Rhic luminosity Upgrade Program
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- Bu səhifədəki naviqasiya:
- INTRODUCTION
- UPGRADES FOR HEAVY IONS
- UPGRADES FOR POLARIZED PROTONS
- ACKNOWLEDGMENTS
| RHIC LUMINOSITY UPGRADE PROGRAM ∗ Wolfram Fischer † , Brookhaven National Laboratory, Upton, New York, USA Abstract The Relativistic Heavy Ion Collider (RHIC) operates with either ions or polarized protons. After increasing the heavy ion luminosity by two orders of magnitude since its commissioning in 2000, the current luminosity upgrade program aims for an increase by another factor of 4 by means of 3D stochastic cooling and a new 56 MHz SRF system. An Electron Beam Ion Source (EBIS) is being commissioned that will allow the use of uranium beams. Electron cooling is considered for collider operation below the current injection energy. For the polarized proton op- eration both luminosity and polarization are important. In addition to ongoing improvements in the AGS injector, the construction of a new high-intensity polarized source has started. In RHIC a number of upgrades are under way to increase the intensity and polarization transmission to 250 GeV beam energy. Electron lenses will be installed to par- tially compensate the head-on beam-beam effect. INTRODUCTION The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has been in operation since 2000. RHIC is the first and one of two existing heavy ion colliders (the LHC has not yet collided heavy ions), and the the only existing polarized proton collider. So far four combinations of particle species collided (Au-Au, d- Au, Cu-Cu, polarized p-p), at 12 different center-of-mass energies [1]. Over the last decade the heavy ion luminos- ity increased by 2 orders of magnitude (Fig. 1) and exceeds the design luminosity by a factor of 10 (Tab. 1). The po- larized proton luminosity increased by more than one or- der of magnitude (Fig. 1), and the average store polariza- tion reached 55% and 34% at 100 GeV and 250 GeV re- spectively. At the highest rigidities the beam is colliding 55% of calendar time (including all interruptions such as setup, maintenance, failures, and accelerator physics ex- periments) [1]. After the RHIC heavy ion design parameters were demonstrated in 2001, enhanced design parameters were formulated calling for a quadrupling of the average store luminosity. The achieved luminosity exceeds this goal, and new goals are set (Tab. 1). The current upgrade program aims to increase the heavy ion luminosity by more than a factor 2 from current levels, to bring into operation a new Electron Beam Ion Source (EBIS), and to extend the oper- ation to energies below the nominal injection energy. For polarized protons it is planned to bring the spin po- larization to the design value of 70% at the highest energies ∗ Work supported by Brookhaven Science Associates, LLC under Con- tract No. DE-AC02-98CH10886 with the U.S. Department of Energy. † Wolfram.Fischer@bnl.gov Figure 1: Time evolution of the integrated luminosity of the RHIC heavy ion runs at 100 GeV/nucleon (top) and polar- ized proton runs at 100 GeV and 250 GeV (bottom) from 2000 to 2010. For heavy ions the nucleon-pair luminosity L NN
1 N 2 is plotted, where L is the luminosity and N 1,2
are the number of nucleons for the species in the two beams respectively. For protons the average store polariza- tion is noted. (250 GeV) and to increase the luminosity by up to a factor of 6 over current levels (Tab. 1).
