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Beam Loss Monitor System

Interface Control Document

Preface

The Beam Loss Monitor (BLM) System includes the distributed Ion Chamber
monitors and the Neutron Detectors (NDs) concentrated in the Linac. Fast
BLMs will be used in the HEBT, Ring and RTBT.

# Overall Description

The BLM System will be distributed throughout SNS to measure the
radiation produced by lost beam. It will provide data on the beam loss
distribution, aide in minimizing those losses, and provide signals to
the Machine Protection System (MPS) in the event of excessive loss.

Approximately 300 cylindrical argon-filled ion chambers will be used as
the detectors, typically with several per Linac cavity, and one per
magnet elsewhere. BLM signals will be conditioned in an analog front end
(AFE) circuit. The signals will rise in a few µsecs (the electron
component) and last up to 100 µsecs after the macro-pulse due to ion
transit times. These signals will be digitized and processed in a
dedicated VME crate. Fast beam inhibit and low-level, long-term loss
warnings will be generated to provide machine protection. The fast loss
data will have a bandwidth of 35 kHz. The low level, long term loss data
will have much higher sensitivity but only be available over a 10 second
average.

NDs will be used in the Linac area. A distribution of 30
photomultipliers will be used for this purpose. Signals from the PMTs
will be conditioned with the same electronics as the ion chambers.

The FBLMs will require HV power supply control but the signals will be
acquired by fast ADCs housed in the BCM PCI crates. The analog signal
handling is not part of this ICD.

External timing is required to synchronize the data acquisition with the
macro-pulse structure and provide time stamping.

# General Capabilities

1.  Detailed time history within the macro-pulse for each BLM.

2.  Total losses for each BLM each macro-pulse.

3.  Long-term history, both for wideband and low level losses for each
    BLM.

4.  Gain setting and readback for each BLM.

5.  Fast Loss output for the Machine Protect System.

6.  Long-term, low level beam loss alarm on exceeding 1 W/m.

7.  Remote/local system test

8.  Remote/local Bias Voltage setting.

9.  System Calibration.

## 1. Detailed time history within the macro-pulse.

The time history within a macro-pulse is useful but transmitting data
for all detectors at the macro-bunch rate would be excessive and
unnecessary. Data will be packed and transmitted at a 6 Hz rate.

## 2. Total losses for each BLM within the macro-pulse.

Data from each BLM will be acquired at 100 kSa/S. These will be summed
at the front end (IOC) and stored for transmission to the console level
at a rate once per second for all BLM channels at each IOC location.
This data will be presented as a "waterfall" display showing detector
location on the horizontal axis, Time on the vertical axis, and
intensity of loss as color. A more conventional display of detector on
the x-axis and amplitude on the y-axis may also be required for update
at once or more per second.

## 3. Long-term loss history for each BLM 

A long term history of the slow, low level losses for each BLM will be
kept at the console level for use in a "strip chart" or "water fall"
like display. This data will be sent up from the IOCs as packets once
every 10 seconds. The data will be acquired on each beam pulse but will
be averaged over 10 seconds to provide the sensitivity required.
Wideband loss data will also be stored at the console level. Total
losses throughout the macro pulse shall be stored in a rolling memory
for the prior 1000 pulses. Detailed losses within the macro pulse shall
be stored for the most recent 100 pulses.

## 4. Gain setting and readback for each BLM 

Each BLM electronics channel will have the capability of 3 jumper
selected gain states which will be used to compensate for known
high-radiation locations, such as at dumps , injection, extraction or
the collimators or, low radiation locations in well shielded areas. A
fourth jumper will be used to select whether the data sent to the ADC
for the fast readback comes from the linear amplifier output or the
integrator output. This is necessary since the sub microsecond pulse
width in the RTBT would be too fast for the linear amplifier and 100
kilo-sampling ADC. A readback of this jumper is provided. In addition,
the "Viewing Gain" can be changed (x1, x10) via the control system
without effecting the loss level or MPS input data provided to the MPS.
Setting this gain may be required on a pulse-to-pulse basis to allow for
different mode cycles. Gain setting in the Ring during the macro-pulse
is not required. Read back of the jumper-selectable gain and Viewing
Gain states will be provided.

## 5. Fast Loss Output for the Machine Protect System 

Each BLM electronics channel will produce an output signal which will be
processed to provide a signal to the Machine Protect System (MPS) to
shut off the beam in the event the fast beam loss exceeds the
programmable threshold level. This signal will represent the beam loss
from each detector integrated over the macro pulse. The signal shall be
provided within 10 μsec of the loss occurrence. Threshold level
comparisons will be done in the MPS-I/F module located in the VME (IOC)
crate, where the signal to the MPS hardware will be generated. The
output of the MPS-I/F module will go directly to the MPS chassis.

