Yaroslav Derbenev

Electron Cooling of RHIC

We report progress on the R&D program for electron-cooling of the Relativistic Heavy Ion Collider (RHIC). This electron cooler is designed to cool 100 GeV/nucleon at storage energy using 54 MeV electrons. The electron source will be a superconducting RF photocathode gun. The accelerator will be a superconducting energy recovery linac. The frequency of the accelerator is set at 703.75 MHz. The maximum electron bunch frequency is 9.38 MHz, with bunch charge of 20 nC. The R&D program has the following components: The photoinjector and its photocathode, the superconducting linac cavity, start-to-end beam dynamics with magnetized electrons, electron cooling calculations including benchmarking experiments and development of a large superconducting solenoid. The photoinjector and linac cavity are being incorporated into an energy recovery linac aimed at demonstrating ampere class current at about 20 MeV.

Status of the R&D towards electron cooling of RHIC

The physics interest in a luminosity upgrade of RHIC requires the development of a cooling-frontier facility. Detailed calculations were made of electron cooling of the stored RHIC beams. This has been followed by beam dynamics simulations to establish the feasibility of creating the necessary electron beam. The electron beam accelerator will be a superconducting Energy Recovery Linac (ERL). An intensive experimental R&D program engages the various elements of the accelerator, as described by 24 contributions to the 2007 PAC.

Low emittance muon colliders

Advances in ionization cooling, phase space manipulations, and technologies to achieve high brightness muon beams are stimulating designs of high- luminosity energy-frontier muon colliders. Simulations of helical cooling channels (HCC) show impressive emittance reductions, new ideas on reverse emittance exchange and muon bunch coalescing are being developed, and high-field superconductors show great promise to improve the effectiveness of ionization cooling. Experiments to study RF cavities pressurized with hydrogen gas in strong magnetic fields have had encouraging results. A 6-dimensional HCC demonstration experiment is being designed and a 1.5 TeV muon collider is being studied at Fermilab. Two new synergies are that very cool muon beams can be accelerated in ILC RF structures and that this capability can be used both for muon colliders and for neutrino factories. These advances are discussed in the context of muon colliders with small transverse emittances and with fewer muons to ease requirements on site boundary radiation, detector backgrounds, and muon production. Compared to studies done 10 years ago where larger bunch intensities were assumed, there now are more possibilities for acceleration, low beta insertions, and detector designs.

Parametric‐Resonance Ionization Cooling and Reverse Emittance Exchange for Muon Colliders

Two new ideas are being developed to reduce the transverse emittance of muon beams in order to increase the luminosity of muon colliders. The first idea involves driving a 12‐integer parametric resonance in a beam line or ring such that particle motion becomes hyperbolic, where xx′=constant. With the proper phase of the resonance driving term, particles move to larger and larger x′ and smaller and smaller x at the position of a thin wedge absorber. The usual mechanism of ionization cooling reduces or constrains the excursion in x′ while the dynamics of the resonance reduces the spread of x. The second idea takes advantage of the large reduction of relative momentum spread with increasing momentum in going from a few hundred MeV/c where the beam is cooled to a few TeV/c for an energy frontier collider. In this case we can use thin wedge absorbers to exchange the transverse and longitudinal emittances to make the transverse emittance smaller. These two ideas depend on careful control of the lattice function...

Helical channel design and technology for cooling of muon beams

Novel magnetic helical channel designs for capture and cooling of bright muon beams are being developed using numerical simulations based on new inventions such as helical solenoid (HS) magnets and hydrogen‐pressurized RF (HPRF) cavities. We are close to the factor of a million six‐dimensional phase space (6D) reduction needed for muon colliders. Recent experimental and simulation results are presented.

Muon bunch coalescing

The idea of coalescing multiple muon bunches at high energy to enhance the luminosity of a muon collider provides many advantages. It circumvents space-charge, beam loading, and wakefield problems of intense low- energy bunches while restoring the synergy between muon colliders and neutrino factories based on muon storage rings. A sampling of initial conceptual design work for a coalescing ring is presented here.

Recent Innovations in Muon Beam Cooling

Eight new ideas are being developed under SBIR/STTR grants to cool muon beams for colliders, neutrino factories, and muon experiments. Analytical and simulation studies have confirmed that a six-dimensional (6D) cooling channel based on helical magnets surrounding RF cavities filled with dense hydrogen gas can provide effective beam cooling. This helical cooling channel (HCC) has solenoidal, helical dipole, helical quadrupole, and helical sextupole magnetic fields to generate emittance exchange and achieve 6D emittance reduction of over 3 orders of magnitude in a 100 m segment. Four such sequential HCC segments, where the RF frequencies are increased and transverse physical dimensions reduced as the beams become cooler, implies a 6D emittance reduction of almost five orders of magnitude. Two new cooling ideas, Parametric-resonance Ionization Cooling and Reverse Emittance Exchange, then can be employed to reduce transverse emittances to a few mm-mr, which allows high luminosity with fewer muons than previously imagined. We describe these new ideas as well as a new precooling idea based on a HCC with z dependent fields that can be used as MANX, an exceptional 6D cooling demonstration experiment.

Recent Innovations in Muon Beam Cooling and Prospects for Muon Colliders

New ideas are being developed to cool muon beams for colliders, neutrino factories, and muon experiments. Analytical and simulation studies have confirmed that a six-dimensional (6D) cooling channel based on helical magnets surrounding RF cavities filled with dense hydrogen gas can be used to achieve very small emittances. This helical cooling channel (HCC) has solenoidal, helical dipole, and helical quadrupole magnetic fields to generate emittance exchange and achieve 6D emittance reduction of over 3 orders of magnitude in a 100 m segment. Three such sequential HCC segments, where the RF frequencies are increased and transverse dimensions reduced as the beams become cooler, implies a 6D emittance reduction of almost six orders of magnitude. Two new post-cooling ideas then can be employed to reduce transverse emittances to one or two mm-mr, which allows high luminosity with fewer muons than previously imagined. We describe the new post-cooling ideas as well as a new precooling idea based on a HCC with z -dependent fields that can be used as an exceptional 6D cooling demonstration experiment.

Image charge undulator: theoretical model and technical issues

A relativistic electron beam undergoes undulating motion due to its image charge wakefields while passing close to a conducting grating surface. A new device, an image charge undulator, has been proposed recently to utilize this mechanism for generating coherent hard radiation. We demonstrate the physics principle of this device by a 2D model of a uniform sheet beam. The transverse image charge wakefields, synchrotron radiation frequency and coherent radiation gain length are presented. We discuss a proof-of-principle experiment that takes into consideration such technical issues as grating fabrication, flat beams and beam alignment.

Polarization Preservation and Control in a Figure-8 Ring

We present a complete scheme for managing the polarization of ion beams in Jefferson Lab’s proposed Medium-energy Electron-Ion Collider (MEIC). It provides preservation of the ion polarization during all stages of beam acceleration and polarization control in the collider’s experimental straights. We discuss characteristic features of the spin motion in accelerators with Siberian snakes and in accelerators of figure-8 shape. We propose 3D spin rotators for polarization control in the MEIC ion collider ring. We provide polarization calculations in the collider with the 3D rotator for deuteron and proton beams. The main polarization control features of the figure-8 design are summarized.