L. Ahrens

Brookhaven National Laboratory, New York City, NY, United States

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Publications (157)173.29 Total impact

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    ABSTRACT: As the world's only high energy polarized proton collider, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) has been provid-ing collisions of polarized proton beams at beam energy from 100 GeV to 255 GeV for the past decade to explore the proton spin structure as well as other spin dependent measurements. With the help of two Siberian Snakes per accelerator plus outstanding beam control, beam polariza-tion is preserved up to 100 GeV. About 10% polarization loss has been observed during the acceleration between 100 GeV and 255 GeV due to several strong depolarizing resonances. Moderate polarization loss was also observed during a typical 8 hour physics store. This presentation will give an overview the achieved per-formance of RHIC, both polarization as well as luminosity. The plan for providing high energy polarized He-3 colli-sions at RHIC will also be covered.
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    ABSTRACT: The 2013 operation of the Relativistic Heavy Ion Collider (RHIC) marks the second year of running under the RHIC II era. This year saw the implementation of several important upgrades designed to push the intensity frontier. Two new electron lenses to compensate beam-beam effects (e-lenses) have been partially installed, along with a new lattice designed for the e-lens operation. A new polarized proton source which generates about factor of 2 more intensity was commissioned as well as a host of RF upgrades ranging from a new longitudinal damper a new Landau cavity in RHIC to a new low level RF system and new beam bunching structure in AGS. We present an overview of the challenges and results from this years run.
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    ABSTRACT: The high energy (T = 250 GeV) polarized proton beam experiments performed in RHIC, require high polarization of the beam. In order to preserve the polarization of the proton beam, during the acceleration in the AGS, which is the pre-injector to RHIC, we have installed in AGS two partial helical magnets which minimize the loss of the beam polarization caused by the various intrinsic spin resonances occurring during the proton acceleration. The minimization of the polarization loss during the acceleration cycle, requires that the vertical tune of the AGS is between the values of 8.97 and 8.985 during the acceleration. With the AGS constrained to run at near integer tune â8.980, the perturbations to the beam caused by the partial helical magnets are large and also result in large beta and dispersion waves. To mitigate the adverse effect of the partial helices on the optics of the AGS, we have installed in specified straight sections of the AGS compensation quads and we have also generated a beam bump at the location of the cold partial helix. In this paper we present the beam optics of the AGS which ameliorates the adverse effect of the two partial helices on the beam optics.
    01/2012;
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    ABSTRACT: During the 2012 summer shutdown a pair of electron lenses will be installed in RHIC, allowing the beam-beam parameter to be increased by roughly 50 percent. To realize the corresponding luminosity increase bunch intensities have to be increased by 50 percent, to 2.5 · 10¹¹ protons per bunch. We list the various RHIC subsystems that are most affected by this increase, and propose beam studies to ensure their readiness. The proton luminosity in RHIC is presently limited by the beam-beam effect. To overcome this limitation, electron lenses will be installed in IR10. With the help of these devices, the headon beam-beam kick experienced during proton-proton collisions will be partially compensated, allowing for a larger beam-beam tuneshift at these collision points, and therefore increasing the luminosity. This will be accomplished by increasing the proton bunch intensity from the presently achieved 1.65 · 10¹¹ protons per bunch in 109 bunches per beam to 2.5 · 10¹¹, thus roughly doubling the luminosity. In a further upgrade we aim for bunch intensities up to 3 · 10¹¹ protons per bunch. With RHIC originally being designed for a bunch intensity of 1 · 10¹¹ protons per bunch in 56 bunches, this six-fold increase in the total beam intensity by far exceeds the design parameters of the machine, and therefore potentially of its subsystems. In this note, we present a list of major subsystems that are of potential concern regarding this intensity upgrade, show their demonstrated performance at present intensities, and propose measures and beam experiments to study their readiness for the projected future intensities.
    01/2012;
