T.L. Grimm

Stanford University, Palo Alto, California, United States

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Publications (69)102.16 Total impact

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    ABSTRACT: Superconducting radio frequency (SRF) cavities used in present-day accelerators for the acceleration of charged particles near the speed of light are based on the axially symmetric TM010 mode of a pillbox cavity. Future accelerators such as the Linear Collider require high accelerating gradients to limit the length of the linac. Two techniques to improve the gradient are being explored: a cavity that is half reentrant to improve the electromagnetic characteristics, and improved heat transfer via cooling channels and surface modification at the helium interface. These changes could potentially increase the gradients and reduce the cryogenic losses. For other applications more important criteria are simplicity, acceleration of high beam current, or the ability to use advanced materials such as Nb<sub>3</sub>Sn or high-T<sub>c</sub> superconductors. A new type of cavity based on the TM01p pillbox mode with p>0 offers such improvements.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: The Rare Isotope Accelerator uses 805 MHz superconducting rf cavities in the linac. Power is transmitted capacitively into the cavities via a high power input coupler. The coupler was designed for greater than 10 kW cw with a VSWR less than 1.05. The design load to the 2 K liquid helium is less than 2 W. The external Q of the coupler is about 2×10<sup>7</sup>. The couplers were conditioned off-line to over 200 kW pulsed before installation into a prototype cryomodule. The cryomodule was tested at 2 K to full accelerating gradients. Multipacting barriers in the coupler were quickly conditioned, and no arcs or discharges were observed during testing. Details of the power couplers performance will be presented.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: Previous end-to-end beam dynamics simulation studies [1] using experimentally-based input beam parameters [2], including alignment and rf errors and variation in charge-stripping foil thickness have indicated that the Rare Isotope Accelerator (RIA) driver linac proposed by Michigan State University (MSU) has transverse and longitudinal acceptances more than adequate to accelerate light and heavy ions to final energies ≥ 400 MeV/u with beam powers of 100 to 400 kW. Further beam dynamics studies [3] were carried out using a new beam envelope code recently developed at MSU to optimize the setting of the rf phase and amplitude of the cavities throughout the linac. During linac operation, equipment loss due to, for example, cavity contamination, problems with cryogenic systems, or failure of rf or power supply systems, can lead to, at least, a temporary loss of some of cavities and focusing elements. To achieve high facility availability, each segment of the linac should be capable of adequate performance even with some failed elements. In order to prove the flexibility and robustness of the driver linac lattice design, beam dynamics studies were performed to evaluate the linac performance under various scenarios of failed cavities and focusing elements with proper correction schemes. The result of these beam dynamics studies is presented in this paper.
    Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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    ABSTRACT: The design and construction of a radio frequency fragment separator (RFFS) kicker system at the National Superconducting Cyclotron Laboratory (NSCL) of Michigan State University (MSU) has been proposed. This RFFS will be used to further purify secondary beams of rare isotopes after the exiting the A1900 Fragment Separator and will open a wide range of possibilities for new experiments at the forefront of nuclear science. The proposed system is studied as an efficient alternative to the traditional approach using a Wien filter. Rare neutron deficient secondary beams are challenging to purify because of the presence of intense contaminants that cannot be removed by the traditional energy loss method. However, velocity differences resulting in time-of-flight (TOF) differences can be used for the effective separation of the beams transversely using the time-varying electromagnetic fields of the RF kicker. Its technical design is presented together with the beam dynamics analysis of a secondary beam in realistic 3D electromagnetic fields. The expected purification improvement of the exotic beams for the foreseen nuclear physics experiments is shown in detail.
    Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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    ABSTRACT: The low energy beam transport (LEBT) system in the front-end of the Rare Isotope Accelerator (RIA) uses a 70 kV platform to pre-accelerate the ion beam from a 30 kV Electron Cyclotron Resonance (ECR) ion source, followed by an achromatic charge selection system. The selected beam is then pre-bunched and matched into the entrance of a Radio Frequency Quadrupole (RFQ) with a multi-harmonic buncher. To meet the beam power requirements for heavy ions, high current (several mA), multi-species beams will be extracted from the ECR. Therefore, it is crucial to control space charge effects in order to obtain the low emittance beam required for RIA. The PARMELA code is used to perform the LEBT simulations for the multi-species beams with 3D space charge calculations. The results of the beam dynamics simulations are presented, and the key issues of emittance growth in the LEBT and its possible compensation are discussed.
