D. Jeon

Higher-order-mode (HOM) power in elliptical superconducting cavities for intense pulsed proton accelerators

In linacs for intense pulsed proton accelerators, the beam has a multiple time-structure, and each beam time-structure generates resonance. When a higher-order mode (HOM) is near these resonance frequencies, the induced voltage could be large and accordingly the resulting HOM power, too. In order to understand the effects of a complex beam time-structure on the mode excitations and the resulting HOM powers in elliptical superconducting cavities, analytic expressions are developed, with which the beam-induced voltage and corresponding power are explored, taking into account the properties of HOM frequency behavior in elliptical superconducting cavities. The results and understandings from this analysis are presented with the beam parameters of the Spallation Neutron Source (SNS) superconducting linac.

SNS HOM damping requirements via bunch tracking

Based on the Higher Order Mode (HOM) field properties, the Spallation Neutron Source (SNS) HOM damping requirements have been determined by bunch tracking simulations. Transverse instabilities are found to be absent and transverse error magnifications are found to be acceptable if the loaded cavity Q for each mode is less than 10/sup 8/ and the expected cavity-to-cavity frequency variation due to manufacturing tolerances is present. Longitudinal instabilities are absent if the loaded cavity Q for each HOM is less than 10/sup 8/, the loaded cavity Q for each non-pi fundamental mode has the expected value, and the expected cavity-to-cavity frequency variation is present. Each of the two cavity types has three longitudinal modes that are close enough to a bunch spacing harmonic that one of the modes may occasionally get tuned onto the resonance; the power deposited as a function of Q has been calculated for this case. Deposited power and transverse instabilities/magnifications have also been determined analytically.

Performance of SNS Front end and warm linac

The Spallation Neutron Source accelerator systems will deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of an H-injector, capable of producing one-ms-long pulses at 60 Hz repetition rate with 38 mA peak current, a 1 GeV linear accelerator, an accumulator ring and associated transport lines. The 2.5 MeV beam from the Front End is accelerated to 86 MeV in the Drift Tube Linac, then to 185 MeV in a Coupled-Cavity Linac and finally to 1 GeV in the Superconducting Linac. With the completion of beam commissioning, the accelerator complex began operation in June 2006 and beam power is being gradually ramped up toward the design goal. Operational experience with the injector and linac will be presented including chopper performance, transverse emittance evolution along the linac, and the results of a beam loss study.

Multipacting on the trailing edge of proton beam bunches in the PSR and SNS

The Proton Storage Ring (PSR) in Los Alamos has a fast intensity-limiting instability, which may result from an electron cloud interaction with the circulating proton beam leading to a transverse mode coupling instability. The most probable mechanism of the electron creation is multipacting. Though the effect depends on many parameters, a model is presented which predicts a large electron creation in the vacuum chamber. A comparison of this effect between the PSR in Los Alamos and the Spallation Neutron Source (SNS) in Oak Ridge is given. In addition, several possibilities to reduce multipactor are discussed.

Spallation Neutron Source Superconducting Linac Commissioning Algorithms

We describe the techniques which will be employed for establishing RF setpoints in the SNS Superconducting linac. The longitudinal tuneup will be accomplished using phase-scan methods, as well as a technique that makes use of the beam induced field in the unpowered cavity [1].

Beam Dynamics for High-Power Superconducting Heavy-Ion Linear Accelerator of RAON

We investigated optimum choices of accelerator parameters for superconducting-technology-based heavy-ion linear accelerator (linac) to achieve the high beam quality at an in-flight target with a high beam power of 400 kW. The superconducting linac system is designed to provide a stable-ion beams from protons to uranium with energies of 600 MeV and 200 MeV/u, respectively. The linac optics is also designed such that the effects of envelope instability, parametric resonance between the transverse and longitudinal planes, beam steering effect due to asymmetric field in quarter-wave resonance (QWR) cavities, and emittance growth in charge strippers due to the straggling effect, parametric resonance, and envelope instability become as small as possible. The dimensions of the warm and cold sections in the low-energy section which mainly determine the longitudinal acceptance of the linac are optimized to achieve a longitudinal acceptance larger than 27 keV/u-ns enabling stable operation. The tracking simulation for the uranium beam was performed and we achieved the transverse normalized rms emittance of 0.095 mm-mrad and longitudinal rms emittance of 2.10 keV/u-ns at the end of designed linac without uncontrolled beam losses along the linac. In order to estimate the degradation of the machine performance due to imperfections such as misalignment, ripple of the magnet power source, and phase and power variation of the RF source, the particle tracking simulation was performed and the uncontrolled beam loss with the machine imperfection without the orbit and optics correction was less than 1 W/m which is a condition required for machine operation. The optimum scheme for orbit correction in the low-energy linac is investigated based on the transfer matrix of the elements and the validation of the scheme is confirmed by the particle tracking simulation using TRACK code.

4.2 K Operation of the SNS Cryomodules

The Spallation Neutron Source being built at Oak Ridge National Laboratory employs eighty one 805 MHz superconducting cavities operated at 2.1 K to accelerate the H-beam from 187 MeV to about 1 GeV. The superconducting cavities and cryomodules with two different values of beta (. 61 and .81) have been designed and constructed at Jefferson Lab for operation at 2.1 K with unloaded Q’s in excess of 5×109. To gain experience in testing cryomodules in the SNS tunnel before the final commissioning of the 2.1 K Central Helium Liquefier, integration tests are being conducted on the cryomodules at 4.2 K. This is the first time that a superconducting cavity system specifically designed for 2.1 K operation has been extensively tested at 4.2 K without superfluid helium.

Transverse beam break-up study of SNS SC linac

Numerical simulation indicates that cumulative beam breakup (BBU) instability is not a concern to the SNS SC linac. First, simulation is carried out for the CW operation mode where the driving harmonics are those with frequency multiples of bunch frequency 402.5 MHz. Even when the median HOM frequency is exactly on resonance with multiples of bunch frequency of 402.5 MHz, the cavity-to-cavity HOM frequency spread can ensure operation of the linac. Second, in the case of the pulsed operation mode, additional driving harmonics of 1 MHz and 60 Hz are added on top of those of the CW mode. The shunt impedance of these additional modes is relatively small. BBU is not a concern also for pulsed mode operation, as is verified for a few most dangerous modes. More systematic analysis of BBU of pulsed mode operation is done by Sundelin et al. and presented at this conference.

Longitudinal tune-up of SNS normal conducting linac

DTL of SNS linac accelerates the 2.5 MeV H/sup -/ beam from the RFQ and the MEBT to 86 MeV. For longitudinal setpoint, two standard phase scan methods will be used, because they are complementary. Numerical simulation using the Parmila code indicates that only the phase scan with the absorber and collector is effective for DTL tank 6. But for the rest of the DTL tanks, both methods are effective.

Comparison of Techniques for Longitudinal Tuning of the SNS Drift Tube Linac

It is important to bring the cavity rf field amplitude and phase to the design for a high intensity linac such as the Spallation Neutron Source (SNS) linac. A few techniques are available such as the longitudinal acceptance scan and phase scan. During the SNS linac commissioning, tuning of cavities was conducted using the acceptance scan and phase scan technique based on multiparticle simulations. The two techniques are compared.