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    ABSTRACT: Proton bunch intensities in RHIC are planned to be increased from 2 · 10¹¹ to 3 · 10¹¹ protons per bunch to increase the luminosity, together with head-on beam-beam compensation using electron lenses. To study the feasibility of the intensity increase, beam experiments are being performed. Recent experimental results are presented.
    01/2012;
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    01/2012;
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    ABSTRACT: Over the last few years, physicists have occasionally observed the presence of noise acting on the RHIC beams leading to emittance growth at high beam energies. While the noise was sporadic in the past, it became persistent during the Run-11 setup period. An investigation diagnosed the source as originating from the RHIC dump kicker system. Once identified the issue was quickly resolved. We report in this paper the investigation result, circuit analysis, measured and simulated waveforms, solutions, and future plans.
    01/2012;
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    ABSTRACT: Through more than a decade of operation, we have noticed the phenomena of beam loss induced kicker instability in the RHIC beam abort systems. In this study, we analyze the short term beam loss before abort kicker pre-fire events and operation conditions before capacitor failures. Beam loss has caused capacitor failures and elevated radiation level concentrated at failed end of capacitor has been observed. We are interested in beam loss induced radiation and heat dissipation in large oil filled capacitors and beam triggered thyratron conduction. We hope the analysis result would lead to better protection of the abort systems and improved stability of the RHIC operation.
    01/2012;
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    ABSTRACT: Following the Fiscal Year (FY) 2010 (Run-10) Relativistic Heavy Ion Collider (RHIC) Au+Au run, RHIC experiment upgrades sought to improve detector capabilities. In turn, accelerator improvements were made to improve the luminosity available to the experiments for this run (Run-11). These improvements included: a redesign of the stochastic cooling systems for improved reliability; a relocation of 'common' RF cavities to alleviate intensity limits due to beam loading; and an improved usage of feedback systems to control orbit, tune and coupling during energy ramps as well as while colliding at top energy. We present an overview of changes to the Collider and review the performance of the collider with respect to instantaneous and integrated luminosity goals. At the conclusion of the FY 2011 polarized proton run, preparations for heavy ion run proceeded on April 18, with Au+Au collisions continuing through June 28. Our standard operations at 100 GeV/nucleon beam energy was bracketed by two shorter periods of collisions at lower energies (9.8 and 13.5 GeV/nucleon), continuing a previously established program of low and medium energy runs. Table 1 summarizes our history of heavy ion operations at RHIC.
    09/2011
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    ABSTRACT: The Siberian snakes are powerful tools in preserving polarization in high energy accelerators has been demonstrated at the Brookhaven Relativistic Heavy Ion Collider (RHIC). Equipped with two full Siberian snakes in each ring, polarization is preserved during acceleration from injection to 100 GeV. However, the Siberian snakes also introduce a new set of depolarization resonances, i.e. snake resonances as first discoverd by Lee and Tepikian [1]. The intrinsic spin resonances above 100 GeV are about a factor of two stronger than those below 100 GeV which raises the challenge to preserve the polarization up to 250 GeV. In 2009, polarized protons collided for the first time at the RHIC design store energy of 250 GeV. This paper presents the experimental measurements of snake resonances at RHIC. The plan for avoiding these resonanances is also presented.
    Journal of Physics Conference Series 05/2011; 295(1):012142.
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    ABSTRACT: As part of the search for a phase transition or critical point on the QCD phase diagram, an energy scan including 5 different energy settings was performed during the 2010 RHIC heavy ion run. While the top beam energy for heavy ions is at 100 GeV/n and the lowest achieved energy setpoint was significantly below RHICs injection energy of approximately 10 GeV/n, we also provided beams for data taking in a medium energy range above injection energy and below top beam energy. This paper reviews RHIC experience and challenges for RHIC medium energy operations that produced full experimental data sets at beam energies of 31.2 GeV/n and 19.5 GeV/n. The medium energy AuAu run covered two beam energies, both above the RHIC injection energy of 9.8 GeV but well below the standard store energy of 100 GeV (see table 1). The low energy and full energy runs with heavy ions in FY10 are summarized in [1] and [2]. Stochastic Cooling ([3]) was only used for 100 GeV beams and not used in the medium energy run. The efficiency of the transition from 100 GeV operation to 31.2 GeV and then to 19.5 GeV was remarkable. Setup took 32 h and 19 h respectively for the two energy settings. The time in store, defined to be the percentage of time RHIC provides beams in physics conditions versus calendar time, was approximately 52% for the entire FY10 heavy ion run. In both medium energy runs it was well above this average, 68% for 31.5 GeV and 82% for 19.5 GeV. For both energies RHIC was filled with 111 bunches with 1.2 10{sup 9} and 1.3 10{sup 9} ions per bunch respectively.
