Search Results (19 total)

Cooling intense high-energy hadron beams poses a major challenge for modern accelerator physics. The synchrotron radiation emitted from such beams is feeble; even in the Large Hadron Collider (LHC) operating with 7 TeV protons, the longitudinal damping time is about 13 hours. None of the traditional cooling methods seem able to cool LHC-class protons beams. In this Letter, we present a novel method of coherent electron cooling based on a high-gain free-electron laser (FEL). This technique could be critical for reaching high luminosities in hadron and electron-hadron colliders.

The fast reduction of the six-dimensional phase space of muon beams is an essential requirement for muon colliders and also of great importance for neutrino factories based on accelerated muon beams. Ionization cooling, where all momentum components are degraded by an energy absorbing material and only the longitudinal momentum is restored by rf cavities, provides a means to quickly reduce transverse beam sizes. However, the beam energy spread cannot be reduced by this method unless the longitudinal emittance can be transformed or exchanged into the transverse emittance. Emittance exchange plans until now have been accomplished by using magnets to disperse the beam along the face of a wedge-shaped absorber such that higher momentum particles pass through thicker parts of the absorber and thus suffer larger ionization energy loss. In the scheme advocated in this paper, a special magnetic channel designed such that higher momentum corresponds to a longer path length, and therefore larger ionization energy loss, provides the desired emittance exchange in a homogeneous absorber without special edge shaping. Normal-conducting rf cavities imbedded in the magnetic field regenerate the energy lost in the absorber. One very attractive example of a cooling channel based on this principle uses a series of high-gradient rf cavities filled with dense hydrogen gas, where the cavities are in a magnetic channel composed of a solenoidal field with superimposed helical transverse dipole and quadrupole fields. In this scheme, the energy loss, the rf energy regeneration, the emittance exchange, and the transverse cooling happen simultaneously. The theory of this helical channel is described in some detail to support the analytical prediction of almost a factor of 10⁶ reduction in six-dimensional phase space volume in a channel about 56 m long. Equations describing the particle beam dynamics are derived and beam stability conditions are explored. Equations describing six-dimensional cooling in this channel are also derived, including explicit expressions for cooling decrements and equilibrium emittances.

In the optics of charged particle beams, circular transverse modes can be introduced; they provide an adequate basis for rotation-invariant transformations. A group of these transformations is shown to be identical to a group of the canonical angular momentum preserving mappings. These mappings and the circular modes are parametrized similar to the Courant-Snyder forms for the conventional uncoupled, or planar, case. The planar-to-circular and reverse transformers (beam adapters) are introduced in terms of the circular and planar modes; their implementation on the basis of skew quadrupole blocks is described. Various kinds of matching for beams, adapters and solenoids are considered. Applications of the planar-to-circular, circular-to-planar and circular-to-circular transformers are discussed. A range of applications includes round beams at the interaction region of circular colliders, flat beams for linear colliders, relativistic electron cooling, and ionization cooling.

We present a method to generate a flat (large horizontal to vertical emittance ratio) electron beam suitable for linear colliders. The concept is based on a round-beam rf photoinjector with finite solenoid field at the cathode together with a special beam optics adapter. Computer simulations of this new type of beam source show that the beam quality required for a linear collider may be obtainable without the need for an electron damping ring.

We recently used an rf dipole magnet to study the spin flipping of a 120 MeV horizontally polarized proton beam stored in the presence of a nearly full Siberian snake in the Indiana University Cyclotron Facility Cooler Ring. We flipped the spin by ramping the rf dipole's frequency through an rf-induced depolarizing resonance. After optimizing the frequency ramp parameters, we used multiple spin flips to measure a spin-flip efficiency of 86.5±0.5%. The spin-flip efficiency was apparently limited by the field strength in the rf dipole. This result indicates that spin flipping a stored polarized proton beam should be possible in high energy rings such as the Brookhaven Relativistic Heavy Ion Collider and HERA where Siberian snakes are certainly needed and only dipole rf-flipper magnets are practical.

In conventional low energy electron coolers, the electron beam is immersed in a continuous solenoid, which provides a calm and tightly focused beam in a cooling section. While suitable for low energies, the continuity of the accompanying magnetic field is hardly realizable at relativistic energies. We consider the possibility of using an extended solenoid in the gun and the cooling section only, applying lumped focusing for the rest of the electron transport line.

The conventional method of spin flipping a stored polarized beam is based on slowly crossing an rf induced depolarizing resonance. This paper discusses a different approach where the polarization reversal is achieved by trapping the beam polarization into a stable spin-flipping motion on top of rf induced resonance at a half-revolution frequency.

We recently created a snake depolarizing resonance using an rf solenoid magnet in a ring containing a nearly 100% Siberian snake. We found that the primary snake rf resonance also had two weaker synchrotron sidebands, which are second-order snake resonances; they were probably caused by the energy-dependent strength of the solenoid snake due to the Lorentz contraction of its longitudinal ∫Ḃ dl. This was the first observation of an rf synchrotron-sideband depolarizing resonance in the presence of a nearly full Siberian snake.

