Pulsed ion beam source

Details Pulsed ion beam source Pulsed ion beam source

1998-06-11

2001-03-20

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ion generation

Field ionization type

C315S111410

Reissue Patent

active

RE037100

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to apparatus for producing ion beams. More particularly it relates to magnetically-confined anode plasma (MAP) ion sources. Still more particularly, it relates to such MAP ion sources as are designed to produce minimum ion beam rotation and precise control over the ion species present in the beam.
BACKGROUND
A variety of ion beam sources exist. MAP ion sources are particularly interesting because of their ability to shield the ion source structure from the destructive effects of the ion plasma by the magnetic shielding created by the magnetic structure of the MAP ion source.
However, most of the prior art MAP ion sources were designed to be used in a beam line that also included downstream steering and confinement of the produced ion beams by various electric and/or magnetic fields. This steering and confinement was necessary because of the beam rotation created by the magnetic structure of these MAP ion sources which imparted significant rotation to the produced ion beam. The downstream electric and/or magnetic fields add complexity, size and expense to a system employing such MAP ion sources.
An example of such a prior art MAP ion beam source, often called an ion diode, is found in a paper by the inventor of the present invention, Greenly, J. B. et al., “Plasma-Anode Ion Diode Research at Cornell: Repetitive-Pulse, and 0.1 TW Single Pulse Experiments,” Proceedings 8th International Conf. on High-Power Particle Beams, pp. 199-206, Novosibinsk, USSR, July, 1990. The RRID ion diode described therein bears some similarity to the present invention, but its magnetic design is such that substantial beam rotation is imparted to the produced ion beam.
It would be desirable to reduce or eliminate the need for such downstream apparatus by designing a new MAP ion source that had minimal or no beam rotation as a result of the creation and acceleration of the ion beam by the ion beam source.
MAP ion beam sources have a variety of uses. The most obvious is use in conventional ion implantation processes. However, a need for a capability to do surface treatments to large areas of various materials with high power, short pulse ion beams has arisen that required a new type of MAP ion beam source with minimal or no beam rotation and precise control over the ion species used. The use of surface treatments to improve properties such as surface hardness, wear resistance, corrosion resistance, and fatigue lifetime add significant value to a wide range of products in industries including automobile manufacture, aerospace, microelectronics, tool and die manufacture, power generation, and the production of steel, aluminum, ceramics, and plastics. This unmet need was the impetus for the creation of the invention disclosed herein.
Although experiments with prior art ion diode sources have indicated that ion beams might be useful for these sort of surface treatments, no commercial implementations have resulted. The use of ion beams for thermally altering the near surface characteristics of a material has been fraught with substantial problems. Most notable of the limitations with existing ion beam technologies have been: 1) high costs per area treated; 2) the inability to generate a large number of pulses without the costly replacement of ion beam generator components; 3) low repetition rates; 4) low average power; and 5) the inability to reliably produce a uniform ion beam of a single selectable ion species.
Typical ion beam generators use dielectric surface arcing on an anode as a source of ions and thereafter magnetically or geometrically direct and focus the generated ion beam onto the material of interest. This surface arcing (also called “flashover”) destroys the anode surface in less than 100 pulses, and produces a mixed species of ions that cannot be adjusted. Other difficulties arising from flashover include: production of large quantities of neutral gas that makes high repetition rate difficult, generation of debris which can contaminate surfaces being treated, and non-uniformity and irreproducibility of the beam in some cases due to the localized and difficult to control nature of flashover, as well as the detrimental beam rotation discussed above.
State-of-the-art ion beam generators are typically “one shot” devices, i.e., they operate at low repetition rates (<<1 Hz). Existing ion beam generators cannot be operated at high repetition rates (>>1 Hz) for a number of reasons. First, existing pulsed power supplies are not able to generate electrical pulses at high repetition rates having the voltage, pulse width (i.e., nominal temporal duration), and power required to generate the ion beams needed (i.e., consistent with the discussion above) for the various beneficial applications described herein. This limitation renders commercial exploitation impractical. However, it should be noted that one shot surface treatments from a robust ion beam source are useful in some applications. Second, the design of existing ion beam generators does not allow repetitive operation for an extended number of operating cycles (>>10
3
) without replacement of major components. This limitation would require a maintenance time—manufacturing time ratio incompatible with routine manufacturing operations. Third, existing ion beam generators generally operate with electrical efficiencies <5%, thus presenting major challenges to the pulsed power supply and the cooling system of the generator. These limitations and others have made it impossible to routinely utilize the ion beam technology described above for surface treating materials.
SUMMARY OF THE INVENTION
The present invention provides a new type of magnetically-confined anode plasma ion beam source. The new MAP ion diode has improvements in a number of areas. Its magnetic fields are designed to have a profile such that the separatrix (B=0) between the fast coil field and the slow coil field is located near the anode to minimize or eliminate ion beam rotation. The gas nozzle is designed to produce a high mach number (supersonic) gas flow rate to efficiently localize the gas puff introduced into the ionizing region proximate the fast coil. Means are also provided to create an adjustable bias field to control the formation position of the plasma. A fast ringing field is imposed on the gas puff as it is delivered to the ionizing region to pre-ionize the gas. These and other improvements contribute to the MAP ion beam source of this invention.
The use of the MAP ion diode of this invention for large scale surface treatment processes requires a pulsed high power source. Some of the detailed description below will be devoted to one such source, the Repetitive High Energy Pulsed Power (RHEPP) system developed at Sandia National Laboratories. For single shot applications other power sources such as Marx generators could also be used.
Further detail concerning the present invention and its scope will become apparent from the detailed discussion of specific embodiments presented below. However, it should be understood that the detailed description and specific examples, while representing the class of embodiments of the invention and its uses, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.


REFERENCES:
patent: 4721889 (1988-01-01), Seidel et al.
“Production and Postacceleration of Intense Ion Beams in Magnetically Insulated Gaps”, S. Humphries, et al., J. Appl. Phys. 51(4), Apr. 1980, pp. 1876-1895.
“Pulsed Plasma Guns for Intense Ion Beam Injectors”, S. Hamphries, et al., Rev. Sci. Instrum. 52 (2), Feb. 1981, pp. 162-171.
“Magnetically Insulated Ion Diode with a Gas-Breakdown Plasma Anode”, J.B. Greenly, et al., Proc. 6th Int'l. Conf. on High Power Particle Beams, Osaka Japan, 1986, pp. 196-199.
“Magnetically Insulated Ion Diode with a Gas-Breakdown Palsma Anode”, J.B. Greenly, et al., J. Appl. Phys. 63(6), Mar. 15, 1998, pp. 1872-1876

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