    03/2011
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    ABSTRACT: During Run-8, the Relativistic Heavy Ion Collider (RHIC) provided collisions of spin-polarized proton beams at two interaction regions. Physics data were taken with vertical orientation of the beam polarization, which in the ``Yellow'' RHIC ring was significantly lower than in previous years. We present recent developments and improvements as well as the luminosity and polarization performance achieved during Run-8, and we discuss possible causes of the not as high as previously achieved polarization performance of the ``Yellow'' ring.
    AIP Conference Proceedings. 08/2009; 1149(1).
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    ABSTRACT: As part of the efforts to increase polarization and luminosity in RHIC during polarized proton operations we have modified the injection optics and stripping foil geometry in the AGS Booster in order to reduce the emittance growth during H injection. In this paper we describe the modifications, the injection process, and present results from beam experiments.
    01/2009;
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    ABSTRACT: The RHIC proton beam polarization has a strong dependence on intensity in Run 2008, whereas the dependence is almost absent in Run 2006. Meanwhile, the RHIC beam transverse emittance also has a dependence on intensity in Run 2008, but little in Run 2006. Using the emittance measurement at the AGS IPM and the BtA multiwires, the source of this difference between 2006 and 2008 runs is traced to the Booster. It is found that at least the degree of the vertical scraping in the Booster is different in 2006 and 2008. The effect of this scraping for the RHIC beam emittance and polarization is studied.
    01/2008;
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    ABSTRACT: The Fast Beam Extraction (FEB) system of the Alternat- ing Gradient synchrotron (AGS) extracts the beam bunches from AGS into the AGS-to-RHIC (AtR) beam transfer line, which injects the beam bunches into the Relativistic Heavy Ion Collider (RHIC). As the beam bunches are extracted from the AGS, they are transported through the fringe field region of three main magnets of the AGS. The optical char- acteristics of this section of the extraction line depend on the trajectory and momentum of the beam, therefore the calculations of the R-matrices in this part of the extraction line requires special attention. To describe accurately the R-matrices of the extraction region along the fringe field of the AGS main magnets, the magnetic field of the AGS main magnets was measured on the median plane, in a) the fringe field region, where the extracted beam is transported, and b) in the circulating beam region. Using these magnetic field maps, we calculate the R-matrices at the beam extrac- tion region for any settings of the AGS. These R-matrices are used to calculate the beam parameters at the starting point of the AtR beam transfer line and subsequently to calculate the required quadrupole settings of the AtR line, to match RHIC's acceptance. In this paper we describe a) the FEB system of the AGS b) the method to calculate the R-matrices in the extraction region along the fringe field, and c) provide an example of the AtR line model which includes the calculated R-matrices.
    01/2008;
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    ABSTRACT: The four electron stripping stages leading to fully stripped gold ions in the Relativistic Heavy Ion Collider (RHIC) are briefly described. The third stripper, which removes 46 electrons from the Au31+ ions leading to heliumlike Au77+, offers the greatest challenges in terms of energy loss and induced energy spread. These problems are described in detail as well as recent advances in the design and performance of this stripper. Measurements performed with several carbon and aluminum strippers show general agreement with a semiempirical model but small systematic deviations suggest that some model adjustments may be in order. The best performance is predicted and obtained with a combined carbon-aluminum foil system. Measurements showing the enhanced performance in the alternating gradient synchrotron are described. The stripper that removes the last two electrons has also been improved and the results of relevant calculations and measurements are presented.
    Review of Modern Physics 01/2008; 11(1). · 44.98 Impact Factor