## 6. Long-term, low-level beam loss alarm on exceeding 1 W/m.

Beam losses should not exceed 1 W/m over the long term. The higher
sensitivity will be achieved by limiting the signal bandwidth to 1 kHz
to reduce noise, and providing additional gain. This data will be
averaged in the IOC over 600 pulses and compared against a reference
representing 1 W/m. Since the signal levels will be very low,
measurements of the signal baseline prior to the beam pulse will be
required. Each BLM shall have an associated slow loss level reference.
The IOC will send warnings as this level is approached and an alarm when
it is exceeded. In addition, the long-term, low-level data will be
presented as a water-fall display or a strip chart style display. This
does not apply to the neutron detector channels.

## 7. Remote/Local System Test

The BLM System can be tested by rapidly cycling the HV Bias Power
Supplies Off and On again. The resulting transient capacitively couples
between the high voltage electrode and the signal electrode and passes
through the BLM electronics. In this way all components of the system,
including the bias an signal cables, are checked.

## 8. Local/Remote Bias Voltage Setting

The ion chambers exhibit a region of nearly constant signal versus bias
voltage from several hundred Volts to 3000 V. During studies periods, it
may be necessary at times to explore the linear region and so the
ability to vary the HV bias Voltage is required. The NDs and FBLM PMTs
will require carefull setting of the High Voltage to properly set their
gain. Readback of the Voltage, current and polarity will also be needed.
Each IOC crate will contain 2 HV Bias Supplies ( 1 dual unit) , each
powering alternate BLM detectors, as well as addition HVPS for the
neutron detectors and FBLMs.

**9. System Calibration**

Each BLM ion chamber will be calibrated in a controlled laboratory test
setup using a certified radiation source. Data will be taken across the
entire operating voltage range since the desired operating point may
change with different operating conditions. Data will provided for each
detector in hardcopy and electronic format. Following installation,
tests of the electronics through the actual cabling will be performed
using a measured 1 G-Ohm resistor and voltage source. These values shall
be used in the electronics calibration database.

# System Configuration

## Distribution of BLMs

**The distribution of loss monitors is shown in Table I.** Table I

  ------------------------------------ ---------- -------- ------- --------
                                       #FBLM      \# BLMs  \# NDs  Total
                                                                   AFE Ch

  •DTL_Diag:Cab401                                12       15      27

  •CCL_Diag:Cab301                                14       4       18

  •CCL_Diag:Cab302                                14       4       18

  •SCL_Diag:HB13Cab01                             42       3       45

  •SCL_Diag:HB1Cab02                              44       4       48

  •HEBT_Diag:Cab07                     3          34               34

  •Ring_Diag:Cab05                     6          55               55

  •Ring_Diag:Cab06                     6          52               52

  •RTBT_Diag:Cab04                     3          28               28

  Totals                               18         295      30      325
  ------------------------------------ ---------- -------- ------- --------

Channels will be grouped according to common region. Typically racks
will be located so that cable runs are not longer than 100 meters.

## Analog Front End Circuit Design

The BLM system must provide a measurement range of approximately 2.5 x
10^4^ in order to cover the span from the high end fast loss (1% of beam
at a single location) to the low-level loss threshold of 1 W/m. In
addition a long-term measurement of low level beam loss extending 2
decades below the 1 W/m level must be made. Including 2-decades at the
low end for resolution below the threshold, results in a 21-bit dynamic
range. The low level measurement will be made by providing a separate 1
kHz limited bandwidth signal to the 24-bit ADC and averaging the signal
over 10 seconds. The 1 kHz low pass filter limits the noise but still
enables measurement of the baseline prior to the beam pulse. This is
necessary since the circuit must be DC coupled and amplifier offset,
though small, is not negligible. An integrated output will also be
provided, which will be used by the MPS-I/F to generate a beam inhibit
signal if a reference threshold is exceeded. The integrated output will
also be used for data logging for the RTBT signal.

## Packaging

The AFE's will be housed in 6U high, 220 mm deep Eurocard (VME w/o the
interface) modules mounted horizontally in a separate chassis, with
linear power supplies. Boards will hold 8 channels each. ADCs, DACs,
Digital I/O, Timing, MPS-I/F and HV Bias Power Supplies modules will be
housed in the IOC VME crates.

**Cabling**

Signals from the detectors will be carried on non-tribo-electric cable
to reduce vibration induced noise. BNC connectors with non-PTFE
insulators will be used. All signal runs must be continuous (no
splices). Patch panel feedthroughs must be insulated from ground. The HV
Bias will be connected to the BLMs using red jacketed RG-59 or
equivalent. Connectors will be SHV type but with non-PTFE insulators. HV
cables will run from the HV PS to the nearest BLM, skip the next BLM and
pick up the following one. A separate HV cable from the second HV supply
will run to alternate detectors to provide some fallback in case of HV
cable or power supply failure.

