MAINTENANCE AT HERA
M.Bieler, DESY, Hamburg, Germany
Abstract
HERA is an electron proton collider with acircumference of 6.3km at the DESY laboratory inHamburg, Germany. The machine was commissioned in1991. The age of the machine and the big number ofcomponents make it necessary to add some preventive
maintenance to the usual scheme of event-triggeredrepairs.
The way maintenance is organized at HERA will be
described in this paper.

1 HERAHERA (Hadron Electron Ring Accelerator) is anelectron proton collider for high energy physics. Twoaccelerator rings are built in one tunnel of 6.3km
circumference: The proton ring with superconducting
magnets reaches an energy of 920GeV with a beam
current of 100mA, distributed in 180 bunches. The
normalconducting electron ring reaches 27.5GeV with abeam current of up to 50mA in 189 bunches.
Two of the four experiments at HERA (ZEUS and H1)
use colliding beams to probe the structure of the proton,
whereas the HERA-B detector uses a wire target in thehalo of the proton beam for fixed target physics. The
fourth experiment, HERMES, uses a polarized gas target
and the polarized electron beam to investigate the spin
structure of the proton.

2 HERA OPERATIONHERA is operated 24 hours per day, 7 days per week, 811 month per year. In the year 2000 HERA has reached an
operational efficiency of roughly 60%. Here efficiency isdefined as the ratio between luminosity operation timeand scheduled luminosity operation time. One of thereasons for this relatively low efficiency is the long filltime of HERA. Both HERA and its preaccelerator PETRA
are slow cycling machines. Therefore it takes 2-3 hours
from the end of a luminosity run until the beginning of thenext luminosity run. Apart from this relatively long filltime the efficiency is determined by the failure rate andrepair time. Breaks due to component failures add up toabout 18% of the scheduled luminosity operation time.
This down time cannot be compared to that of asynchrotron radiation source, being operated at fixedenergy and having only a low number of components.
HERA has for example 1500 superconducting magnets,
2500 normalconducting magnets, 1220 magnet power
supplies, 16 klystrons (500MHz) with 1MW output each,
6 proton RF transmitters, 500 normalconducting cavity
cells with 82 couplers, 80 superconducting RF cells with16 couplers, 550 beam position monitors, 470 beam loss
monitors and a quench protection system with some

20000 cable connections. Another reason for delays is thetime it takes the repair crew to reach the faulty component.
From the accelerator control room it takes approximately
15 minutes to reach the most distant hall of HERA.
Afterwards the repair crew may have to walk up to 750m
through the tunnel, depending on the location of the faulty
component.

3 MAINTENANCE
3.1 Preventive MaintenanceEvery first Thursday of the month HERA operation isinterrupted for a service day. The tunnel is open foraccess from 6am to 3pm, the experimental areas are openfrom 6am to 10pm. During 9 hours access to the tunnel
there is time for new installations, for regular service
work, for preventive maintenance and for a lot of not sourgent repairs, which have piled up during the previousmonth. Regular service work is for example theregeneration of NEG pumps and titan sublimation pumps.
Examples for preventive maintenance are the cleaning offeedthroughs on superconducting magnets, checks forwater leaks on magnets and RF or the regular switchingof relays in safety circuits to prevent them from getting
stuck in the same position when never operated. Not sourgent repair work in the tunnel is collected on a whiteboard in the accelerator control room and is scheduledeither on short notice in parallel to urgent repairs or
shifted to the next service day. Last but not least theaccess to the tunnel is used for ‘tunnel tourism’. As there
are only about ten opportunities per year to enter thetunnel, there is always a need to show new colleagues orguests the tunnel.
Every access to the tunnel has to be announced inadvance to the technical coordinator. The work in thetunnel is planned and coordinated in order to minimize
disturbances between different tasks in the same area,
avoid rush hours at the tunnel gates and check forconsequences of certain actions for other components (i.e.
repairs on cooling water or mains power). The work
scheduled for the next service day is published on a web
page.
The experiments get 16 hours access to their detectors,
while the tunnel is closed after 9 hours. The remaining
time is used for a thorough search of the tunnel, to switch
on all power supplies and for component tests.
Problems occur either during a service day (test of the‘emergency off’ buttons without prior notice,…) or at theend of a service day, when everything is switched onagain (remote control switched to ‘local’,…). Now it isuseful to have a list of the work that was foreseen and alist of the people that were in the tunnel. This helps forexample to locate a ground fault in a 6.3km dipole circuit.

3.2 Event-triggered Maintenance
If an accelerator component breaks unexpectedly
during operation, the shift crew is able to respond
relatively quickly, because the shift crew is recruited from
20 different technical support groups of the DESY
machine department [1]. Due to this mixture of hardware
experts on shift, chances are high that there is either asystem expert on shift or there is at least one colleague inthe shift crew who has taken part in a similar repair before.
All power supply failures are handled by the two power
supply experts on shift and the mixture of people fromdifferent groups on shift helps to solve almost all otherproblems without external help. However, several groups
have people on call (quench protection, vacuum, RF,superconducting cavities, cooling water, power
distribution), who can give advice on the phone, checktheir systems from their computers at home or come to
DESY and help to solve the problem. Although most shift
crews take pride in solving problems themselves, the shift
leaders do not hesitate to call even people who are not ‘on
call’ to solve severe problems and get the machines
running again.
If a quick repair is impossible (broken magnet, broken
10kV mains power cable,…) the repair is scheduled forthe next working day. In most cases HERA is still able todeliver at least one beam (p+ or e+/e-), which can be usedby the experiments for calibration runs. If the repair takes
several hours, some of the not so urgent repair work listed
at the white board in the control room is scheduled inparallel. However, the amount of work done during suchbreaks should be limited to the necessary minimum inorder to reduce the aftermath of such an unscheduled
service day.
Repairs of broken components are generally managed
by the shift leader. The shift leader will call in experts, if
necessary, and will discuss the situation with the physicist
on shift, the shift leaders of the four experiments, thetechnical coordinator of HERA, the coordinator of HERAor the head of the machine department, depending on theseverity of the problem. If the shift leader is convincedthat all necessary support is available and the repair justtakes time, there is no need for the shift leader to call anygroup leaders or coordinators, no matter how long therepair takes.

4 REFERENCES[1] M. Bieler, “How we do Business at DESY,” theseproceedings.