    AIP Conference Proceedings. 03/2005; 749(1).
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    ABSTRACT: Two superconducting quarter-wave resonator (QWR) prototypes have been fabricated and tested. They operate at 80.5 MHz and 161 MHz and are optimised for beta = 0.085 and beta = 0.16, respectively. The prototypes are simplified versions without integrated helium vessels. In the first RF tests, the beta = 0.085 QWR reached a peak surface electric field (Ep) in excess of 30 MV/m, with an intrinsic quality factor (Q0) in excess of 1E9 at the design field of Ep = 20 MV/m. The beta = 0.16 QWR reached Ep = 20 MV/m with Q0 = 2.5E9. It is suspected that the performance of the latter cavity can be improved via better cooling of the Nb tuning plate and a better RF contact between the plate and the outer conductor.
    01/2005;
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    ABSTRACT: A control method, known as adaptive feedforward cancellation (AFC), is applied to damp sinusoidal disturbances due to microphonics in superconducting radio frequency (SRF) cavities. AFC provides a method for damping internal and external sinusoidal disturbances with known frequencies. It is preferred over other schemes because it uses rudimentary information about the frequency response at the disturbance frequencies, without the necessity for an analytic model (transfer function) of the system. It estimates the magnitude and phase of the sinusoidal disturbance inputs and generates a control signal to cancel their effect. AFC, along with a frequency estimation process, is shown to be very successful in the cancellation of sinusoidal signals from different sources. The results of this research may significantly reduce the power requirements and increase the stability for lightly loaded continuous-wave SRF systems.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2005; · 1.14 Impact Factor
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    ABSTRACT: The Rare Isotope Accelerator (RIA) driver linac will use a superconducting, cw linac with independently phased superconducting rf cavities for acceleration and utilize beams of multiple-charge-states (multi-q) for the heavier ions. Given the acceleration of multi-q beams and a stringent beam loss requirement in the RIA driver linac, a new beam dynamics code capable of simulating nonlinearities of the multi-q beam envelopes in the longitudinal phase space was developed. Using optimization routines, the code is able to maximize the linearity of the longitudinal phase space motion and thereby to minimize beam loss by optimizing values for the amplitude and phase of the cavities for a given accelerating lattice. Relative motion of the multi-q beams is also taken into account so that superposition of the beam centroids and matching of their Twiss parameters are automatically controlled. The new tuning procedure and its benefit on the performance of the beam dynamics in the longitudinal plane are discussed in the paper.
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    ABSTRACT: Three prototype 6-cell superconducting cavities for acceleration in the velocity range of 0.40 to 0.53 times the speed of light have been fabricated. The quality factor (Q) of the first prototype cavity was above 1E10 for accelerating gradients up to 11 MV/m. The highest gradient reached was about 16 MV/m; the Q was about 3E9 at the maximum gradient.
    12/2003;
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    ABSTRACT: The effects of Electron Beam Welding on solidification microstructure, texture, microhardness and mechanical properties were investigated in high purity niobium weld specimens. The welds have an equiaxed microstructure with a 1 mm grain size in the fusion zone, 100 μm in the heat affected zone (HAZ) and 50 μm in the parent metal. The fusion zone had slightly higher microhardness values despite having a large grain size, while the unaffected material had the lowest microhardness. The texture in the weld consisted of a strong {111} fiber texture in the center and a mix of {111} - {100} components on the surface. Tensile tests of specimens gave σ<sub>y</sub> = 60 MPa, but the UTS and elongation for weld specimens were lower than the parent material (137 vs. 165 MPa, 32% vs. 58%). The properties and microstructure of the weld are discussed in terms of optimizing the SRF cavity.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: The Rare Isotope Accelerator (RIA) driver linac will produce >400 MeV/u proton through uranium beams using many types of superconducting accelerating cavities such as quarter wave, spoke, and elliptical cavities. A cryomodule design that can accommodate all of the superconducting cavity and magnet types is presented. Alignment of the cold mass uses a titanium rail system, which minimizes cryomodule size, and decreases both the tunnel cross-section and length. The titanium rail is supported from the top vacuum plate by an adjustable tri-link, which is similar to existing Michigan State University magnet technology. A prototype cryomodule is under construction for testing 805 MHz, v/c=0.47, six-cell niobium cavities in realistic operating conditions. Details of the design and progress to date are presented.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: The Rare Isotope Accelerator (RIA) will accelerate heavy ions to >400 MeV/u using an array of superconducting cavities. A proposed linac design based on harmonics of 80.5 MHz will require six cavity types to cover the entire velocity range: three quarter wave resonators, one spoke cavity (half wave resonator), and two 6-cell elliptical cavities. A prototype 322 MHz niobium spoke with optimum velocity of 0.28 c has been fabricated. Each spoke would generate over 1 MV at 4 K for acceleration from v/c=0.20 to 0.40. Details of the design and experimental study are presented.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    S.E. Musser, T.L. Grimm, W. Hartung
