C. Piller

Oak Ridge National Laboratory, Oak Ridge, FL, United States

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Publications (19)7.65 Total impact

  • Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2014; 763:610 - 673. · 1.32 Impact Factor
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    ABSTRACT: Recent measurements of the H(-) beam current show that SNS is injecting about 55 mA into the RFQ compared to ∼45 mA in 2010. Since 2010, the H(-) beam exiting the RFQ dropped from ∼40 mA to ∼34 mA, which is sufficient for 1 MW of beam power. To minimize the impact of the RFQ degradation, the service cycle of the best performing source was extended to 6 weeks. The only degradation is fluctuations in the electron dump voltage towards the end of some service cycles, a problem that is being investigated. Very recently, the RFQ was retuned, which partly restored its transmission. In addition, the electrostatic low-energy beam transport system was reengineered to double its heat sinking and equipped with a thermocouple that monitors the temperature of the ground electrode between the two Einzel lenses. The recorded data show that emissions from the source at high voltage dominate the heat load. Emissions from the partly Cs-covered first lens cause the temperature to peak several hours after starting up. On rare occasions, the temperature can also peak due to corona discharges between the center ground electrode and one of the lenses.
    The Review of scientific instruments 02/2014; 85(2):02B137. · 1.58 Impact Factor
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    ABSTRACT: Extraction of negative ions from a saddle antenna radio-frequency surface plasma source is considered. Several versions of new plasma generators with different antennas and magnetic field configurations were tested in the smal Oak Ridge National Laboratory Spallation Neutron Source Test Stand. The efficiency of positive ion generation in plasma has been improved to 200 mA/cm(2) kW from 2.5 mA/cm(2) kW. A small oven was developed for cesiation by cesium compounds and alloy decomposition. After cesiation, a current of negative ions to the collector was increased from 1 mA to 10 mA with 1.5 kW RF power in the plasma and longitudinal magnetic field Bl ∼ 250 G. The specific efficiency of H(-) production was increased to 20 mA/cm(2) kW from 2.5 mA/cm(2) kW.
    The Review of scientific instruments 02/2014; 85(2):02B111. · 1.58 Impact Factor
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    ABSTRACT: Progress on the development of RF H- surface plasma source (SPS) with saddle antenna radio frequency (SA RF) discharge is considered. Several versions of new plasma generators with different antennas and magnetic field configurations were tested in the SNS small Test Stand. The efficiency of positive ion generation in plasma has been improved up to 0.18 A/cm2 per 1 kW of RF power at 13.56 MHz. A first prototype SA SPS with AlN chamber was installed in the SNS Test Stand that achieved current of H- ions up to 67 mA with an apparent efficiency of up to 1.6 mA/kW at RF frequency 2 MHz. A new version of the RF assisted triggering plasma source (TPS) has been designed, fabricated and tested. After cesiation emission of negative ions was increased from 1 mA to 5 mA with RF power 2.5 kW.
    02/2013;
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    ABSTRACT: Improving efficiency of plasma generation in RF H -surface plasma source (SPS) with saddle (SA) RF antenna is considered. Several versions of new plasma generators with different antennas and magnetic field configurations were tested in the SNS small Test Stand. The efficiency of positive ion plasma generation has been improved ~4x times up to 0.18 A/cm 2 per 1 kW of RF power 13.56 MHz. A first prototype SA SPS with AlN chamber was installed in the SNS Test that achieved current of H-ions up to 67 mA with an apparent efficiency of up to 1.6 mA/kW at RF frequency 2 MHz. A new version of the RF assisted triggering plasma source (TPS) has been designed, fabricated and tested. A Saddle antenna SPS with water cooling is being fabricated for high duty factor have been tested.
    RU PAC 2012, http://accelconf.web.cern.ch/AccelConf/rupac2012/papers/tuppb058.pdf; 09/2012
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    ABSTRACT: Since 2009, the Spallation Neutron Source (SNS) has been producing neutrons with ion beam powers near 1 MW, which requires the extraction of ∼50 mA H(-) ions from the ion source with a ∼5% duty factor. The 50 mA are achieved after an initial dose of ∼3 mg of Cs and heating the Cs collar to ∼170 °C. The 50 mA normally persist for the entire 4-week source service cycles. Fundamental processes are reviewed to elucidate the persistence of the SNS H(-) beams without a steady feed of Cs and why the Cs collar temperature may have to be kept near 170 °C.
    The Review of scientific instruments 02/2012; 83(2):02A732. · 1.58 Impact Factor