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height="2.4784722222222224in"}

**FIGURE 2. 32 channel BLM layout**

The various signals to be interfaced to the Control System are shown in
Table II.

**TABLE II**

+---------------+---------+--------+------+------------+-------------+
| **Signal**    | **Qua   | **     | *    | **Charac   | **IOC       |
|               | ntity** | Type** | *Ran | teristic** | Rate**      |
|               |         |        | ge** |            |             |
+---------------+---------+--------+------+------------+-------------+
| Wideband Loss | 1/BLM   | ADC    | 24-  | 100        | 6/sec       |
|               |         |        | bits | kSa/s/ch   |             |
|               |         |        |      |            |             |
|               |         |        | 0    |            |             |
|               |         |        | -±10 |            |             |
|               |         |        | V    |            |             |
+---------------+---------+--------+------+------------+-------------+
| Low-level     | 1/BLM   | ADC    | 24-  | 100        | 600 pulses  |
| Loss          |         |        | bits | kSa/s/ch   |             |
|               |         |        |      |            |             |
|               |         |        | 0    |            |             |
|               |         |        | -±10 |            |             |
|               |         |        | V    |            |             |
+---------------+---------+--------+------+------------+-------------+
| Fast Loss     | 1/BLM   | DAC    | 16   | \> 60 Hz   | 60 Hz       |
| Reference     |         |        | Bits |            |             |
|               |         |        |      |            |             |
|               |         |        | 0-10 |            |             |
|               |         |        | V    |            |             |
+---------------+---------+--------+------+------------+-------------+
| AFE Gain/Mode | 6       | Dig    | \-   | TTL        | 60 Hz       |
| Readback      | b       | Input  |      |            |             |
|               | its/BLM |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| AFE Gain CMD  | 1       | Dig    | \-   | TTL        | 60 Hz       |
|               | bit/BLM | Output |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| AFE Channel   | 3       | Dig    | \-   | TTL        | 60 Hz x 32  |
| Select        | b       | Output |      |            |             |
|               | its/AFE |        |      |            |             |
|               | Channel |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| AFE Module    | 3       | Dig    | \-   | TTL        | 60 Hz x 32  |
| Select        | b       | Output |      |            |             |
|               | its/AFE |        |      |            |             |
|               | Module  |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| AFE Module    | 8       | Dig    | \-   | TTL        | 60 Hz x 32  |
| Serial Number | b       | Input  |      |            |             |
|               | its/AFE |        |      |            |             |
|               | Module  |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| HV PS Inhibit | 2/HVPS  | D      | \-   | TTL        | 60 Hz       |
| CMD           |         | igital |      |            |             |
|               |         | Output |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| HV PS         | 2/HVPS  | VME    | \-   | VME        | 60 Hz       |
| Over-current  |         |        |      |            |             |
| Status        |         |        |      |            |             |
| Readback      |         |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| HV PS Voltage | 2/HVPS  | VME    | \-   | VME        | 6 Hz        |
| CMD           |         |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| HV PS Voltage | 2/HVPS  | VME    | \-   | VME        | 6 Hz        |
| Readback      |         |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| HV PS Current | 2/HVPS  | VME    | \-   | VME        | 6 Hz        |
| Readback      |         |        |      |            |             |
+---------------+---------+--------+------+------------+-------------+
| Start of Beam | 1/L     | Timing | 1    | TTL        | 60 Hz       |
|               | ocation |        | turn |            |             |
+---------------+---------+--------+------+------------+-------------+
| Sample Delay  | 4/L     | Timing | 1    | TTL        | 60 Hz       |
|               | ocation |        | turn |            |             |
+---------------+---------+--------+------+------------+-------------+

Based on this table the total number of modules by type can be
estimated. This is shown in Table III.

**TABLE III**

  ------------------------------------------------------ ----------------
  \# IOCs                                                14

  \# Util Mod                                            14

  \# Dig I/O                                             14

  \# IP Carrier Modules                                  14

  \# DAC IP Modules                                      42

  \# ADC                                                 23

  \# AFE                                                 45

  \# MPS/IF                                              25

  \# HV PS (BLM)                                         14

  \# HV PS (ND)                                          16

  \# HV PS (FBLM)                                        10

  VME Bus Extenders^\*^                                  2

  \# Exp CTLs Crates                                     1
  ------------------------------------------------------ ----------------

^\*\ A\ second\ IOC\ may\ be\ substituted\ for\ the\ Bus\ Extenders^

**Analog Data Acquisition**

Analog data from the fast and slow loss outputs will be acquired using
commercial 24-bit, 100 kSa/s/ch, VME ADC modules. Data will be taken for
1 msec prior to beam to provide baseline acquisition, and for 200 μsec
after the beam to allow for ion collection for the fast loss, and 2 msec
for the slow loss circuit signal decay.