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    ABSTRACT: Field emission loading limits the performance of a significant fraction of the cavities in existing superconducting accelerators. The field emission produces an additional load to the cryogenic system; it is a source of dark current and background radiation in the accelerator; and it can lead to RF breakdown if the cavity is pushed to its limits. The field-emitted electrons are accelerated by the RF field and strike the cavity wall, generating Bremsstrahlung X-rays. The regions of X-ray emission (intensity and energy spectrum) can be located by using a collimated NaI detector placed outside the cryostat and radiation shield. The X-ray emission sites can be reconstructed using tomographic techniques. Particle tracking simulations can be used to trace the field emission electrons back to their source in order to help identify the locations of the surface defects.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: The Rare Isotope Accelerator (RIA) is being designed to supply an intense beam of exotic isotopes for nuclear physics research. Superconducting cavities are to be used to accelerate the CW beam of heavy ions to 400 MeV per nucleon, with a beam power of up to 400 kW. Because of the varying beam velocity, several types of superconducting structures are needed. This paper covers the fabrication of three prototype RIA 6-cell β<sub>g</sub> = 0.47 cavities and the RF tests on the first and second of these cavities.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: The National Superconducting Cyclotron Laboratory (NSCL) design for the Rare Isotope Accelerator (RIA) driver linac uses superconducting quarter-wave, half-wave and 6-cell elliptical cavities with rf frequencies ranging from 80.5 MHz to 805 MHz with two charge-stripping chicanes. The driver linac requirements include acceleration of light and heavy ions to final beam energies of ≥400 MeV/nucleon with final beam powers of 100 to 400 kW. The impact of simultaneous misalignment and rf errors for the full RIA driver linac, including the charge-stripping chicanes, on the 6-dimensional beam emittance was evaluated by simulation. Beam loss and large-amplitude beam behaviors were also studied.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: Beam dynamics simulations of the proposed Rare Isotope Accelerator (RIA) driver linac have been done. The RIA driver linac is designed to accelerate stable ion beams from proton to uranium to final energies of 400 MeV/u for the heaviest and about 900 MeV/u for the lightest ions with beam powers of 100 to 400 kW. Two stripping sections are used to increase the charge state of heavy ions and minimize the total accelerating voltage required. To achieve the final beam power and to reduce the ion source requirements, multi-charge state beam acceleration is used. Multi-spoke structures in the high-energy part of the driver linac have been proposed as an alternative to the baseline design of 6-cell elliptical structures. A comparative analysis of this alternative is explored including beam dynamics, error constraints, and manufacturing issues.
    Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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    ABSTRACT: A superferric quadrupole for use in a superconducting radio frequency (SRF) cryomodule has been designed and constructed. Even though the quad has a poor aspect ratio of length-to-aperture, the external fringe field is required to be small so that the SRF can operate at the high gradients required for the ISCL. Calculations of the external fringe fields required an iron flux clamp to obtain the necessary attenuation. Room temperature testing of the magnet confirmed the magnetic length.
    IEEE Transactions on Applied Superconductivity 04/2002; · 1.20 Impact Factor
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    ABSTRACT: This paper will cover the design of a 6-cell β<sub>g</sub> = 0.47 cavity for the Rare Isotope Accelerator, as well as the fabrication and RF testing of single-cell prototypes. Single-cell prototypes were chosen as a first step, as they provide a quick and inexpensive way to find out whether the desired field level and Q can be reached, and to check for problems with multipacting. An accelerating gradient of 8 MV/m was chosen as a goal for the β<sub>g</sub> = 0.47 cavity
    Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001; 02/2001