The RHIC polarized source upgrade

A novel polarization technique has been successfully implemented for the upgrade of the RHIC polarized H ion source to higher intensity and polarization. In this technique, a proton beam inside the high magnetic field solenoid is produced by ionization of the atomic hydrogen beam (from an external source) in the He-gas ionizer cell. Proton polarization is produced by the process of polarized electron capture from the optically-pumped Rb vapor. Polarized beam intensity produced in the source exceeds 4.0 mA. Strong space-charge effects cause significant beam losses in the LEBT (Low Energy Beam Transport, 35.0 keV beam energy) line. The LEBT was modified to reduce losses. As a result, 1.4 mA of polarized beam was transported to the RFQ and 0.7 mA was accelerated in linac to 200 MeV. A maximum polarization of 84% (in the 200 MeV polarimeter) was measured at 0.3 mA beam intensity and 80% polarization was measured at 0.5 mA. The upgraded source reliably delivered beam for the 2013 polarized run in RHIC at √S=510 GeV. This was a major factor contributing to the increase in RHIC polarization to over 60 % for colliding beams. OPPIS UPGRADE WITH THE ATOMIC HYDROGEN BEAM INJECTOR The polarized beam for the RHIC spin physics experimental program is produced in the OpticallyPumped Polarized H Ion Source (OPPIS) [1]. An Electron Cyclotron Resonance (ECR) ion source was used as the primary proton source in the old operational polarized source. The ECR source was operated in a high magnetic field. The proton beam produced in the ECR source had a comparatively low emission current density and high beam divergence. In pulsed operation, suitable for application at highenergy accelerators and colliders, the ECR source limitations can be overcome by using a high brightness proton source outside the magnetic field instead of the ECR source. In this technique (which was implemented for the first time at INR, Moscow [2]), the proton beam is focussed and neutralized in a hydrogen cell producing the high brightness 6.0-8.0 keV atomic H 0 beam. The atomic H 0 beam is injected into the superconducting solenoid, where both the He ionizer cell and the optically-pumped Rb cell are situated in the 25-30 kG solenoid field. The solenoid field is produced by a new superconducting solenoid with a re-condensing cooling system. The injected H atoms are ionized in the He cell with 60-80% efficiency to form a low emittance intense proton beam and then enter the polarized Rb vapour cell (see Figure 1). The protons pick up polarized electrons from the Rb atoms to become a beam of electron-spin polarized H atoms (similar to the ECR based OPPIS). A negative bias of about 3.0-5.0 kV applied to the He cell decelerate the proton beam produced in the cell to the 3.0 keV beam energy, optimal for the charge– exchange collisions in the rubidium and sodium cells. This allows energy separation of the polarized hydrogen atoms produced after lower energy proton neutralization in Rb-vapour and residual hydrogen atoms of the primary beam. Figure 1: A new polarized source layout: 1-atomic hydrogen injector; 2pulsed He –gaseous ionizer cell; 3 -optically-pumped Rb-vapour cell; 4Sona-transition; 5-Na-jet ionizer cell. Residual atomic H beam is converted to un-polarized H ion beam with lower yield (3-4% at 6.0-8.0 keV atomic beam energy). The H ion beam acceleration (by applying a pulsed -32 kV voltage to the ionizer cell) produces polarized H ions with beam energy of 35 keV and un-polarized beam with energy of 38-40 keV. Further suppression of un-polarized higher energy ion beam is produced by magnetic separation in the LEBT. Atomic hydrogen beam currents of equivalent densities in excess of a 100 mA/cm 2 were obtained at the Na jet ionizer location (about 240 cm from the source) by using a high brightness fast atomic beam source which was developed in collaboration with BINP, Novosibirsk. Estimated polarized H ion beam current of about 5-10 mA is expected in this source after upgrade completion (assuming 50% ionization efficiency in He-cell and 50 % neutralization efficiency in the optically-pumped Rbvapour cell). In feasibility studies of this technique (performed at TRIUMF) in excess of 10 mA polarized H and 50 mA proton beam intensities were demonstrated [3]. The beam losses during proton beam deceleration introduced additional losses. Higher polarization is also expected with the fast atomic beam source due to: a) elimination of neutralization in residual hydrogen; b) better Sona-transition efficiency for the smaller ~ 1.5 cm diameter beam; c) use of higher ionizer field (up to 3.0 kG). All these factors combined should increase polarization in the pulsed OPPIS to over 85%.