SNS visit 18-22 November

K. Hanke, B. Mikulec, JL Sanchez-Alvarez, JF Comblin, F. Chapuis

objective was to see a machine with H- injection in real day-to-day operation, find out what the issues are, frequency and nature of problems, operating model, etc.

accelerator complex

main machine and beam parameters

• rep. rate 60 Hz, one injection every 16.66 ms • number of turns injected into ring: 1000 • the actual cycle within this basic period is only 1 ms• revolution frequency 1.045883 MHz• beam energy nominal 1.0 GeV, presently running slightly below that (932 MeV) due to

limited modulator power (capacitor problems + overheating); energy upgrade to come (linac); will go up to 1.3 GeV by installing 9 more cryo-modules, space left in the linac tunnel for more cavities

• ring does not accelerate, only bunch compression (presumably required by target)

linac

• source: 2 RF power supplies (2MHz and 13MHz)• RFQ: old one gives some problems (bad transmission ~60-70%); unclear

why; new one had just arrived• chopper: 2 devices: in the LEBT and in the MEBT; they said that they

would not even need the second one, only if full beam has to be removed (extinction factor of 1st one ~10-4-10-5)

• normal conducting linac gives occasional problems (flashovers, vacuum windows weak point)

• super conducting linac perfectly stable

ring

• only accumulating and bunching, no acceleration• magnets are DC• RF 2 harmonics (H1 = 1.05 MHz; H2=2.10 MHz)• ferrite cavities• 4 cavities (2 running in H1 mode, 1 spare H1, 1 running in H2 mode)• H1 mode gives bunch structure• H2 mode to avoid peak intensity in the bunch• 13 extraction kickers (+1 hot spare) all pulsing together, 200 ns gap

provided by the choppers for the extraction kicker rise-time

injection process and hardware

• horizontal and vertical painting (4 horizontal kickers and 4 vertical kickers)

• no longitudinal painting• foil flatters; cause not very well known (mechanical, thermal shock,

discharge etc.); under investigation but not conclusive• foil needs conditioning after a foil change; need to start up with reduced

intensity for ~hours (but no issue at 1 Hz)• foil: 350 μg/cm2, actual thickness 1 um, size 17 x 54 mm• 3% beam loss (~1-1.5 mA on external dump)• stripping made by 2 foils (1st thin foil strips H-, 2nd thick foil strips

remaining H- and H0)• stripping foil damage or failure is diagnosed by the increase of BLM and

beam intensity on the dump; results in stop of the MPS• Preventive foil exchange every 4-6 weeks

diagnostics and tools

• diagnostics do not acquire every pulse, but only 1/10 of them• BEAST = Best Ever Alarm System Tool• unstripped ions measured by a BCT, ~1.5 mA, modulation on the signal

visible which is often due to foil shaking, also visible on the other BCT; 1.5 mA is the sum of the H- and H0

• laser stripping being developed; issue is to generate short bunch length (~ps); 1st result 90% stripping efficiency for 6-7 ns @ 1Hz; needs machine studies; may need dedicated cavity for bunch rotation (currently using 2 last cavity modules); requires 2 MW laser; want to measure the H0; still in MD mode; HW gives lots of prbs. (HV not at required level etc.)

• wire scanners: concept of super Gaussian (during the painting the profile becomes non-Gaussian, with steep edges and flat; combination of Gaussians); wire scanners can do simultaneous scans; this causes beam loss and can trigger a beam stop; used during setting up and once per month

• BLMs: main tool; they do all the setting up with BLMs (hundreds)• BPMs: there is an equivalent to YASP• SEM grids: HV applied to remove stripped electrons, says that works well

but do not see any difference in the profile with and w/o HV• camera on the foil (was broken)

BLMs

current on injection dump

safety and interlocks

• PPS (Personnel Protection System): 4 access points equipped with video cameras; controlled by two redundant PLC ways; no hardware loop; PPS communicating with MPS (Machine Protection System)

• MPS (Machine Protection System) acts on source and choppers.

controls

• controls of the diagnostics done a lot in LABView (mainly for BI), JAVA, X-Motif

• 3 levels of colors: green = system OK, yellow = system drifting (intervention not required), red = system failing (intervention required)

• low level: EPICS (equivalent to FESA)• GUI mainly provided by operators; based on synoptic• Java starts to replace the older GUI application

issues

• anything shorter than 1 min not accounted for• power glitches• power supplies• controls• external faults (storms) • normal conducting linac has prbs.; sparking, vacuum issues• super conducting linac perfectly stable• depending on the breakdown duration need to ramp up slowly to its

maximum power• foil flickering, foil temperature high, camera broken• vacuum issues in the DTL

operations model

• very calm (one user, one beam type)• 12 hours shift periods (06:00 to 18:00, 18:00 to 06:00)• normal operation: 3 people on shift (+ 1 for training; team composed of

one shift Leader and two operators• troubleshooting: - troubleshooting on site not very often - mainly based on best effort service via call-out list - usually one specialist per equipment family• an Accelerator Physics/Maintenance Period is always followed by a start-

up/restore phase of 8 hours • FY2014: operational efficiency for neutron production 92.3%