Blake P. Wood (P-24), firstname.lastname@example.org, (505)665-6524
For more photos of the CHAMP system, click here.
The material in this web page is excerpted from LAUR-99-991.
Background and Research Objectives
Intense Pulsed Ion Beams (IPIB) have been under investigation for a number of years in Japan, the Former Soviet Union, Germany, and the United States for materials processing applications [1-7]. Such IPIB devices typically produce beams of 10's of kiloamps current at 100's to 1000's of kilovolts, in pulses of 50 -1000 ns. Although these are called "ion beams", they are, in fact beams of neutral plasma - although only the ions are initially accelerated, electrons are pulled off surfaces to provide quasi-neutrality.
For approximately eight years, LANL has operated the Anaconda IPIB . Anaconda utilizes the ballistically focused, magnetically insulated ion diode shown at left, a common anode design. Ions are formed by the flashover of a conical annulus of Lucite attached to the anode electrode. Ions are accelerated across a 300 kV gap toward nested, truncated metal cones that form the cathode. Field-emitted cathode electrons are prevented from shorting the anode-cathode gap by a transverse magnetic field generated by pulsed electromagnets ("slow coils") which surround the cathode cone. The azimuthal E(B drift of electrons in the anode-cathode gap causes them to stay confined. The diode is directly connected to a Marx generator operated to produce a 300 keV, 30 kA, 400 ns beam. The ballistic focus of the diode produces a minimum spot size of about 100 cm2, yielding energy fluences at focus of about 30 J/cm2.
There are several limitations of this solid flashover ion diode design. The beam produced consists of carbon, hydrogen, and oxygen ions, with little composition control possible. Anode debris (molten bits of anode material) is produced, which impacts and contaminates the object being treated. Such debris is shown at left. Finally, due to the large amount of vapor and anode debris produced, it requires a significant amount of time (seconds to minutes) to pump the vacuum chamber down in preparation for the next shot.
A better anode design is the active anode plasma diode developed by John Greenly at Cornell University.[10, 11]. This diode looks very much like the Anaconda diode shown above, but replaces the solid flashover anode with a plasma produced by breaking down gas which has been puffed across the face of the anode from a fast valve on the diode axis. The anode face consists of a "fast" inductive coil potted in epoxy. Pulsing the fast coil breaks down the gas into a plasma. This plasma is stagnated against the transverse magnetic field produced by the slow coils, and ions are subsequently extracted by the high voltage accelerating pulse. The active anode plasma diode designed and constructed for CHAMP is shown at left.
The active plasma anode diode has a number of important advantages. It can produce a high purity beam of any gaseous ion. Since the ion production is separated from extraction of the beam, greater control over beam characteristics is possible, and higher electrical efficiencies can be achieved. Such a beam can be rep-rated, since far less ancillary gas is produced (Quantum Manufacturing Techologies, Inc. routinely operates their ion beam, which has this type of anode, at 10 Hz.)
The primary research objective of this project is to design and construct a compact, rep-rateable IPIB utilizing an active plasma anode diode. This device is known as the Continuous High Averge power Microsecond Pulser (CHAMP), and when completed should produce a beam of 10-15 kA at 200-250 keV, in pulses of 1 us duration, repeated at 1-2 Hz. CHAMP utilizes a simple four stage Blumlein (shown at left) to produce its 1 ms pulses, resulting in a compact machine.
Importance to LANL's Science and Technology Base and National R&D Needs
Applications of IPIBs to treatment of materials are well documented in the published literature, and include improved hardness and corrosion resistance through surface microstructure refinement by rapid melt and resolidification, high rate production of coatings by congruent condensation of material ablated by the ion beam, nanoparticle production, and enhancement of surface catalytic action . In this section, we will review several other applications of particular interest to LANL's science and technology base and the national R&D needs: 1) use as a portable, intense flash neutron source for neutron resonance spectroscopy (NRS) of a dense plasma, neutron radiography, and criticality measurements, and 2) use as an intense diagnostic neutral beam (IDNB) on an advanced tokamak. Strong interest has been expressed within LANL for all of these applications, and some funding has been obtained to pursue two of them (NRS and IDNB).
Neutron Resonance Spectroscopy
Dynamic NRS has been demonstrated at the LANSCE accelerator facility at LANL , but a portable source is needed for making similar measurements on a variety of experiments which cannot be moved to LANSCE, for example, the Atlas imploding liner facility scheduled to be completed in late 2000. NRS is the only known method to make temperature measurements in the several-eV range for strongly coupled plasma (SCP) experiments. Experimental determination of SCP viscosity and equation-of-state has been identified by LANL's Thermonuclear Applications Group as a critical component in improving our understanding of secondary weapons physics. Once CHAMP is completed, LDRD funding has been identified to characterize the intense deuteron beam, produce D-D neutrons (and hopefully D-T neutrons in the future), and demonstrate the NRS technique. The goal is to have a working high temperature diagnostic when Atlas comes on line in late-2000. A pulse of >1012 14 MeV neutrons would be produced by hitting a tritiated metal target with a 12 kA, 250 keV, 1 ms intense deuteron beam. These neutrons, after moderation to epithermal energy, would irradiate a 4 cm3, 1-10 eV dense plasma containing a 1% solid density of indium or tantalum dopant. Neutrons are resonantly absorbed, resulting in "holes" in the time-of-flight energy spectrum detected some meters away. The temperature of the sample can be determined from the width of the "holes". Calculations show that such an experiment fielded on Atlas would could allow a few-percent accurate measurement of temperature in the 10 eV range. More information on this application can be found here.