The goal of the heavy ion luminosity upgrade is to bring the luminosity close to a level where the dominant beam loss is from burn-off, i.e. particle loss from collisions with the other beam. With the highest luminosities listed in Tab. 1 the luminosity lifetime from burn-off is 9 h. When burn-off is the dominant beam loss the luminosity can be increase further only by storing and colliding more beam of the same density, not by reducing β ∗ or the emittance. The luminosity upgrade at full energy has 3 main compo- nents: a reduction of β ∗ from currently 0.75 m to 0.50 m, the full implementation of stochastic cooling in all 3 di- mensions, and the installation of a 56 MHz superconduct- ing radio frequency system. The reduction of β ∗ is an ongoing effort. β ∗ values of Proceedings of IPAC’10, Kyoto, Japan TUXMH01 01 Circular Colliders A01 Hadron Colliders 1227
Table 1: Enhanced design, achieved and further RHIC upgrade parameters. The design average Au-Au luminosity is 2 × 10
26 cm −2 s −1 , the design peak p-p luminosity is 2 × 10 32 cm −2 s −1 , and the the design spin polarization for proton beams is 70%. parameter unit
enhanced achieved
next achieved
enhanced next
design upgrade
design upgrade
2010 ≥2012
2009 ≥2012
≥2014 Au-Au operation polarized p-p operation particle energy E GeV/n
— 100 — 100 / 250 100 / 250 250
no of bunches N ... — 111 — — 109 —
bunch intensity N b ... 1.1
1.1 1.0 × 10
9 1.3 / 1.1 1.3 / 1.5 2.0 × 10
11 IP envelope function β ∗
1.0 0.75
0.5 0.7 / 0.7 0.85 / 0.5 0.5
norm. rms emittance n mm ·mrad 2.5
2.8 2.5
3.0 / 2.5 2.5
2.5 rms bunch length σ s
0.3 0.3
0.3 0.8 / 0.6 0.55 0.3
hourglass factor h ... 0.96 0.93
0.88 0.72 / 0.80 0.86 / 0.88 0.88
beam-beam parameter ξ/IP
10 −3 1.6 1.5 1.5
6.5 / 4.7 6.5 / 7.2 10 peak luminosity L peak
cm −2 s −1 36 40 55 × 10 26 50 / 85 50 / 250 500 × 10
30 average luminosity L avg
cm −2 s −1 8 20 40 × 10 26 28 / 55 30 † / 150 300 × 10 30 average polarization P % ——— 55 / 34 70 70 calendar time in store % 60 53 55 60 / 53 55 55 integrated L per week ...
300 650
1300 μb 8.3 / 18 10 / 50 100 pb
−1 † Until the 2009 polarized proton run, the enhanced design goal for the average store luminosity at 100 GeV was 60 × 10 30 cm −2 s −1 . In the current machine configuration this appears unachievable due beam lifetime limitations. 0.75 m were reached in the most recent run [2], far below the design value of 2.0 m. β ∗
beam lifetime of off-momentum particles prevented stable operation. Low β ∗
tions are implemented at the lattice design stage [3] since a small radial aperture prevents beam-based correction of an otherwise uncorrected lattice. Figure 2: First Au store with vertical stochastic cooling. When the cooling starts the emittance of both transverse planes is reduced through cooling of the vertical plane and coupling. This is observable with an Ionization Profile Monitor. (left scale). The luminosity signal of both ex- periments (right scale) increases visibly [4, 5]. The main luminosity lifetime limit for heavy ions is in- trabeam scattering. To overcome the emittance increase and particle loss from that effect, bunched beam stochas- tic cooling was implemented in the longitudinal plane in Figure 3: Vertical stochastic cooling kicker. The kicker is open for injection and acceleration (shown) and closed for cooling operation at store. 2007 [4, 5] and in the vertical plane in 2010 (Fig. 2). Fig- ure 3 shows one of the vertical kickers installed in both rings. Horizontal cooling systems are under construction. With stochastic cooling an increase in the average luminos- ity of a factor 4 is planned (Fig. 4), half of which has been realized in the most recent Au-Au run [2]. The cooling system operates in the 4-8 GHz range, and cooling times in both the longitudinal and transverse dimension are of order 1 h. Even with longitudinal stochastic cooling intrabeam scattering still drives particles out of the RF buckets. This effect can be ameliorated through stronger longitudinal fo- TUXMH01 Proceedings of IPAC’10, Kyoto, Japan 1228 01 Circular Colliders A01 Hadron Colliders
