T.L. Grimm

Stanford University, Palo Alto, California, United States

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Publications (80)148.74 Total impact

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    ABSTRACT: A study was conducted to characterize the microstructure of electron beam welds in high purity niobium and its effect on creep behaviour at room temperature. The parent material was 2mm sheet with a 50μm grain size. The weld fusion zone had ∼1mm grains, implying that these grains all intersected the free surface. The parent material showed no room temperature creep deformation below the yield stress, but room temperature creep of weld specimens caused up to 10% strain in the weld region at ∼75% of the yield strength, over 1–2 months. Creep deformation was not smooth or continuous; the strain saturated at some value, and then after an incubation time, the strain increased and saturated again several times over 1–2 months. The magnitude of the strain for several specimens was similar but the creep deformation behavior was highly dependent on the actual microstructure and loading history. An initial prestrain with unloading shut down the creep deformation mechanism at the prior stress due to a dislocation-locking effect. The local stresses in the weld fusion zone arose from anisotropic elastic interactions due to different crystal orientations that caused local regions to exceed the yield strength.
    Physica C Superconductivity 01/2006; 441(1):122-125. · 0.72 Impact Factor
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    M. Meidlinger, T. L. Grimm, W. Hartung
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    ABSTRACT: The shape of a TeSLA inner cell can be improved to lower the peak surface magnetic field at the expense of a higher peak surface electric field by making the cell reentrant. Such a single-cell cavity was designed and tested at Cornell, setting a world record accelerating gradient [V. Shemelin et al., An optimized shape cavity for TESLA: concept and fabrication, 11th Workshop on RF Superconductivity, Travemünde, Germany, September 8–12, 2003; R. Geng, H. Padamsee, Reentrant cavity and first test result, Pushing the Limits of RF Superconductivity Workshop, Argonne National Laboratory, September 22–24, 2004]. However, the disadvantage to a cavity is that liquids become trapped in the reentrant portion when it is vertically hung during high pressure rinsing. While this was overcome for Cornell’s single-cell cavity by flipping it several times between high pressure rinse cycles, this may not be feasible for a multi-cell cavity. One solution to this problem is to make the cavity reentrant on only one side, leaving the opposite wall angle at six degrees for fluid drainage. This idea was first presented in 2004 [T.L. Grimm et al., IEEE Transactions on Applied Superconductivity 15(6) (2005) 2393]. Preliminary designs of two new half-reentrant (HR) inner cells have since been completed, one at a high cell-to-cell coupling of 2.1% (high-kcc HR) and the other at 1.5% (low-kcc HR). The parameters of a HR cavity are comparable to a fully reentrant cavity, with the added benefit that a HR cavity can be easily cleaned with current technology.
    Physica C Superconductivity 01/2006; 441(1):155-158. · 0.72 Impact Factor
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    ABSTRACT: A tapered wedge niobium specimen was rolled at room temperature with multiple passes in the same direction without lubricant and then annealed at 750°C for 1h. The crystal orientation distribution of the 50%, 70%, 80%, 90% deformed samples was investigated using X-rays to obtain a quantitative texture analysis. The initial rotated cube {001}〈110〉 texture was largely retained up to about 70% reduction in the interior of the samples. After 80% rolling deformation the initial texture vanished and revealed a {111} fiber texture in the interior, which remained stable during annealing. With 90% reduction, the {111} fiber texture become somewhat stronger. In the surface layer, the 001 fiber orientation remained stable but after annealing, the surface texture sharpened to become {001}〈110〉.
    Physica C Superconductivity 01/2006; 441(1):118-121. · 0.72 Impact Factor
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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 09/2005; · 1.14 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: 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: 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.
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    ABSTRACT: Three 6-cell 805 MHz superconducting cavity prototypes for acceleration in the velocity range of about 0.4 to 0.53 times the speed of light have been fabricated and tested. The quality factors (Q0) were between 7×109 and 1.4×1010 at the design field (accelerating gradient of 8 10 MV/m). The maximum gradients reached were between 11 and 16 MV/m; in each case, the Q0 values were >=3×109 at the maximum gradient. The design, fabrication, surface preparation, and rf testing of the 6-cell cavities are reported in this paper.
    Review of Modern Physics 01/2005; 8(4). · 44.98 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

Publication Stats

140 Citations
148.74 Total Impact Points

Institutions

  • 2010
    • Stanford University
      Palo Alto, California, United States
  • 2008
    • Northern Illinois University
      DeKalb, Illinois, United States
  • 1996–2007
    • Michigan State University
      • • Department of Chemical Engineering and Materials Science
      • • National Superconducting Cyclotron Laboratory (NSCL)
      East Lansing, MI, United States