[en] The Kicker system is used to extract beam from the Rapid Cycling Synchrotron (RCS). The Kicker consists of four identical pulse circuits, each providing over 3.8 kA to each magnet winding. The magnet length is restricted to the space between vacuum bellows attached to the ring magnets. This leaves 0.89 m for the magnet. To keep the voltage low the magnet conductor is broke up into 4, 1/4-turn magnet windings. Each pulse circuit consists of a Pulse Forming Network (PFN) that is charged to 50 KV. The PFN is discharged through a thyratron into a 6.3-ohm transmission line to one of the magnet windings. Our system has always had marginal rise time of around 100 ns. Although the thyratron switching time is much faster than this, losses in the transmission lines cause the slower response. By using ferrite to make a fast switch between the transmission lines and the magnet, the rise time in the magnet can be reduced. To make a fast ferrite switch, the saturation point must carefully be chosen. Parameters related to choosing the proper ferrite to provide fast saturation, at the correct current will be discussed.

[en] A TEM' and soo. Nd:glass laser system operating at incident wavelengths of 1053 and 527 nm was used to perform Thomson-scattering diagnosis of compact toroid (CT) plasmas generated within the Compact Toroid Transport Experiment (CTTX). Thomson scattering was employed in order to obtain local plasma measurements necessary for CTTX confinement-physics studies. CT plasmas were created in initial static-fill pressures of 20, 100, and 150 mT of deuterium. Maximum CTTX forward and reversed magnetic-field induction values were 3.5 and 0.9 kG, respectively; the main field was crowbarred with a 1/3 decay time of 36 μs. CT plasmas generated within CTTX were found to have peak densities and temperatures of 2.5 x 10' and S16. cm' and S-3. and 20 eV, respectively. Plasma transport was dominated by classical processes in all three pressure cases. Thomson-scattering results were combined with data from other diagnostics; these included diamagnetic and magnetic field probes, a high-speed, image-converter camera, and a Mach-Zender interferometer. Combined diagnostic results corroborated CT behavior such as axial contraction after field-line reconnection

[en] Numerical simulations indicate that it should be possible to use an electron beam to strip 1+ DC radioactive ion beams to 2+ or higher charge states with on the order of 50% efficiency. The device, which the authors call an Electron-Beam Charge-State Amplifier, is similar to an Electron Beam Ion Source, except that it is not pulsed, the beams are continuous. The 2+ beams are obtained in a single pass through a magnetic solenoid while higher charge states may be reached via multiple passes. An unexpected result of the ion optics simulations is that the normalized transverse emittance of the ion beam is reduced in proportion to the charge-state gain. Ion beams with realistic emittances and zero angular momentum relative to the optic axis before entering the solenoid will travel though the solenoid on helical orbits which intercept the axis once per cycle. With an ion beam about 2 mm in diameter and an electron beam about 0.2 mm in diameter, the ion stripping only occurs very near the optic axis, resulting in the emittance reduction

[en] The Intense Pulsed Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS) accelerates 3.2 x 10E¹² protons from 50 MeV to 450 MeV in a single bunch (h=1) at 30 Hz. The rf frequency varies from 2.21 MHz to 5.14 MHz during the 14.2 ms acceleration interval. To maintain stability of the bunch, phase modulation is introduced to the rf at approximately twice the synchrotron frequency (synchrotron tune is 0.0014). This phase modulation causes a parametric quadrupole oscillation to develop in the bunch, and as this occurs, the bunch spectrum shows a significant increase in high frequency content. Without phase modulation, the beam experiences an instability which results in the loss of a large fraction of the charge 2-4 ms prior to extraction. It is unclear if the stability imparted to the beam by phase modulation comes from the quadrupole oscillation or from the high frequency excitation. A longitudinal tracking code has been modified to include amplitude and phase modulation of the bunch. The numerical analysis is used to compare growth rates with those observed in the machine. The results of this analysis will be important as we introduce second harmonic rf with a new third cavity in the RCS later in 2005.