Figure 4: Calculated average store luminosity as a func- tion of the longitudinal vertex cut for longitudinal stochas- tic cooling only, and 3-D cooling with different RF systems (courtesy M. Blaskiewicz). cusing. A 56 MHz beam-driven superconducting RF cav- ity, common to both beams, is under construction. This cavity is expected to increases the luminosity by another 30-50% (Fig. 4) [6]. The 56 MHz RF system ( h = 2×360) is in addition to the existing normal conducting 28 MHz ( h = 360) accelerating, and the 197 MHz (h = 7 × 360) storage RF systems. Bunches are filled in every third bucket of the 28 MHz system. The ion beam intensity is currently limited by beam loading effects in the four 197 MHz normal conducting storage cavities common to both beams (there are 3 more 197 MHz cavities in each ring). The common cavities will be removed and placed in both rings separately. This is possible because new RF windows allow for higher volt- ages [7]. The beam intensity is also limited by instabilities at tran- sition [8, 9], driven by the machine impedance and elec- tron clouds [10]. In the 2010 run the intensity threshold for the instability was found to be higher than in previous years. This could be due to 2 short scrubbing runs with high intensity proton beams in 2009 [11], which could have cleaned parts of the beam pipe surface in the cold arcs. Most of the warm sections are coated by NEG material, and to reduce the electron cloud density further the sec- ondary electron yield (SEY) in the cold arcs also needs to be reduced. Scrubbing with beam is expected to be time consuming [12], and tests with protons showed that the beam losses associated with scrubbing need to be con- trolled in order not to upset any electronic equipment in the tunnel [11]. An in-situ coating technology for the arcs is under development [13]. A new Electron Beam Ion Source (EBIS) (Fig. 5) is under commissioning. EBIS is followed by an RFQ and a short linac, both also new, before the ions are injected into the AGS Booster [14]. This setup replaces the cur- rently used two Tandem Van de Graaff electrostatic pre- accelerators, in service since 1970 and upgraded several times. Without EBIS the Tandems would need to undergo a comprehensive reliability upgrade EBIS will also be able to deliver U beams at intensities comparable to Au, which is not possible with the Tandems. With U collisions larger densities of nuclear matter can be achieved than with Au collision due to the shape and mass of the U nuclei. EBIS can be also be used as an ionizer for spin polarized 3 He gas. Figure 5: Electron Beam Ion Source (EBIS) under com- missioning [14]. The search for a critical point in the QCD phase diagram requires operation at several energies below the nominal in- jection energy. At these low energies, magnet field errors from persistent currents in the superconducting magnets are particularly pronounced, the beam size is large, both intrabeam scattering and space charge effects are strong, and beam-beam effects are present. This presents unique challenges for colliding beams [15]. Beam and luminosity lifetimes are only minutes, and store lengths are limited to 20 min. Frequent refills are essential to produce a good av- erage luminosity. Event rates in the detectors are of order 1 Hz only. Figure 6 shows the ion intensities in the 2 ring during a day (30 April) in the 2010 operation at a beam en- ergy of 3.85 GeV/nucleon. To operate at this energy, the defocusing sextupole polarities need to be reversed, and octupoles were found to improve the beam lifetime. Due to the low luminosity lifetime, interception of the lost ions in well controlled areas (collimators, abort) is required to avoid material activation in uncontrolled areas. The exper- imental program has these low energies has just started and electron cooling is considered to increase the luminosity by up to an order of magnitude in future years [16]. Heavy ion operation also requires frequent changes in particle species and collision energy [1]. For ramp develop- ment simultaneous orbit, tune, coupling, and chromaticity feedbacks are now available, which considerably shorten the time to commission a new ramp [17]. Proceedings of IPAC’10, Kyoto, Japan TUXMH01 01 Circular Colliders A01 Hadron Colliders 1229
Figure 6: Beam intensity of stores of gold beam at 3.85 GeV/nucleon (total energy), corresponding to 38% of the nominal injection rigidity.
RHIC has stored and collided the highest energy spin polarized proton beams [18]. For the experiments the fig- ure of merit with longitudinally polarized beams, the main operating mode, is LP 2 B L 2 P where
L is the luminosity and P B,Y the polarization of the Blue and Yellow beam respec- tively.