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    ABSTRACT: A prototype RF H(-) surface plasma source (SPS) with saddle (SA) RF antenna is developed which will provide better power efficiency for high pulsed and average current, higher brightness with longer lifetime and higher reliability. Several versions of new plasma generators with small AlN discharge chambers and different antennas and magnetic field configurations were tested in the plasma source test stand. A prototype SA SPS was installed in the Spallation Neutron Source (SNS) ion source test stand with a larger, normal-sized SNS AlN chamber that achieved unanalyzed peak currents of up to 67 mA with an apparent efficiency up to 1.6 mA∕kW. Control experiments with H(-) beam produced by SNS SPS with internal and external antennas were conducted. A new version of the RF triggering plasma gun has been designed. A saddle antenna SPS with water cooling is fabricated for high duty factor testing.
    The Review of scientific instruments 02/2012; 83(2):02A712. · 1.58 Impact Factor
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    ABSTRACT: High-power RF ion sources produce intense beams of negative H- ions for high-power accelerators using charge-changing injection schemes. The inductively induced RF plasma produces copious amounts of positive ions, electrons, and excited molecules. Energetic electrons rapidly destroy H- ions with their 0.75 eV electron affinity. A ˜250 G filter field reflects the energetic electron while the cold electrons, the ions, and the molecules can drift towards the outlet. There slow electrons colliding with highly excited vibrating molecules form H- ions that can be extracted. However, production yields suggest that most negative ions are formed on a conical Mo converter surface, which surrounds the outlet. This appears to be especially true when the surface is covered with a fractional layer of Cs. The persistence of the extracted H- beam suggests that the Cs layer is persistent, likely due to low levels of impurities and hydrogen being to light to sputter Cs atoms from the metallically clean surface. Experimental evidence, data, and simple models will be presented to support our findings.
    11/2011;
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    ABSTRACT: The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory reached 1‐MW of beam power in September 2009, and now routinely operates near 1‐MW for the production of neutrons. This paper reviews the performance, operational issues, implemented and planned mitigations of the SNS H− ion source to support such high power‐level beams with high availability. Some results from R&D activities are also briefly described.
    AIP Conference Proceedings. 09/2011; 1390(1):216-225.
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    ABSTRACT: Pulsed, high-power RF ion sources are needed to produce copious amounts of negative H- ions for high-power accelerators with charge-changing injection schemes. When increasing the RF power, the plasma inductance changes the RF resonance, which drifts away from the low-power resonance. When the RF circuit is tuned to maximize the (pulsed) plasma power, the (off- resonance) power at the beginning of the pulse is reduced. If the induced electric fields fall below the breakdown strength of the hydrogen gas, the plasma fails to develop. This can be avoided with a compromise tune and/or by increasing the inductance of the resonant circuit. However, the breakdown strength of the hydrogen gas increases with time due to the gradual decrease of the electron-rich plasma impurities, which causes plasma outages after weeks of reliable operation. In this paper we discuss the success of different mitigations that were tested and implemented to overcome this fundamental problem of pulsed, high-power RF hydrogen ion sources.
    04/2011;
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    ABSTRACT: This paper reports the development of 350 MHz superconducting cavities of a spoke-loaded geometry, intended for the velocity range 0.2<v/c<0.6. Two prototype single-cell cavities have been designed, one optimised for velocity v/c=0.4, and the other for v/c=0.29. Construction of the prototype niobium cavities is nearly complete. Details of the design and construction are discussed, along with the results of cold tests
    Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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    ABSTRACT: A system has been developed at Jefferson Lab for measuring transverse position of very low current beams delivered to the Experimental Hall B of the Continuous Electron Beam Accelerator Facility (CEBAF). At the heart of the system is a position sensitive cavity operating at 1497 MHz. The cavity utilizes a unique design which achieves a high sensitivity to beam position at relatively low cavity Q. The cavity output RF signal is processed using a down-converter and a commercial lock-in amplifier operating at 100 kHz. The system interfaces with a VME based EPICS control system using the IEEE 488 bus. The main features of the system are simple and robust design, and wide dynamic range capable of handling beam currents from 1 nA to 1000 nA with an expected resolution better than 100 μm. This paper outlines the design of the system
    Particle Accelerator Conference, 1997. Proceedings of the 1997; 06/1997
