The inner surface of the ring vacuum chambers of the US Spallation Neutron Source (SNS) will be coated with ~100 nm of Titanium Nitride (TiN). This is to minimize the secondary electron yield (SEY) from the chamber wall, and thus avoid the so-called e-p instability caused by electron multipacting as observed in a few high-intensity proton storage rings. Both DC sputtering and DCmagnetron sputtering were conducted in a test chamber of relevant geometry to SNS ring vacuum chambers. Auger Electron Spectroscopy (AES) and Rutherford Back Scattering (RBS) were used to analyze the coatings for thickness, stoichiometry and impurity. Excellent results were obtained with magnetron sputtering. The development of the parameters for the coating process and the surface analysis results are presented.
Phone: (800) 553-6847 Facsimile: (703) 605-6900 Online ordering: http://www.ntis.gov/ordering.htm Abstract Beam induced pressure rise in RHIC warm sections is one of the machine luminosity limits. The RHIC electron cloud and the beam transition pressure rise are discussed. Countermeasures and studies for RHIC pressure rise and RHIC upgrade are reported.
The Facility for Rare Isotope Beams (FRIB) Project has entered the phase of beam commissioning starting from the room-temperature front end and the superconducting linac segment of first 15 cryomodules. With the newly commissioned helium refrigeration system supplying 4.5[Formula: see text]K liquid helium to the quarter-wave resonators and solenoids, the FRIB accelerator team achieved the sectional key performance parameters as designed ahead of schedule accelerating heavy ion beams above 20[Formula: see text]MeV/u energy. Thus, FRIB accelerator becomes world’s highest-energy heavy ion linear accelerator. We also validated machine protection and personnel protection systems that will be crucial to the next phase of commissioning. FRIB is on track towards a national user facility at the power frontier with a beam power two orders of magnitude higher than operating heavy-ion facilities. This paper summarizes the status of accelerator design, technology development, construction, commissioning as well as path to operations and upgrades.
The stainless steel vacuum chambers of the 248 m accumulator ring of the Spallation Neutron Source are to be coated with ∼100 nm of titanium nitride (TiN). This is to minimize the secondary electron yield from the chamber wall, and thus avoid the so-called e–p instability caused by electron multipacting as observed in a few high-intensity proton storage rings. Reports in the literature suggest that a TiN coating, by acting as a hydrogen permeation barrier, may also reduce the ultimate outgassing rate. The outgassing rate of TiN coated chambers deposited at various sputtering pressures was measured and compared to uncoated chambers, both with and without 250 °C in situ bake. Some coated chambers were subjected to glow discharge treatment (GDT). It was found that the surface roughness, analyzed with a scanning electron microscope, depends on the deposition pressure and is also influenced by GDT. The outgassing rate varies as a function of the surface roughness of the TiN layer, with rougher coatings more hydroscopic in nature. The in situ postbake outgassing rate was reduced ∼30% for a chamber coated with TiN at low pressure and subsequently subjected to GDT, thus giving evidence that the TiN layer acts as a permeation barrier to hydrogen diffusion. It was also found that a 450 °C vacuum degas reduced the hydrogen outgassing rate one order of magnitude, although the amount of reduction does not agree with the value predicted by standard diffusion equations.
OBSERVATIONSSince 2081 M I C has experienced electron cloud effects, which have Iimited the beam intensity. These include dynamic pressure rises -including pressure instabilities, tune shifts: electrons, a reduction of the stability threshold for bunches crossing the transition energy, and possibly slow emittance growth. We summarize the main observations in operation and dedicated experiments, as well as countermeasures including baking, NEG coated warm beam pipes, solenoids, bunch patterns, anti-grazing rings, pre-pumped cold beam pipes, and scrubbing. This article is a condensed version of Ref. El].Dynamic pressure rise from electron-impact desorption after an electron cloud has been formed was the first, and still is the most common electron cloud observation in RHIC [2,4,5]. It is also the operationally most relevant electron cloud effect in RHIC. This pressure rise is particularly pronounced at transition crossing when the ion bunches are short [6,73. In some cases the electron cloud switched off spontaneously, like in a second order phase transition [S], which can be explained if the both and electron and ion cloud is assumed 195.
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