Integrated loss data will be used as an input to the comparators which
generate the signals sent to the MPS to shut off the beam. However,
since the RTBT beam pulse is so short, the integrator signal will also
be used for the ADC input for the fast data. A jumper will reconfigure
those channels for this purpose with its status read back as the MODE.
The comparators will have individually programmable references provided
by the DACs. The outputs of the comparators in the MPS-I/F modules will
be sent to the MPS using standard MPS interface circuitry. No provision
has been made to digitize the integrator output except for the RTBT, as
stated above. The same data is available by digitally integrating the
fast loss data.

HV Power supply voltage and current will also be read back via the VME
interface. Data readback at once per second would be sufficient.

ND data will bethe same as for the BLMs except that the slow loss data
will not be acquired. PMT gain for the NDs and FBLMs will be set from an
LUT of HV bias vs gain for each detector.

**Digital Data Flow**

The gain and mode (integrator/linear) readback bits will be acquired by
applying a 3 bit board select and a 3 bit channel select code to the
digital data bus on each card in an AFE crate. The 5-bits corresponding
to each channel will be read back along with one bit per channel of the
8 bits board serial number. Similarly, the Viewing Gain Command bit can
be set when the appropriate channel and board code is applied. Since
there are only a maximum of 4 boards per AFE crate, 2 AFE crates can be
addressed by the 3-bit code from each IOC.

**Front End Data Processing**

1.  [Data scaling]{.underline}. Each BLM and ND channel will have gain
    and offset parameters for the AFE and ADC, and sensitivity of the
    BLM or ND detector, which must be applied to the data. The offsets
    should be stored in a separate array. Data must be scaled for
    presentation in either engineering (Volts) or physics (Rad) units.

2.  [Detailed Time History]{.underline}. Fast loss data from each BLM
    and ND channel will be sent to the console at a 6 Hz update rate.
    The array will include the pre- and post- macropulse data. At the
    console, this data may be used to make a waterfall display with
    detector location along the X-axis, time on the Y-axis and intensity
    shown as color. The same data shall be available for display as a
    "scope trace".

3.  [Total loss per Macropulse]{.underline}. Each fast loss data channel
    shall have the data summed over the period of interest (macropulse
    plus 0.1 or 2 msec). The summed data shall be sent to the console at
    a 1 Hz rate. At the console, this data may be used to make a
    waterfall display with detector location along the X-axis, time on
    the Y-axis and intensity shown as color. The same data shall be
    available for display as a "scope trace".

4.  [Long Term History for Each BLM.]{.underline} A 1000 pulse FIFO
    history of the long term, low level losses for each BLM will be kept
    at the console level for use in a "strip chart" or "water fall" like
    display. This data will be sent up from the IOCs once per 10
    seconds. The fast loss data for the most recent 100 pulses will also
    be stored at the console level. Total losses throughout the macro
    pulse shall be stored in a rolling memory for the prior 1000 pulses.

5.  [Slow Loss Data.]{.underline} The slow loss data will be used to
    warn the operators that the 1 W/m loss level is being approached and
    will annunciate an alarm when the level is reached. Since the 1 W/m
    level applies to long term losses and uses data with a bandwidth of
    a few Hz, the warnings and alarm will be generated from an average
    of taken over a 1 minute period. This monitoring, warning and
    alarming shall be done at the console level. Additional processing
    will be required at the IOC level. The slow loss data will be
    acquired through 24-bit ADCs at a 100 kSa/s/ch rate. A baseline will
    be computed from data taken during 1 msec prior to beam and will be
    subtracted from the beam time data. Total loss during the 1 msec
    beam pulse and for 2 msec following will be accumulated and averaged
    over a 10 sec period to determine if the slow losses exceed the 1
    W/m threshold.

6.  [Bias Voltage Setting and Readback]{.underline}. The high voltage
    bias for the BLMs shall be set at the console level. The power
    supply current shall be monitored and compared to a standard value
    at the IOC. A warning shall be generated if the current exceeds the
    reference value for a 1 second period. This time requirement is
    needed to allow for the current draw during turn on.

7.  [System Test]{.underline}. The BLM system can be tested by switching
    the HV bias supply on and monitoring the resulting transient. Since
    the voltage capacitively couples from the HV electrode to the signal
    electrode, the entire system is tested. A table shall be made of
    expected values for each channel and each gain in response to a
    defined voltage step. This shall serve as reference for future tests
    of the system and be stored in the IOC. The IOC shall, on command,
    perform the test by turning off the system and waiting for a (to be
    defined) period, then turning on the HV to the prescribed voltage.
    The outputs will be compared to the table and discrepancies will be
    sent to the console level.