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    ABSTRACT: An ion linac formed of superconducting rf cavities can provide a multi-beam driver accelerator for the production of nuclei far from stability. A multi-beam driver supports a wide variety of production reactions and methods. This paper outlines a concept for a 1.3 GV linac capable of delivering several hundred kilowatts of uranium beam at an energy of 400 MeV per nucleon. The linac would accelerate the full mass range of ions, and provide higher velocities for the lighter ions, for example 730 MeV for protons. The accelerator will consist of an ECR ion source injecting a normally conducting RFQ and four short IH structures, then feeding an array of more than 400 superconducting cavities of six different types, which range in frequency from 58 to 700 MHz. A novel feature of the linac is the acceleration of beams containing more than one charge state through portions of the linac, in order to maximize beam current for the heavier ions. Such operation is made feasible by the large transverse and longitudinal acceptance provided by the large aperture and high gradient which are characteristic of superconducting rf cavities.
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    ABSTRACT: This paper reports the development of 350 MHz niobium superconducting cavities for the velocity range 0.2< v/c
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    Yoon W Kang, Chip Piller, Aly Fathy
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    ABSTRACT: The RF ion source at Spallation Neutron Source has been upgraded to meet higher beam power requirement. One important subsystem for efficient operation of the ion source is the 2MHz RF impedance matching network. The real part of the antenna impedance is very small and is affected by plasma density for 2MHz operating frequency. Previous impedance matching network for the antenna has limited tuning capability to cover this potential variation of the antenna impedance since it employed a single tuning element and an impedance transformer. A new matching network with two tunable capacitors has been built and tested. This network can allow precision matching and increase the tunable range without using a transformer. A 5-element broadband matching network also has been designed, built and tested. The 5-element network allows wide band matching up to 50 kHz bandwidth from the resonance center of 2 MHz. The design procedure, simulation and test results are presented.
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    ABSTRACT: The 4 GeV CEBAF accelerator at Thomas Jefferson National Accelerator Facility (Jefferson Lab) is arranged in a five-pass racetrack configuration, with two superconducting radio-frequency (SRF) linacs joined by independent magnetic transport arcs. The 1497 MHz continuous electron beam is composed of 3 interlaced variable-intensity 499 MHz beams that can be independently directed from any of the five passes to any of the three experimental halls. Beam extraction is made possible by a system of nine warm sub-harmonic separator cavities capable of delivering a 100 microrad kick to any pass at a maximum machine energy of 6 GeV. Each separator cavity is a half-wavelength, two cell design with a high transverse shunt impedance and a small transverse dimension. The cavities are powered by 1kW solid state amplifiers operating at 499 MHz. Cavity phase and gradient control are provided through a modified version of the same control module used for the CEBAF SRF cavity controls. The system has recently been tested while delivering beam to Hall C. In this paper the authors present a description of the RF separator system and recent test results with beam.
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    ABSTRACT: The Spallation Neutron Source (SNS) * linear accelerator low-level RF control system has been developed within a collaboration of Lawrence Berkeley, Los Alamos, and Oak Ridge national laboratories. Three distinct generations of the system, previously described, have been used to support beam commissioning at Oak Ridge. The third generation system went into production in early 2004, with installation in the coupled-cavity and superconducting linacs to span the remainder of the year. The final design of this system will be presented along with results of performance measurements.
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    ABSTRACT: Jefferson Lab's CEBAF electron accelerator has recently begun delivering spin-polarized electrons for nuclear experiments. Spin-polarized electrons are emitted from a GaAs photocathode that is illuminated with pulsed laser light from a diode laser synchronized to the 3 rd subharmonic (499 MHz) of the accelerating cavity frequency (1497 MHz). Up to three experimental halls (A, B and C) are served by the photoinjector each with their own beam requirements. To accomplish this, three independent diode lasers are synchronized and combined to illuminate the GaAs photocathode emitting a 1497 MHz pulse train of electrons. In addition an RF bunching cavity approximately 2 m down stream from the photocathode is used to compensate for space charge effects at the higher beam currents. The RF system that controls these elements is a modified VME based system. Custom RF VME modules control phase and amplitude for each laser diode and the bunching cavity. Power requirements were satisfied with commercial RF amplifiers, 5 W for the diode lasers and 10 W for the bunching cavity. Simple software algorithms using the EPICS control system correct phase and amplitude drifts. The RF system is compact, simple and allows for easy hardware or software modifications.

Publication Stats

50 Citations
7.65 Total Impact Points

Institutions

  • 2011–2012
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, FL, United States
  • 1999
    • Argonne National Laboratory
      • Division of Physics
      Lemont, Illinois, United States