The development of a high-resolution 14 MeV neutron radiography capability for explosively driven experiments will provide an enhanced diagnostic capability for the weapons community. Fast neutrons readily penetrate thick layers of heavy metals, while at the same time being easily scattered by low-atomic-number materials, particularly hydrogen bearing materials. This makes them an ideal tool for diagnosing shock phenomena in mixed metal-explosive systems such as those of interest in nuclear weapons. Neutron radiography is widely used as a non-evasive diagnostic for static systems, but has not been used extensively for dynamic systems because of the lack of adequate neutron sources. Dynamic applications require high-flux sources with millimeter-diameter-spot size for adequate spatial resolution. An available neutron source employed in a recent Nevada shot generated 109 neutrons in a several square centimeter spot over multi-microsecond time duration. A system adapted from CHAMP would be capable of several times 1012 neutrons in few-hundred-nanoseconds burst focused to a 4-mm-diameter spot. Spatial resolution from this source would be less than 1 mm for a typical experiment. Work by LLNL suggests that 1010 neutrons/cm2 on target is sufficient to resolve 1 mm3 voids in LiH sandwiched between depleted uranium plates, and larger features can be resolved with neutron fluxes approximately 30x lower. By utilizing fast capacitor banks which exist at LANL, coupled with well-understood pulse compression techniques, the duration of the neutron pulse could be reduced to 10 ns. A team at KFK (Karlsruhe, Germany) has generated a 50 kJ, 60 ns, beam of 1.4 MeV protons focused to a 4-mm-diam. spot . For this application, we only need a 200 keV beam and factor of 5 less beam energy (about 10 kJ), so our beam requirements have been already been exceeded. CHAMP can serve as a starting point and proof of principle, from which a faster pulse and smaller spot size can be developed.
Although underground testing has ended, our responsibility for maintaining the safety and reliability of our weapons stockpile has not. Subcritical experiments have taken on a high importance. A method of measuring instantaneous neutron multiplication (keff, or equivalently, g) in an HE driven dynamic subcritical assembly is needed to understand the effects of SNM alloying, aging, and fabrication techniques on the performance of primaries. Determination of keff is one of the primary methods of matching experimental weapons data to modeling results. In the past (for instance, during the first nuclear test moratorium), similar measurements were made using steady-state neutron sources. Such measurements are difficult to interpret because a time-integrated value of keff is produced. In addition, cross talk with neutrons derived from and scattered from surrounding non-SNM is difficult to eliminate. A clean measurement is possible by looking directly at g's produced by fission events derived from a very short (1-10 ns) neutron pulse. These g's are discriminated from the neutrons by time-of-flight. This is effectively a measurement of the impulse response of the system. Such measurements are possible with a modest extension of the state-of-the-art. Using an intense deuteron ion beam impinging upon a tritiated target, it should be possible to produce 1010-1012 neutrons in a 10 ns pulse. Such a beam can be produced by a magnetized ion diode similar to that developed for CHAMP, driven by a fast capacitor bank lower neutron fluxes (from D-D reactions) over longer times have already been demonstrated with this technology on the Anaconda IPIB at LANL. This technology is appropriate for downhole testing at NTS only modest cost equipment (the diode and detectors) need be blown up.
Intense Diagnostic Neutral Beam for Advanced Tokamaks
Intense ion beam technology has been proposed to make a diagnostic neutral beam source for active spectroscopic measurements on ITER . Instead of the usual steady (or slowly modulated) beams with less than 1 A/cm2 current density, microsecond pulses of 100 A/cm2 would be used. Operated at 100 keV/AMU, near the peak of the charge-exchange recombination spectroscopy (CXRS) cross section, these beams would increase signal-to-noise ratio because their high-intensity coupled with very short gating times on the detectors reduce the bremsstrahlung background. Not only would these beams provide vital measurements in the plasma core, but they would do so with time resolution governed by the beam repetition rate. Such a single shot neutral beam was developed, and it was demonstrated that effective charge neutralization could be achieved in a gas cell at current densities of 20 A/cm2 (only a factor of 3-7 below the required value depending on the diode diameter) . The remaining essential issues in the application of this technology to CXRS involve the development of a suitable high-power, repetitive source that can be integrated into the ITER environment. CHAMP is an good prototype of such a source.
For more photos of the CHAMP system, click here.
 V.M. Bystritskii and A.N. Didenko, High-Power Ion Beams, American Institute of Physics, New York, 1989.
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 For a comprehensive review of material treatment with IPIBs, see Davis, H.A., Remnev, G.E., Stinnett, R.W., and Yatsui, K, "Intense Ion Beam Treatment of Materials," Mater. Res. Soc. Bull., 21 58 (1996).
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 Bluhm, H.J., Hoppe, P.J.W., Laqua, H.P., and Rusch, D., "Production and Investigation of TW Proton-Bemas from an Annular Diode Using Strong Radial Magnetic Insulation Fields and a Preformed Anode Plasma Source," Proc. IEEE, 80 995 (1992).
 Rej, D.J., Henins, I., Fonck, R.J., and Kim, Y.J., "Intense Diagnostic Neutral Beam Development for ITER," Rev. Sci. Instrum., 63 4934 (1992).
 Bartsch, R.R., Davis, H.A., Henins, I., and Greenly, J.B., "High-Intensity Neutral Beam for Beam-Spectroscopy Diagnostics," Rev. Sci. Instrum., 66 306 (1995).
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