Speaker
Description
Summary
The primary purpose of the ALICE experiment is to study
heavy ion
collisions. Under these conditions the luminosity will be far
lower
than in proton proton collisions (L = 10^27 cm-2 s-1 in Pb-Pb
collisions). The ALICE experiment programme will include
runs with
several different ion species, and also both proton nucleus
and pp
runs. Note that several of the detector subsystems would
not be able
to follow the high luminosity pp running conditions, and for
this
reason ALICE receives a lower pp luminosity of around
10^30 cm-2
s-1.
The ALICE trigger system operates with interaction rates
between
about 8 kHz and 300kHz. It must provide a number of
different
services. There are three different trigger levels (L0, L1 and
L2)
with latencies from 1.2 microseconds to 88 microseconds.
The system
allows dynamic partitioning in order to make optimum use of
detector
readout. The system has a flexible provision for past-future
protection. The system also allows for special provision for
high
priority ("rare") triggers. These features have been
described in
detail at previous workshops.
The trigger system was built in 2005 and has been tested in
considerable detail. There are six different types of 6U VME
board
in the CTP system, and two further boards required for fan-
in and
trigger distribution. Apart from the fan-in board, which is
passive,
these share a common architecture base on ALTERA
CYCLONE FPGAs.
These FPGAs are loaded from a flash memory, which is
loaded via VME.
This feature makes it easy to distribute firmware upgrades,
as has
been done in the case of the Local Trigger Unit (LTU), where
units
are in use at ALICE institutes around the world.
The three trigger levels involve several signal types from the
CTP.
The L0 trigger is sent as an LVDS signal; the L1 signal is
sent on
channel A of the TTC system; trigger data associated with
level 1 is
sent as a message on channel B of the TTC system; the L2
trigger is
sent as a message on the TTC system after a delay,
currently 88
microseconds, to allow for the longest required past-future
protection interval. Additional possibilities exist in the case of
calibration triggers. Examples of these sequences from CTP
measurements will be shown, and in addition a
measurement of the CTP
internal decision time.
In order to control the CTP an extensive software
development was
required. The ALICE CTP is in the experiment cavern, and
therefore
is inaccessible during running time. For this reason particular
attention has been paid to monitoring and debugging
facilities.
Configuration of the system is possible in a two-tier system,
with
experts able to change all parameters while normal users
select
already prepared configurations from a database. Trigger
data is
sent to dedicated monitoring processors, to the data
acquisition and
to the detector control system. A protocol based on SMI++ is
being
developed to make an interface to the experiment control
system
(ECS). All these systems will be reviewed. A more detailed
description of the CTP software is the subject of a separate
contribution to this conference.
