DEFINING PRIMARY FAILURE MODES

The beam current in a storage ring is a primary parameter for beam delivery. We can define two simple failure modes: ‘no-beam’ and ‘low-beam-current’ (see Fig. \ref{fig:primary-failure-modes}). These two modes are discussed in the next sections with examples. In general each facility can define beam limits \(I_{\hbox{nom}} \ge I_{\hbox{tol}} \ge I_{\hbox{min}}\) for each operation mode.

No-beam

When the beam current is below \(I_{\hbox{min}}\) the ‘no-beam’ mode starts. It stops when the nominal beam current \(I_{\hbox{nom}}\) is reached again.

Low-beam-current

This mode starts when the beam current drops below \(I_{\hbox{tol}}\), but only if the machine is not in the ‘no-beam’ failure mode, in order to have both modes disjoint. The ‘low-beam-current’ mode stops when the beam current reaches the nominal beam current \(I_{\hbox{nom}}\) . For facilities in top-up mode the current limit \(I_{\hbox{tol}}\) should be slightly below the target value for the top-up current. For non-top-up facilities \(I_{\hbox{tol}}\) is below the current when a re-fill is scheduled.

Primary Failure Mode in Practice

ALBA has no defined ‘no-beam’ mode, rather beam trips and the time in which all shutters are closed during user time are regarded as ‘downtime’; a typical example includes the malfunction of the injector which requires closing of the shutters. When operating in decay mode at 120\(\,\)mA, a ‘low-beam-current’ event is defined when the current in the machine is lower than 72\(\,\)mA, then a re-injection is required. In top-up mode, with 100\(\,\)mA in the machine and injections every 20 minutes, a low beam current starts when the current is below 95\(\,\)mA. Below this current it is required to close the front end shutters for re-injection up to 100\(\,\)mA.

BESSY II users do not distinguish between lost beam and closed beamshutters: in either case they do not receive photons on sample. Thus, a ‘no-beam’ event (‘dark’ time) starts either when the beam current is below \(I_{\hbox{min}}\), or the moment, the BESSY top-up interlock enforces a closure of the beam shutters. Whatever happens first will trigger the start of the ‘no-beam’ event; it will stop when the nominal beam current \(I_{\hbox{nom}}\) is restored. Short interruptions of the injector system are neglected, as long as the beam current is not decaying to less than \(I_{\hbox{tol}}=91\%\) of \(I_{\hbox{nom}}\). Below that threshold a ‘beamdrop’ event is recorded. Numbers for the limits are (a) in multi-bunch mode: \( I_{\hbox{nom}}= 299\,\)mA, \( I_{\hbox{min}}= 200\,\)mA and \( I_{\hbox{tol}}= 272\,\)mA and (b) in single-bunch mode: \( I_{\hbox{nom}}= 13.5\,\)mA, \( I_{\hbox{min}}= 8\,\)mA and \( I_{\hbox{tol}}= 12\,\)mA.

If BESSY II operates in decaying beam mode (low-\(\alpha\), degraded single bunch) no ‘low-beam-current’ is defined. Independent from the re-fill schedule, insufficient beam current is accounted as ‘no- beam’ by \(I_{\hbox{min}}\). In low-α mode \(I_{\hbox{min}}\) =5mA, in degraded single bunch mode \(I_{\hbox{min}}\) =3mA

Elettra uses the term ‘fatal failure’ for ‘no-beam’ events. It starts when the beam current is zero and stops when the software control over the insertion device gaps is given back to the users. The nominal beam current is kept constant during top-up with a delta of 1\(\,\)mA. To maintain a current of 310 \(\pm 1\,\)mA at 2\(\,\)GeV, top-up is required every 6 minutes. At 2.4\(\,\)GeV, the beam current of 160\(\pm 1\,\)mA requires top-up every 13 minutes. Thresholds of 307.5\(\,\)mA at 2\(\,\)GeV or 158\(\,\)mA at 2.4\(\,\)GeV are used to initiate a ‘low-beam-current’ event. After the ‘low-beam-current’ issue has been solved, the nominal beam current mode is recovered above 240\(\,\)mA at 2\(\,\)GeV or 110\(\,\)mA at 2.4\(\,\)GeV. For lower currents, top-up can be activated to maintain constant beam current. To restore the nominal beam current a new injection must be agreed with the users.

