Date June 8, 2000 Attendees S. Danilov, B. Davis, J. Galambos, S. Kim, J. Wei, K. Woody J. Galambos introduced the new summer students Brian Davis and Kathy Woody. J. Wei discussed acitvities going on at LANL. The first end-to-end simulation is nearly complete and will be reported on at the mini-ASAC. Loyd Young is doing commisioning studies for the superconducting linac, and has a proposal for using beam-generated signals to get RF amplitude and RF phase information. LANL needs more beta = 0.61 cavity information, and we will provide this. The capability of the linac is expected to be between 1.23 and 1.27 GeV, for the nominal cavity gradients. A DTL cold model is expected to be at LANL in July, and several ORNL people (Sang ho, Marc DLeons, Dong O and J. Galambos) should plan on observing the tuning of it. Several LANL personel are going to visit cavity fabrication shops in Europe, Saclay, and Rutherford after EPAC. Also, LANL is hiring a Saclay person. J. Wei also discussed ongoing Ring activities. Operation of the second target while the first target is undergoing maintenance causes safety concerns. Deepak is looking into ways to get a bend in thge RTBT betweent he target and the kicker to the second target station. Regarding the extraction kicker, the Note shown below was discussed. J. Galambos reported on the efforts being done with the new students. B. Davis is proceeding on the DTL corrector studies, and Kathy Woody is starting convergence studies for the longitudinal impedance studies. Slava Danilov has discussed the kicker impednace issues with BNL personnel. He is now starting to look into modleing of longitudinal impedance effects and is helping Kathyy Woody with a benchmark of the numerical model.He is also thinking about ways to model transverse impedance effects. Sang ho is assisting the JLab cavity design work. He noted that the beta = 0.61 end cell changed again, and now there is better field flattnes (~ 2.5%). He is helping JLab determine the die parameters, including the effects of cooldown and chemical processing. 3 out of the 4 dies need to be re-done. He is also doing some 3-D modeling of the power coupler for JLab, and looking at optimizing the cavity end diameter to get the right Q_ext. Finally he is invloved in calculating up to 150 HOMs with MAFIA. ========================================= Note on the ring extraction kicker issues ========================================= Recollection of Ring extraction kicker related issues (draft, 6/6/00) The following is a collection of thoughts on SNS ring extraction kickers. As shown below, since R&D study (A) is not conclusive, efforts should be made on practical measures (B) to minimize potential maintenance/instability problems. (A) R&D STUDIES (1) Theoretical understanding of instability threshold & damping mechanism Comparing with AGS Booster, SNS is estimated to be closer to possible transverse instability threshold by a factor of 5 (10 times intensity, 1/5 energy, 2.5 times kicker number). So, no claim can be made on SNS ring even though Booster is found to be mostly stable. On the other hand, beam is accumulated in SNS for a much shorter time of 1 ms and head-tail instability is difficult to develop. The understand of instability & damping mechanism is on-going and verification with existing machine data is not performed. No breakthrough is expected in the coming months. (2) Experimental test & verification Measurements/experiments can help the understanding: (a) Impedance measurement This work has begun more than a year ago. Progress has been made but the comparison result is not conclusive. (b) machine experience Possible experiments can be arranged on AGS Booster to test the condition (chromaticity setting, etc) when instability occurs. Quantitative calibration is however difficult. (B) PRACTICAL MEASURES (1) Options for kicker impedance reduction Several options are explored to reduce kicker impedance (a) shortening of ferrite module Since modules are near-by, ferrite core length can be reduced without significantly reducing the integral kick strength. Each ferrite module is shortened from 40 to 30 cm in length. Further reduction will give engineering difficulty without due benefit (Nick, YY, et al). (b) use of ferrite with lower permeability (mu) value ISIS uses ferrite mu of 1000, while LBL uses 100. AGS/Booster uses a ferrite mu of 2600 and so does SNS ring's. Slava et al suggested that mu should be reduced to push up resonance frequency but reducing mu can result in vacuum out-gassing problem. This discussion is under-going (Slava, Sergey, SY, Mike, et al). (c) half the number of kicker modules while each powered with two PFNs This double powering is used by ISIS and LBL. The concern is whether we are still tolerable to 1-module failure. One may argue that most failure modes are with drivers and that corresponds to half-kicker failure. (Evaluation is needed here.) If this is possible, the 7 - 8 modules will be in the long drift space possibly powered with higher voltage (oil & outside of tunnel). (d) low termination impedance/ISIS design ISIS's termination is 7 Ohm while SNS's is kilo Ohms. Adopting ISIS's design implies approximately doubling the present driver power. According to Jon and SY, the current can remain the same. The extra voltage can be handled if the PFN's are moved outside of tunnel and operates in oil. The cost implication is not clear and some R&D may be needed. (e) make ferrite external of beam chamber / ceramic pipe & shielding inside This requires much higher power and one may reserve this option as a future upgrade after operational experience is collected. (f) sandwich ferrite material with ceramic in thin slices There are reliability issues with this scheme. (2) Maintainability concern Presently, PFN's are placed in the tunnel, similar to the situation of Booster. For Booster, maintenance is difficult due to high radiation level. As for SNS, the lifetime of thyrotron is about 6 months. Replacing 14 units every half a year can be a major task in a potential high-radiation area. We need to seriously consider and evaluate moving the PFN's out of tunnel to a separate location similar to ISIS' practice. Moving PFNs out of tunnel can help on: (a) easy maintain of PFN units. (b) reduce beam induced signal on kicker feeders (possibly determined by the termination). (c) allow oil, thus higher driver voltage than present 35 kV in air. The additional cost for cable is about $200k. There are questions on possible cable degradation and termination problem. According to ISIS experience, termination is ok and redundancy can be designed to withstand one cable failure.