The polarization is limited by the source, the polariza- tion transmission in the AGS, the polarization transmission in RHIC (for energies above 100 GeV), and the polariza- tion lifetime in RHIC. The main luminosity limit comes from the beam-beam effect that creates tune spread. The beam-beam effect together with other nonlinear effects and parameter modulations limits the bunch intensity and lu- minosity lifetime. At 100 GeV this is now a hard limit (Tab. 1) [19]. An upgrade of the OPPIS source has started, to increase the current by an order of magnitude to 10 mA, and the polarization by about 5% to 85-90% [20, 21]. In the Booster no polarization is lost during acceleration. In the AGS, however, the polarization transmission in only about 75% for bunch intensities of 1.5 × 10
11 leading for a polarization of up to 65% for that intensity [22]. The lowest order depolarizing resonances are addressed with 2 partial snakes (one normal conducting and one superconducting). With partial snakes the stable spin direction is not verti- cal any more and polarization is also lost due to weaker horizontal resonances. A tune jump system has been built and tested to cross a total of 82 horizontal resonances in 100
μs, much faster than with the normal ramp rate. This is expected to yield up to 5% more polarization (absolute). Figure 7 shows one of the two AGS tune jump quadrupoles. In RHIC the polarization is preserved with two Siberian snakes [24] that create a constant spin tune of 0.5. No polarization loss is observed from injection up to 100 GeV [18]. With acceleration to 250 GeV not all of the polarization was preserved, due to depolarizing resonances Figure 7: One of two AGS tune jump quadrupoles [23]. above 100 GeV, which are about twice as strong as the res- onances at the lower energies. In experiments it was also found that the polarization transmission is strongly depen- dent on the vertical betatron tune (Fig. 8) and that acceler- ation near a vertical tune of 2/3 will preserve the polariza- tion to 250 GeV. To accelerate a high-intensity beam near a low order resonance requires upgrades to the main power supplies, an improved control of orbit, tune, coupling, and chromaticity on the ramp (set through feedbacks [17]), and collimation on the ramp. Polarized proton operation also requires a vertically well aligned machine, and polarimetry. Over the first few years the machine has settled several mm (depending on the lo- cation), and is realigned every few years. In RHIC there are 2 polarimeters: a polarized hydrogen jet that delivers absolute polarization but needs measurement periods of at least a store, and a Carbon Nuclear Interference (CNI) po- larimeter that delivers instantaneous polarization but needs calibration with the hydrogen jet. The CNI polarimeter has been upgraded, and needs further upgrade in order to cope with the rates of higher intensity beams. A spin flipper is under construction, based on an AC dipole, to flip the spin of all bunches. This way system- atic effects in the experiments can be reduced. The luminosity upgrade for polarized protons has a two components: The reduction of β ∗ from currently 0.7 m to 0.5 m, and the increase of the bunch intensity from 1.5 to 2.0 × 10
11 (Tab. 1) and perhaps beyond. As for the ion lattices, chromatic aberrations have to be corrected for the reduction of β ∗
bunch intensity three problems must be addressed. First, with higher bunch intensity the polarization drops. This can be mitigated by the source upgrade (see above), or im- provements in the AGS (see above). Second, the higher bunch intensity also requires acceleration of a higher to- tal intensity. In one of the rings (Yellow) the total inten- sity is currently limited, likely because of electron cloud effects [26]. Acceleration with a 9 MHz system ( h = 120), which has been tested in 2009, would reduce the electron cloud density. The 9 MHz system also allows to preserve the longitudinal emittance better through injection match- TUXMH01 Proceedings of IPAC’10, Kyoto, Japan 1230 01 Circular Colliders A01 Hadron Colliders
0.67 0.68
0.69 0.7
0.71 0.72
vertical tune 0 0.2 0.4 0.6
0.8 1 Polarization transmission efficiency(CNI #1) Blue Yellow
Figure 8: Polarization transmission from injection to 250 GeV as a function of the vertical tune (courtesy M. Bai).