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    ABSTRACT: There is significant interest in RHIC heavy ion collisions at $\sqrt{s_{NN}}=$5--50 GeV, motivated by a search for the QCD phase transition critical point. The lowest energies for this search are well below the nominal RHIC gold injection collision energy of $\sqrt{s_{NN}}=19.6$ GeV. There are several operations challenges at RHIC in this regime, including longitudinal acceptance, magnet field quality, lattice control, and luminosity monitoring. We report on the status of work to address these challenges, including results from beam tests of low energy RHIC operations with protons and gold, and potential improvements from different beam cooling scenarios.
    11/2007;
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    ABSTRACT: The alternating gradient synchrotron (AGS) employs two partial helical snakes[1] to preserve the polarization of the proton beam during acceleration. In order to compensate for the focusing effect of the partial helical snakes on the beam optics in the AGS during acceleration of the beam,we introduced eight quadrupoles in straight sections of the AGS at the proximity of the partial snakes. At injection energies, the strength of each quad is set at a high value,and is ramped down to zero as the effect of the snakes diminishes by the square of beam's rigidity. Four of the eight compensation quadrupoles had to be placed in very short straight sections ~30 cm in length,therefore the quadupoles had be thin with an overall length of less than 30 cm. In this paper we will discuss: a. the mechanical and magnetic specifications of the "thin" quadrupole. b. the method to minimize the strength of the dodecapole harmonic,c. the method to optimize the thickness of the laminations that the magnet iron is made,d. mechanical tolerances of the magnet,e. comparison of the measured and calculated magnetic multipoles of the quadrupole.
    Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007
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    ABSTRACT: After the last successful RHIC Au-Au run in 2004 (Run-4), RHIC experiments now require significantly enhanced luminosity to study very rare events in heavy ion collisions. RHIC has demonstrated its capability to operate routinely above its design average luminosity per store of 2times10<sup>26</sup> cm<sup>-2</sup> s<sup>-1</sup>. In Run-4 we already achieved 2.5 times the design luminosity in RHIC. This luminosity was achieved with only 40% of the total possible number of bunches filled, and with beta* = 1 m. However, the goal is to reach 4 times the design luminosity, an average of 8times10<sup>26</sup> cm<sup>-2</sup> s<sup>-1</sup>, by reducing the beta* value and increasing the number of bunches to the accelerator maximum of 111. In addition, the average time at store was expected to be increased by a factor of 1.1 to about 60% of calendar time. We present an overview of the changes that increased the instantaneous luminosity, luminosity lifetime and integrated luminosity of RHIC Au-Au operations during Run-7 even though the goal of 60% time at store could not be reached.
    Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007
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    ABSTRACT: The relativistic heavy ion collider (RHIC) as the first high energy polarized proton collider was designed to provide polarized proton collisions at a maximum beam energy of 250 GeV. It has been providing collisions at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during the acceleration from injection to 100 GeV with careful control of the betatron tunes and the vertical orbit distortions. However, the intrinsic spin resonances beyond 100 GeV are about a factor of two stronger than those below 100 GeV making it important to examine the impact of these strong intrinsic spin resonances on polarization survival and the tolerance for vertical orbit distortions. Polarized protons were accelerated to the record energy of 250 GeV in RHIC with a polarization of 46% measured at top energy in 2006. The polarization measurement as a function of beam energy also shows some polarization loss around 136 GeV, the first strong intrinsic resonance above 100 GeV. This paper presents the results and discusses the sensitivity of the polarization survival to orbit distortions.
    Particle Accelerator Conference, 2007. PAC. IEEE; 07/2007

Publication Stats

305 Citations
173.29 Total Impact Points

Institutions

  • 1985–2011
    • Brookhaven National Laboratory
      • Collider-Accelerator Department
      New York City, NY, United States
  • 2006
    • Deutsches Elektronen-Synchrotron
      Hamburg, Hamburg, Germany
    • Stony Brook University
      Stony Brook, New York, United States
  • 2003
    • RIKEN
      Вако, Saitama, Japan
  • 2000–2003
    • Indiana University East
      Indiana, United States
  • 1997–1999
    • Indiana University Bloomington
      • Department of Physics
      Bloomington, IN, United States