasma channel 24, it also pulls in free electrons 32 from the channel. This tends to establish charge equilibrium within beam 26, which is necessary to eliminate strong electrostatic self-repulsion of the beam ions. The net current I.sub.n, i.e. the sum of the ion beam current and the current carried by plasma electrons, remains essentially frozen at the magnitude initially set by the precursor electron flow. The conditions under which a charged particle beam will be pinched are generally known to workers in this field, and are given by the pinch equation: EQU I.sub.n &gt;(17kA)(.epsilon./a).sup.2 .beta..gamma.(m.sub.i /m.sub.e)(1/Q.sub.i) where: a is the beam radius. PA1 .epsilon. is the emittance of the particle beam, a standard measure of the quality of such a beam (i.e. of how well the velocities of the beam particles are aligned, and hence how much the beam will tend to diverge during propagation). p1 .beta.=v/c, where v is the mean velocity of the beam, and c is the speed of light. Thus .beta. is the speed at which the beam travels, expressed as a fraction of the speed of light. PA1 .gamma.=(1-.beta..sup.2).sup.-1/2 PA1 m.sub.i is the mass of the particles which constitute the beam. PA1 m.sub.e is the mass of an electron. PA1 Q.sub.i is the charge of the particles which constitute the beam, expressed in multiples of electron charge. (For example, for heavy ion fusion it may be appropriate to use ions such as bismuth, which are stripped to an average ionization state Q.sub.i =50.) PA1 Ion beam energy of 10 GeV, roughly corresponding to .beta.=0.3 for bismuth. PA1 I.sub.b Q.sub.b =5 kAmp upon entry into the channel. PA1 m.sub.i =209 times the proton mass (i.e. bismuth). PA1 .epsilon.=10.sup.-3 rad-cm. PA1 Beam pulse duration of 10 nsec. PA1 Radius of ion beam: 1.0 cm. PA1 Radius of the plasma channel: 1.5 cm. PA1 The channel electron charge, per unit length, was five times the beam charge. To ensure pinching, the charge density .rho..sub.p of plasma channel 24 must be larger than the charge density .rho..sub.b of beam 26. A minimum condition for this would be that the density .rho..sub.g of atoms in gas 22 exceed .rho..sub.b /Q.sub.p, where Q.sub.p is the average number of electrons removed from atoms in gas 22. (Beam 26 may itself contribute to the ionization of channel 24 by collisions, thus relaxing the demands on laser 12 to fully ionize the channel.) One skilled in the art will know how to create these conditions, after having been instructed by this application in the desirability of so doing. Nominally, a gas pressure in chamber 20 of between 10.sup.-3 to 1 Torr should suffice. The term .beta. must not be so close to the speed of light that the electric field from beam 26 cannot significantly outrun the beam itself. Numerical simulations indicate that useful pinching will occur at least within the range .beta.=0.3 to 0.8, corresponding to an energy of 0.05 to 0.66 A measured in GeV, where A is the atomic weight of an ion in the beam. The channel radius should be one to a few times the beam radius in order to supply electrons outside the beam for charge neutralization, and a well-collimated precursor electron current for pinching. Gas 22 and the constituents of ion beam 26 can be any molecular or atomic species. Member 12 can be any type of laser which effectively ionizes the gas 22. This will occur if the laser frequency is well matched to the quantum states of gas 22, for example a KrF laser used to create a plasma channel in an organic gas such as benzine, or a device such as a free electron laser which can be tuned to the optimal frequency for the gas 22 in chamber 20. Alternatively, a laser or microwaves source can be used to trigger an avalanche breakdown in the gas in order to create a plasma channel. A third technique is to use a low-energy (about a few hundred volts), low current (about a few amps) electron beam, guided by a weak magnetic field (about 50 G) to create the plasma channel. A numerical simulation was done to investigate the working of the invention. The simulation used the FRIEZR beam simulation code, which was developed by workers at the Naval Research Laboratory in Washington, D.C., in support of their research. It is one of a number of numerical codes available for simulating charged particle beams. The parameters of the simulated beam were: FIGS. 3 and 4 show the results of that simulation. In FIG. 3, the solid line indicates beam current, and the dashed line net current, at a time 6 nsec after the tail of the ion pulse had passed point z=0. As seen in the figure, the two are of the same order. The net current is well in excess of the requirement from the pinch equation (for I.sub.n, above), which should indicate good pinching. FIG. 4 plots the "half radius" of the beam, i.e. the radius which contains half the beam current, at the time when the beam tail had passed z=0 (solid curve), and 6 nsec thereafter (dashed curve). As seen from these curves, the half radius stayed roughly the same during this time, and in fact the half radius contracted, indicating good pinching. Referring again to FIG. 1, the laser could be positioned differently, for example at the opposite end of chamber 20 as illustrated in FIG. 1 as laser 12'. Here, laser output 14' goes directly into chamber 20, where it creates plasma channel 24 in the manner discussed above concerning laser 12. Although laser 12' is advantageously positioned closer to chamber 20, it suffers the disadvantage that target 28 obscures its output 14'. This would be unacceptable if target 28 is sensitive to light at the frequency of laser 12', or if it is desired that the diameter of plasma channel 24 be close to that of the target. The invention has been described in what is considered to be the most practical and preferred embodiments. It is recognized, however, that obvious modifications may occur to those with skill in this art. Accordingly, the scope of the invention is to be discerned solely by reference to the appended claims, wherein: