Sasha Alexandrov, Sarah Cousineau, Slava Danilov, Marc Doleans, Sang Ho Kim, Jeff Holmes, Dong-o Jeon, Jie Wei
Don Rej has been named the new Senior Team Leader for the linac team at Los Alamos National Laboratory, replacing Bruce Miller who has resigned to pursue other activities.
Weishi Wan will be coming from Fermilab to join the staff at ORNL in the next couple of weeks.
Paul Chu will visit ORNL for discussions and a physical exam in the next couple of weeks.
Jie Wei visited KEK, Jaeri, and China. Discussions at KEK focused on the 50 GeV ring and specifically momentum collimation in the high dispersion region, injection matching, and the 150 MeV FFAG study. At Jaeri, discussions focused on the SC linac, which at present calls for two beta cavities from 400-600 MeV. There is consideration for extending to 200 MeV with the possible use of up to four additional beta cavities. Also discussed were full aperture negative ion stripping, phase shifts, and equipartitioning. SACLAY suggests that the constraining guideline in cavity design be Hpeak rather than Epeak. A particular concern at KEK is momentum collimation.
The use of wire scanners for commissioning in the SC linac must be addressed. Sasha Alexandrov and Tom Shea will work with the JLAB and LANAL teams in resolving this issue.
Sang Ho Kim is working on higher order modes in the high beta SC cavities. Some small HOMs may be difficult to extract and could be dangerous if they become trapped and hybridize. To study this will require ~1000 MAFIA runs at approximately 6 hr/run. This number of runs is required to cover the range of cavity deformations and HOM types.
Marc Doleans is studying end-cell tuning and external Q for the high beta cavities On end-cell tuning, he finds that to obtain acceptable field flatness, for equatorial diameters of 67 mm or less the right hand cell diameter can be the same as the mid cells, allowing a 3-DIE cavity. On Qex behavior with fixed coupler geometry, he finds that 1) there is a large depedence of Qex, which decreases with increasing antenna penetration distance; 2) there is a large dependence of Qex on beam pipe radius below about 6.5 cm, but the dependence is mild at larger radii; 3) ellipse dimensions at the iris affect Qex; and 4) Qex increases as the distance from end cell to coupler increases.
Sasha Alexandrov is studying the magnetic fields of the closely-spaced MEBT quadrupoles. Because of the close spacing of the MEBT quadrupole triplets, nonlinear interaction leads to a reduction of up to 10% in the magnetic fields at high currents. The specifications for the power supplies calls for a 10% reserve.
Slava Danilov will work on on analytical modeling for the beam-in-gap problem, transverse impedance modeling for ring beam dynamnics and instabilities, and collaborate with YY Lee and Sergei Kurenoy on the extraction kicker design modifications.
Dong-o Jeon has been working on beam breakup calculations for the SC linac using the TDBBU Code from JLAB. After benchmarking the code with analytic results from a Gluckstern paper on beam breakup in electron linacs, Dong-o studied the effect of HOMs by examining the transverse deflection vs frequency. His conclusion is that for SNS, cumulative beam breakup is not a concern due to the large mass of the H-minus.
Jeff Holmes reported the results of longitudinal dynamics calculations with impedance carried out by summer student Kathy Woody in collaboration with Holmes, Slava Danilov, and John Galambos. An important result is that inclusion of the extraction kicker longitudinal impedance measured by J. G. Wang leads to increased bunching, lowering the bunch factor by ~10%. This could increase the space charge tune shift. It is noteworthy that the beam is stable until the extraction kicker impedance is increased by a factor of 4. Holmes is also working with Dong-o Jeon and Alexei Fedotov to understand the exact nature of the observed "23/4 resonance" in certain spacecharge calculations with FODO lattice and KV distribution using ACCSIM and with BNL lattice designers to consolidate lattice results and to study matching and robustness of alternative working points.
Sarah Cousineau has begun working with Jeff Holmes, Slava Danilov, Nuria Catalan-Lasheras, and Pete Cameron to model the beam-in-gap kicking problem. A small, self-contained, demonstration code has been developed to assist with the intuitive understanding of the process, and the BIG kicker modeling in ORBIT has begun.