ing and thereby reduce the hourglass effect at store. Third, the total intensity is now also limited by the beam abort system. Ramps that were aborted at the highest energies have repeatedly quenched the superconducting quadrupole that follows the (internal) beam dump [27]. After analy- sis and simulations [28] the beam pipe in the dump is now thickened to increase the acceptable intensity. To mitigate the fundamental problem of beam-beam generated tune spread in head-on collisions, two electron lenses are under construction (Fig. 9) [29]. Electron lenses are installed in the Tevatron and used as abort gap clean- ers [30]. The partial compensation of the head-on beam- beam effect, together the polarized source upgrade (see above), are expected increase the luminosity by a factor of two. Figure 9: Layout of the RHIC electron lens. The electron beam travels from right to left, the proton beam in the op- posite direction (courtesy J. Hock). SUMMARY The Relativistic Heavy Ion Collider is in operation for 10 years and the heavy ion average store luminosity has reached 10 times the design value (Tab. 1). This was achieved through an increase in the bunch intensity and number of bunches, a reduction in β ∗ , and most recently, the implementation of longitudinal and transverse stochas- tic cooling during stores. A further upgrade of the stochas- tic cooling system and a beam-driven 56 MHz supercon- ducting RF system are expected to yield another factor of 2. The heavy ion program is now extended to energies below the nominal injection energy, where a luminosity increase through electron cooling is planned. An Electron Beam Ion Source is under commissioning allowing for high-intensity U beams.
RHIC has operated with polarized protons at 100 GeV and 250 GeV (Tab. 1). A source upgrade is expected to increase both the polarization and the bunch intensity. A tune jump system in the AGS is under commissioning to improve the polarization transmission in that machine. Ac- celeration with the vertical tune near a 2/3 resonance is planned to increase the polarization transmission to the highest energies. A 9 MHz RF system will allow longi- tudinal injection matching and acceleration with reduced electron cloud effects thereby better preserving both the longitudinal and transverse emittances. Electron lenses are under construction for both beams to mitigate the head-on beam-beam effect.
The author is thankful to the members of the Collider- Accelerator Department at Brookhaven National Labora- tory whose collective work is reported here. REFERENCES [1] www.rhichome.bnl.gov/RHIC/Runs [2] K. Brown et al., MOPEC023, these proceedings. [3] Y. Luo et al., BNL C-A/AP/348 (2009). [4] J.M. Brennan, M. Blaskiewicz, and F. Severino, Phys. Rev. Lett. 100, 174802 (2008). [5] J.M. Brennan and M. Blaskiewicz, PAC’09, WE3GRI01. [6] A. Fedotov, PAC’09, WE6PFP004. [7] A. Zaltsman, private communication (2010). [8] C. Montag et al., Phys. Rev. ST – Accel. Beams 5 084401 (2002). [9] V. Ptitsyn et al., TUPEB053, these proceedings. [10] W. Fischer et al., Phys. Rev. ST Accel. Beams 11, 041002 (2008).
[11] W. MacKay, private communication (2009). [12] P. He and W. Fischer, PAC’09 TH5PFP002. [13] A. Hershcovitch et al., these proceedings. [14] J. Alessi et al., PAC’09, MO6RFP025. [15] T. Satogata et al., PoS(CPOD 2009)052 (2009). [16] A. Fedotov et al., BNL C-A/AP/307. [17] M. Minty et al., MOPEC030, these proceedings. [18] M. Bai et al., PAC’09, MO4RAC04 (2009). [19] C. Montag et al., MOPEC033, these proceedings. [20] A. Zelenski, Rev. of Sc. Instrum., 81, 02B308 (2010). [21] A. Zelenski, proceedings PST2009 (2009). [22] H. Huang et al., PAC’09, FR1GRC04. [23] H. Huang et al., PAC’09, TU6RFP066. [24] V. Ptitsin and Y.M. Shatunov, Nucl. Instrum. Methods A398, pp. 126-130 (1997) [25] V. Ptitsyn et al., THPE054, these proceedings. [26] V. Schoefer, private communication (2010). [27] L. Ahrens, private communication (2010). [28] K. Yip, private communication (2010). [29] W. Fischer et al., MOPEC026, these proceedings. [30] X.-L. Zhang et al., Phys. Rev. ST Accel. Beams 11, 051002 (2008).
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