We have demonstrated for the first time spin flipping of a polarized proton beam stored in a ring containing a nearly 100% Siberian snake; we did this using a “snake” depolarizing resonance induced by an rf solenoid magnet. By varying the rf solenoid's ramp time, frequency range, and voltage, we reached a spin-flip efficiency of about 91%. This spin-flip efficiency was probably reduced because the horizontal stable spin direction was not perpendicular to the longitudinal field of the rf solenoid, and was possibly reduced by nearby synchrotron sideband resonances. The planned use of a vertical rf dipole may improve the spin-flip efficiency.

Using an rf solenoid magnet, we studied the depolarization of a stored 104.1 MeV vertically polarized proton beam. The two primary rf depolarizing resonances were properly centered around the protons’ circulation frequency f_c, at f_c(3-ν_s) and f_c(ν_s-1), where ν_s is the spin tune; moreover, each resonance was roughly consistent with the expected width of about 720 Hz. Each primary rf resonance had two synchrotron sideband resonances at the expected frequencies. The two ν_s-1 sidebands were deep dips while the two 3-ν_s sidebands were very shallow; this was not expected. Moreover, all four sideband resonances were unexpectedly wider than the two primary resonances.

We recently accelerated a polarized proton beam from 95 to 380 MeV through both the Gγ=2 imperfection depolarizing resonance and the Gγ=7-ν_y intrinsic depolarizing resonance. The imperfection resonance flipped the spin, while the intrinsic resonance initially caused partial depolarization. We then pulsed a vertical kicker magnet for about 500 ns to increase the beam's vertical betatron amplitude; this made the intrinsic resonance much stronger. By varying the strength and start time of the kicker, we observed a sharp spin flip due to the now very strong intrinsic depolarizing resonance.

We recently studied the spin flipping of a vertically polarized, stored 139-MeV proton beam. To flip the spin, we induced an rf depolarizing resonance by sweeping our rf solenoid magnet's frequency through the resonance frequency. With multiple spin flips, we found a polarization loss of 0.0000 ± 0.0005 per spin flip under the best conditions; this loss increased significantly for small changes in the conditions. Minimizing the depolarization during each spin flip is especially important because frequent spin flipping could significantly reduce the systematic errors in stored polarized-beam experiments.

We recently studied the first acceleration of a spin-polarized beam through a depolarizing resonance using a partial Siberian snake. We accelerated polarized protons from 95 to 140 MeV while ramping a 10% partial Siberian snake along with the acceleration cycle. The 10% partial snake suppressed all observable depolarization due to the Gγ=2 imperfection depolarizing resonance which occurred near 108 MeV during acceleration. However, 20% and 30% partial Siberian snakes apparently moved the Gγ=7-ν_y intrinsic depolarizing resonance from near 177 MeV into our energy range; this caused some interesting but not-yet-fully understood depolarization.

A new configuration is proposed wherein a low-current beam is accelerated to high energies (tens of amps, tens of MeV) by a driver beam of high current and low energy (a few kiloamps, <1 MeV). The annular driver beam excites the TM₀₂₀ cavity mode of an accelerating structure which transfers its rf power to the on-axis secondary beam. Systematic variation of the driver beam radius provides the secondary beam with phase focusing and adjustable acceleration gradient. A proof-of-principle experiment is suggested.

A recent experiment in the IUCF cooler ring studied the adiabatic turn-on of a partial Siberian snake at 370 MeV, where the spin tune, ν_s is 21/2 for all snake strengths. The snake consisted of two rampable warm solenoid magnets in series with a superconducting solenoid; this combination allowed varying the snake strength between about 0 and 25% at 370 MeV. We measured the beam polaraization after varying the snake either 1, 2, or 10 times; we found with good precision that no polarization was lost. This supports the conjecture that a Siberian snake can be ramped adiabatically at an energy where the spin tune is a half integer.

The behavior of ‘‘overlapping’’ depolarizing resonances was studied using a stored 106.4-MeV polarized proton beam. We used an rf solenoid magnet to create an ‘‘rf induced’’ depolarizing resonance which was then forced to overlap with the Gγ=2 imperfection depolarizing resonance. We found significant interactions between the two resonances by varying their frequencies and strengths; the frequency of the rf resonance strongly affected the response of the imperfection resonance to correction magnetic fields. The Siberian snake overcame all observable depolarization due to the overlapping resonances and maintained full polarization for all observed frequencies and strengths.

A recent experiment studied the effect of an rf solenoid magnet and a partial Siberian snake on a 120-MeV polarized proton beam. We measured the frequencies of the ‘‘rf-induced’’depolarizing resonance for different values of the snake strength; this frequency measurement determined the spin tune ν_sp, which is the number of spin processions in one turn around the ring. A 4% snake increased the frequency of the rf-induced depolarizing resonance by the predicted 11 kHz and thus shifted the spin tune by the predicted Δν_sp=0.006 88±0.000 04; as expected the 4% snake also tilted the stable spin direction by more than 38° from the vertical.

We recently studied the proton beam polarization in the Indiana University Cyclotron Facility Cooler Ring at five energies. We found that the Gγ=2 imperfection depolarizing resonance occurred about 1.9 MeV below the expected resonance energy of 108.4 MeV. This energy shift could be due to a shift of about +0.0036 in the spin tune ν_s, which is the number of spin rotations in each turn around the ring. We then demonstrated that this spin tune shift is consistent with the Cooler Ring containing a partial type-3 Siberian snake, which is apparently caused by the magnets that confine the electron and proton beams in the cooling region.