T.L. Grimm

Stanford University, Palo Alto, California, United States

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Publications (86)117.96 Total impact

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    ABSTRACT: The overall mechanical properties of the high purity niobium (RRR) sheets, which are used in fabrication of the superconducting accelerator cavities, were investigated in this study. High purity niobium has a microstructure that leads to a rough surface on the free surfaces after plastic deformation. High purity niobium is a rate sensitive material and its strength and ductility increases with increasing strain rate. Due to the presence of an unstable texture, the r values in high purity niobium sheets decrease with plastic deformation. From tensile tests in different in-plane directions and bi-axial bulge tests, a new mathematical model was developed to describe the evolving yield function that predicts the variation of the r values with respect to the effective strain and angle to the rolling direction. From this analysis, suggestions for optimal microstructure and texture for fabricating cavities from high purity niobium are proposed.
    Full-text · Article · Nov 2006 · Materials Science and Engineering A
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    ABSTRACT: Secondary beams at the National Superconducting Cyclotron Laboratory (NSCL) are separated through a combined application of magnetic rigidity and energy loss filtering. Design and construction of a Radio Frequency Fragment Separator (RFFS) for further beam purification is underway. The RFFS will apply a time-varying electromagnetic field to induce transverse beam separation. This method relies on velocity differences of the beam species to selectivey apply separation to unwanted fragments. The technical design of the RFFS and the expected purification of exotic beams are shown in detail[1]. [1] Gorelev, D. et al., ``RF Kicker System for Secondary Beams at the NSCL'' Proc of Part Accel Conf 2005, Knoxville, TN
    No preview · Article · Oct 2006
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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.
    Preview · Article · Jul 2006 · Physica C Superconductivity
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    H. Jiang · T.R. Bieler · C. Compton · T.L. Grimm
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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〉.
    Full-text · Article · Jul 2006 · Physica C Superconductivity
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    D. Meidlinger · T.L. Grimm · W. Hartung
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    ABSTRACT: For high current applications, it is desirable for the cavity shape to have a low longitudinal loss factor and to have a high beam-breakup threshold current. This paper briefly describes three different cavities designed for this purpose: a six-cell elliptical cavity for particles traveling at the speed of light, a two-cell elliptical cavity for subluminal particle speeds, and a single cell cavity which uses the TM012 mode for acceleration. SUPERFISH simulations predict the peak fields in both of the elliptical cavities will not exceed the TeSLA values by more than 10% but both will have 28.7% larger apertures. The elliptical designs assume the bunch frequency equals the accelerating mode frequency. The beam pipe radius is chosen so that the cutoff frequency is less than twice that of the accelerating mode. Hence all of the monopole and dipole higher-order modes (HOMs) that can be driven by the beam have low loaded Q values. This simplifies the problem of HOM damping. The TM012 cavity is predicted to have much higher peak fields than a π-mode elliptical cavity, but offers potential advantages from its simplified shape; it is essentially a circular waveguide with curved end plates. This basic shape results in easier fabrication and simplified tuning.
    Preview · Article · Jul 2006 · Physica C Superconductivity
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    H. Jiang · T.R. Bieler · C. Compton · T.L. Grimm
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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.
    Full-text · Article · Jul 2006 · Physica C Superconductivity
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    A. Aizaz · T. L. Grimm
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    ABSTRACT: The accelerating gradients in superconducting RF cavities can be limited by excessive temperature rise on the inner surface. In some circumstances, such as high RF frequency or anomalous losses, improved heat transfer at the niobium‐helium interface can increase the achievable gradient. Different surface morphology techniques have been applied to reduce the limitation at the thermal interface. These techniques include varying surface roughness and forming cooling channels (embedded fins) in the surface. Heat transfer measurements on several niobium samples of different surface states, but same bulk purity at both Helium I and II (super‐fluid) temperature regimes are planned. Initial measurements to validate the experimental setup on one sample are presented. Comparison of the test measurements is made with the existing literature data. The surface characterization uses a high‐resolution 3D optical nano‐scope. Finally, the influence of these interface surface techniques on the performance of the cavities will be discussed.
    Preview · Article · Apr 2006 · AIP Conference Proceedings
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    ABSTRACT: Structural analyses of half-reentrant mid-cell and multi- cell niobium cavities were performed, including the static and dynamic response. The effects of a helium vessel and stiffening rings were explored. Results were compared to other cell shapes. With the proper stiffening system, the structural properties of the various cavities are similar.
    Full-text · Article · Jan 2006
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    ABSTRACT: As superconducting niobium cavities achieve higher gra- dients, it is anticipated they will reach a performance limit as the peak surface magnetic field approaches the critical magnetic field. Low loss (1) and reentrant (2) cavity de- signs are being studied at CEBAF, Cornell, DESY, and KEK, with the goal of reaching higher gradients via lower surface magnetic field, at the expense of higher surface electric field. At present, cavities must undergo chemical etching and high-pressure water rinsing to achieve good performance. It is not clear whether this can be done ef- fectively and reliably for multi-cell low loss or reentrant cavities using traditional techniques. A half-reentrant cav- ity shape has been developed with RF parameters similar to the low loss and reentrant cavities, but with the advantage that the surface preparation can be done easily with existing methods. Two 1.3 GHz prototype single-cell half-reentrant cavities have been fabricated; the non-reentrant wall angle is 8◦, the beam tube radius is 29 mm, and the cell-to-cell coupling is 1.47%. The half-reentrant cavity design and status of the prototyping effort is presented.
    Full-text · Article · Jan 2006
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    ABSTRACT: An L-band superconducting cavity has been designed for acceleration of particles travelling at 81% the speed of light (β = 0.81). Four single-cell prototypes have been being fabricated and tested. Two of these cavities were formed from standard high purity fine grain niobium sheet. The rest were fabricated from large grain niobium. The RF per- formance of the single-cell cavities indicate that the design is suitable for use in a proton linac; the highest measured accelerating gradient was about 28 MV/m. The fabrication of two 7-cell cavity prototypes is in progress.
    Full-text · Article · Jan 2006
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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.
    Full-text · Article · Sep 2005 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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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.
    No preview · Article · Jul 2005 · IEEE Transactions on Applied Superconductivity
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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.
    Full-text · Article · Jul 2005 · IEEE Transactions on Applied Superconductivity
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    X. Wu · M. Doleans · D. Gorelov · T.L. Grimm · F. Marti · R.C. York
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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.
    Full-text · Conference Paper · Jun 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.
    Full-text · Conference Paper · Jun 2005
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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.
    Full-text · Article · Apr 2005 · Review of Modern Physics
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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.
    Full-text · Article · Mar 2005
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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.
    Full-text · Article · Jan 2005
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    M Doleans · D Gorelov · TL Grimm · F Marti · X Wu · RC York
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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.
    Full-text · Article · Jan 2005
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    Full-text · Article · Jan 2005

Publication Stats

212 Citations
117.96 Total Impact Points

Institutions

  • 2010
    • Stanford University
      • Department of Physics
      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
      • • Department of Physics and Astronomy
      • • National Superconducting Cyclotron Laboratory (NSCL)
      East Lansing, MI, United States