[en] The Intense Pulse Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS) accelerates 50 MeV protons to 450 MeV 30 times per second for spallation neutron production. Average current from the RCS has recently exceeded 16 μA with peak instantaneous current approaching 15 A. The RCS makes efficient use of 21 kV of RF accelerating voltage and uses phase-modulation between the two rf cavities to damp vertical instabilities. Split-ring electrodes in the ring suggest an anomalous tune shift that increases with time in the acceleration cycle. Based on a background gas pressure of 1 μTorr, the neutralization time for the beam is approximately 0.5 ms at injection suggesting the beam becomes fully neutralized relatively quickly in the cycle. Over-neutralization of the beam can lead to a positive tune shift that is presumably incoherent. Studies are underway to characterize the ionization within the RCS using the existing Profile and Position System (PAPS) and a newly installed Retarding Field Analyzer (RFA). Also a newly installed fast, deep-memory digitizing oscilloscope allows the entire history of a single acceleration cycle to be recorded from all four components of the split ring electrodes simultaneously at a rate of 250 MS/s

[en] A high-resolution Fabry--Perot interferometer (FPI) has been developed to diagnose plasma with low electron line densities. The technique is based on the fact that a cavity which is undergoing a small oscillation in length will periodically pass through a point of maximum sensitivity; in the present work the duration of this condition is about 100 μs. Line densities of approximately 10¹⁵ cm^-2 were examined; this corresponded to shifts on the order of one-thousandth of a fringe. A scanning FPI was used to tune in one axial mode of a 632.8-nm He--Ne laser with a linewidth of 2.0 x 10^-5 nm. The plasma was generated in an arc discharge formed in 50-mT air with electron densities of about 10¹⁴ cm^-3. A double Langmuir probe was used as the reference diagnostic. In this case, sensitivities to electron densities were 2--5 times greater than could be achieved with a standard double-pass device; order of magnitude improvements can be made with refinements

[en] Beam properties of a twice Q-switched Nd:glass laser operated at 1053 and 527 nm have been examined with respect to energy, pulse width, divergence, and conversion efficiency at a pulse separation of 75 μs. Q switching occurred twice within a flashlamp burst (FWHM: 160--175 μs). Maximum IR energies of 24 joules per pulse (Jpp) were measured with pulse widths of 25 and 40 ns and divergence values of 0.6 and 0.7 mrad for the first and second pulses, respectively. Maximum second-harmonic energies of 14 J (single pulse) and 4 Jpp were measured with peak power-density conversion efficiencies of 45% and 30%. Use of this laser at 1053 nm, in conjunction with generation III GaAs detectors, offers improvement over current source/detection systems (ruby/S-20) for diagnosis of high-temperature plasmas (T/sub e/ >10 keV)

[en] A helium atom beam has been extracted from a rf plasma source with beam energies ranging from 22 to 35 kV. At optimum conditions, a beam of approximately 15 mA equivalent current of 30-keV neutral atoms displays a minimum angular beam divergence of 0.3 degree. The beam system can be operated continuously in a chopped mode with a 50% duty factor by square-wave modulation of the extraction voltage. Frequency of the modulation can be varied up to 500 Hz. The beam system is also capable of dc operation, having been run in this mode for periods of time exceeding 2 h

[en] A beam of energetic H^- or D^- ions is efficiently converted to the uncharged state upon impact on a dense plasma. Neutralization efficiencies of the emerging beam can exceed 80%. This paper reports on a design approach to a plasma-stripping cell, illucidating several practical aspects. The plasma is formed by a low-frequency (1.4 MHz), high-power (20--60 kW) rf discharge in a multipole ring-cusp magnetic field. A mild steel rectangular box is used as a low-reluctance return path for the magnetic flux, a heat-transfer medium, and mechanical support. The working gases used in these studies are H₂, He, Ar, and Xe. The central density found in the resulting plasma ranged from about 10¹³ to >10¹⁴ cm^-3 with the heavier species exhibiting the higher densities. Test results performed over a pressure range of 2--10 mTorr are presented. The rf power required to achieve a given plasma density with this design should scale with the cell length for a given gas

[en] In the process of accelerating protons from 50 to 450 MeV at 30 Hz, low-energy electrons are generated within the IPNS RCS vacuum chamber. Electrons from background gas stripping are detected using an Ionization Profile Monitor (IPM) to generate integrated, horizontal charge distributions of the single-harmonic bunch during acceleration. Recently, a Retarding Field Analyzer (RFA) was installed in the RCS to look for evidence of beam-induced multipacting by measuring the electrons ejected by the space charge of the beam. A wide-band, high-gain transimpedance amplifier has been built to observe time structure in the electron signal detected with the RFA. Though a noisy power supply prevented full I-V characteristics from being obtained, interesting features are observed; especially, after the period of phase modulation between the rf cavities that is deliberately introduced during the cycle. The phase modulation generates a longitudinal quadrupole oscillation in the bunch, which is believed to enhance beam stability. Preliminary results indicate that electron multipacting is not significant in the RCS. The effects of background gas neutralization are considered and details of the RFA measurements are presented.