MAINTAINING THE ACCELERATOR SAFETY ENVELOPE
P. Ingrassia, L. Ahrens, R. Olsen, BNL, Upton, NY 11973, USAAbstract
The Accelerator Safety Envelope (ASE) defines
boundaries for accelerator operation that may not be
exceeded. Examples of parameters that are limited by theASE [3] include beam intensity and particle loss, theradiation safety access control system, the fire protection
system, and control room staffing. This paper will focus
on BLAM, the Beam Loss Accounting Manager, a novel
approach that is used to monitor beam loss at theRelativistic Heavy Ion Collider (RHIC).

1 INTRODUCTIONThe US Department Of Energy order 420.2A [1],entitled Safety of Accelerator Facilities establishes
accelerator-specific safety requirements. The Accelerator
Safety Envelope defines physical and administrative
bounding conditions for safe operation based on the safety
analysis documented in the Safety Assessment Document(SAD), another requirement of the order. Accelerator
Operations is charged with watching over a subset of theparameters that require control, most notably particle
losses. We use software to monitor specific beam current
transformers, to determine when losses occur, and to alertthe operators when the losses approach thresholds that are
judged to be significant.

2 RADIOLOGICAL AREAS
The accelerators at the Collider – Accelerator
Department of the Brookhaven National Laboratory
include two Tandem Van de Graff accelerators, a Linearaccelerator, a booster synchrotron, the Alternating
Gradient Synchrotron (AGS), and the two super
conducting synchrotrons that make up the RHIC. Given
the range of species and particle intensities accelerated atBNL, one finds the all of the types radiological areas
throughout the complex. Radiological area classification
determines the dose rates expected and protective
measures required for that area. Uncontrolled areas
require unrestricted access. A great deal of thought andeffort has gone into guarding individuals in theuncontrolled area at the RHIC.

Figure 1: Controlled & Uncontrolled areas at RHIC
Protection of the uncontrolled area is challenging because
it lies in the path of the injected beam for the RHIC
yellow accelerator (see figure 1) and because theuncontrolled area classification requires that individuals
who work in that area receive a dose less than 25 mRemper year (see table 1).

3 BLAM
BLAM is the “watchdog” used by the operators tomonitor losses at the RHIC and in the injection arcs
leading into the RHIC. Based upon the information from
BLAM the operators decide whether to delay injecting
beams in order to limit the hourly average loss. BLAM isonly concerned with losses that occur when the beams are
dumped and not with losses during the ramp up to fullenergy or during the store.

Very High Radiation> 500 mRem/hr

Knowing the allowable dose rates permitted in an
uncontrolled area one could, in principal, calculate themaximum number of particles which, when lost at fullenergy, would produce the radiation field.
Thiscalculation is the source of the limiting value for the ASE
for beam losses. BLAM learns about the loss limits fromexternal servers. Unless told otherwise BLAM assumes
that losses occur such that they are concentrated in thesextant that produces radiation in the uncontrolled area.

High Radiation> 100 mRem/hr

3.1 BLAM Dependencies

Radiation> 5 mRem/hrTable 1: Radiological Area Definitions
Area

Allowable Dose RateControlled

25 mRem/yr<d<100 mRem/yr

Uncontrolled< 25 mRem/yr

BLAM depends upon a number of server processes.
_________________________________________

The Post Mortem (PM) server writes files containing loss
monitor data when it sees an “abort” or “dump” event on
the time line. The Loss Analysis (LA) server reads thefiles written by the PM server and determines whether thedump was a clean or “dirty” one based upon where losses
are seen and whether the losses are correlated in time withthe dump/abort event. If the LA server determines thedump is clean then it will issue a “credit” to BLAM forthe number of ions previous circulating in the ring basedon measurements from beam current transformers. The
specification manager “specMan” is used by BLAM todetermine which ion species is in each ring and theinjection energy for each species. (See Fig. 2).

The dark blue horizontal line represents 50% of theallowable “dose ions” that may be lost. The light blue
horizontal line represents 90% of the allowable dose ionsthat may be lost. During the evolution more than 100% of
the allowable dose ions were injected. The other darkblue line represents the dose ions circulating in the bluering, while the light blue line represents the “blue loss
hourly sum”. Coincident with the loss of the beam one
sees an increase in the blue hourly loss sum. BLAM
issued a critical alarm telling the operators that 90% of theallowable dose ions were lost. Five minutes later the blueloss hourly sum returns to normal levels and the alarm
disappears from the screen after BLAM learns from theLA server that the dump was a clean one. A clean dump
is defined as one where the entire beam is dumped in thebeam dumps designed for that purpose. A dirty dump
assumes all the beam is lost such that the dose in theuncontrolled area will be unacceptable if operation isallowed to continue. An hour after the “loss credit” fromthe LA server the blue loss hourly sum is seen to go
negative because the loss has expired. Five minutes later
when the credit expires, the blue loss hourly sum is seen
to return to normal. Each point on the blue (yellow) loss
hourly sum is the sum of any losses or credits that occur
at that time, plus the previous loss value, minus the valueof the blue (yellow) loss hourly sum one hour ago.

3.4 Operator Response
The operator’s response to a loss of 50% of theallowable dose ions is to do nothing. The alarm will goaway after five minutes if the loss was clean, or if an hour
goes by without significant losses. The operator’s
response to an alarm corresponding to a loss of 90% of
the allowable dose ions is measured. If the alarm does notclear after five minutes then the dump or abort was a dirty
one. The operators must not inject beams into the RHIC
until enough time passes to ensure that the “limitingnumber” of ions cannot be lost in an hour.
Figure 2: BLAM dependencies

3.2 Dose Arithmetic
BLAM calculates an effective dose for the beam as afunction of beam energy according to the formula.

 Energy− RESTenergy DoseIons = Ions *  InjEnergy− RESTenergy

0.8

BLAM takes into the account the energy of the beam andgives less weight in the accounting process to beam that isaborted before it reaches full energy.

3.3 Example of an evolution
Figure 3 shows and example of an evolution in theRHIC blue accelerator that lasted 65 minutes before thebeam was aborted. The yellow accelerator is not shown.

Figure 3. BLAM evolution in the Blue ring

4 CONCLUSION
The task of ensuring that the accelerator safety envelope
is not violated becomes impossible without the BLAM
software. BLAM is a tool that makes a potentiallychallenging task a relatively easy one for the operators.
The author wishes to thank and acknowledge R. Olsen
the principal programmer behind BLAM and L. Ahrens,
the accelerator physicist who specified the application.

5 REFERENCES[1]DOE O 420.2A
http://www.directives.doe.gov/pdfs/doe/doetext/neword/420/o4202a.pdf[2]BLAM Program overview
http://www.rhichome.bnl.gov/Controls/doc/blam/blam.html[3] C-AD OPM 2.5.2 RHIC Accelerator Safety Envelope
Parameters.
http://www.rhichome.bnl.gov/AGS/Accel/SND/OPM/Ch02/02-0502.PDF[4] C-AD TPL 03-06 Temporary Procedure to Monitor the
RHIC ASE.
http://www.rhichome.bnl.gov/AGS/Accel/SND/OPM/TPL/03-04.PDF