LNLS-UVX combines ‘beam trip’ and ‘delayed beam delivery’ into ‘no-beam’ events. Such events occur when the beam current falls below the ‘no beam’ threshold of 60\(\,\)mA for multibunch operation, or when the beamline shutters are closed during user time. The injection period is a ‘no beam’ period and lasts until the interlock is cleared that prevents the beamline shutters from opening. Early delivery is not considered for reliability purposes but is considered for yearly beam availability, which is calculated based on the recorded events in the database.

‘Low-beam-current’ events may happen after electrical power surges or power supply faults. Usually the current is completely lost but in a few rare occasions a ‘low-beam-current’ situation may happen, for instance when a sextupole or a steering magnet power supply trips. In these events the current is still above the ‘no-beam’ threshold but is low compared to the expected current. When this kind of event occurs the users are consulted about re-injection and the problem is accounted for as a ‘no-beam’ event.

PETRA III starts ‘no-beam’ events (or ‘downtime’) when the beam current is below 75\(\,\)mA (75% of the nominal beam current of 100\(\,\)mA). It stops 1 hour or the length of the downtime (whichever is shorter) after the nominal beam current of 100\(\,\)mA is reached again. This time is added to allow for the warm-up of the optical components in the beamlines. The nominal beam current is kept constant by top-up: The beam is accumulated to 101\(\,\)mA. As soon as it decays to 100\(\,\)mA, the injection is started automatically. Injection stops as soon as 101\(\,\)mA are reached again. Short interruptions of the injector system in the order of a few minutes are neglected, as long as the beam current is not decaying to less than 75\(\,\)mA. Below that threshold this is called a ‘no-beam’ event. ’Low-beam-current’ is not a criteria in the statistics of PETRA III.

SPring-8 starts ‘no-beam’ events (labeled ‘downtime’) when the beam is aborted and stops ‘downtime’ when the main beam shutters (MBS) for photon beam lines are unlocked. This is because the users cannot open the MBS during the beam injection for full current storage due to radiation safety. The nominal beam current of the SPring-8 is also kept constant within 0.03% by top-up. For top-up the injection current is kept at 0.03\(\,\)mA, and the shot number per one injection is one. SPring-8 records the ‘low-beam-current’ when the beam current decays below 0.1\(\,\)mA of the nominal 99.5\(\,\)mA. To keep one shot per one injection, the interval of the injection is not fixed but variable. SPring-8 defines \(I_{\hbox{min}} = 0\,\)mA, since low current does not happen during user operation except for the beam abort.

SLS starts ‘no-beam’ events (‘downtime’) when the beam current is below 20\(\,\)mA and stops when the control over the insertion device gaps is given back to the users; that normally happens after the nominal beam current of 402\(\,\)mA is reached. The yearly beam availability is not calculated based on ‘no-beam’ events, rather it is calculated from the beam current, as archived by the control system. The reason is that the automated availability calculation were used long before the automatic event logging system was in place. The beam current is kept constant above 400\(\,\)mA during top-up: as soon as it decays to 400\(\,\)mA, the injection is started automatically. Injection stops at 402\(\,\)mA. Short interruptions of the injector system on the order of minutes are neglected, as long as the beam current is not decaying to less than 399\(\,\)mA. Below that threshold we record a ‘low-beam-current’ event, called a ‘beamdrop’. The total duration of injector outages for the yearly statistics is calculated from ‘beamdrop’ events.

Primary failure mode limit comparison

Table \ref{tab:pf-limits} shows the current limits for the primary failure modes for the seven facilities described above. The column \(I_{\hbox{nom}}\) shows the maximum current in the given mode. The current \(I_{\hbox{inj}}\) is the typical value when injection would start in this mode, below \(I_{\hbox{tol}}\) it would be considered a ‘low-beam-current’ event. The column \(I_{\hbox{min}}\) shows the condition under which a ‘no-beam’ event would start. Most facilities do not just have a current limit, but a combination of either the current being below a limit or the photon shutters being closed.