**IPM System Brochure**

**Preface**

This interface control document (ICD) represents the current design
concepts and presents plans for capabilities for the ionization profile
monitor system (IPM).

**Overall Description**

The Ionization Profile Monitor is a VME-based instrument which acquires
data from the Ionization Profile Transducers located in the accumulator
ring of SNS. Its purpose is to measure the transverse profile of the
beam. It supports both local and remote operation via a local console
and EPICS (Experimental Physics and Industrial Control System) channel
access.

An IPM transducer collects and measures the distribution of electrons in
the beamline resulting from residual gas ionization during beam passage.
The electrons are swept transversely from the beamline, amplified by a
microchannel plate (MCP), and collected on 64 strip anodes oriented
parallel to the beam axis. During each bunch passage the current is
amplified and digitized. Approximately six profiles will be recorded
during the passage of the beam bunch once through the detector.

The current from each channel is digitized at 10 MSPS by one channel of
a VME ADC module. During beam measurement the data are stored in memory
on the VME module. The data from all 64 channels are then read into the
analysis application which presents the user with a menu of display
options.

The instrument is a stand-alone device with a few exceptions. External
timing for triggering the digitizers is required to synchronize the data
acquisition with the accelerator pulse structure.

The instrument will measure the beam profiles on every turn of the
accumulation process. For beam studies of the first 100 turns there will
be averaging over several machine cycles to increase the signal to a
statiscally usable level.

**General Capabilities**

**The ionization profile monitor will provide the following
capabilities:**

1\. Ten separate profiles at 100ns intervals during each \~1μs
accumulator revolution. Each profile consists of 64 channels each
digitized with 14-bit resolution. The digitizer trigger will be a
subharmonic of the revolution frequency so each profile will be at the
same place in the beam bunch as its corresponding profile on other
turns. All turns on an accumulation cycle will be measured.

2\. The beam from one injected turn will produce 15 electrons/turn in
the detector. For this reason it will be necessary to average data from
several machine cycles to study the early accumulation process (first
100 turns).

**Display options**

1\. Average profile for any one turn. The average profiles from
consecutive turns can be displayed in a mountain-range plot.

2\. Mountain-range plot showing the profile vs. position in the bunch
for any one turn.

3\. Mountain-range plot showing the profile at one position in the bunch
vs. turn.

**Signal processing**

The amount of beam injected into the ring on each turn will generate 15
electrons/turn. On turn 1000 there will be 15,000 electrons collected on
64 channels in 600ns giving an input signal to the MCP of 60pA/anode
channel. Each anode collection channel will be 2.8mm wide x 75mm long
giving a collection area of 210mm^2^. An extended dynamic range MCP can
deliver an average current of 1.4nA/mm^2^ or 300nA/anode channel. The
MCP gain needed is therefore 5,000 which is easily obtained with one
MCP.

The signal structure is a square wave with \"on\" portions of length
600ns increasing linearly with time over the fill and \"off\" portions
of 400ns. The signal from each channel will be amplified by an amplifier
mounted near the beamline, fig.1. The amplifiers will have gains of 1V
out for 300nA in. The output of the amplifier is sent over an
individually-shield twisted pair cable to a VME digitizer, fig. 2. The
64 ADC channels are triggered by a common signal at ten times the
revolution frequency giving 5-7 profiles/turn.

For an applied field of 2500 V/cm, 50 kV ions will have a typical
transit time of the order of a microsecond while electrons will be
collected in 2 ns, suitable for turn-by-turn measurements. The electrons
will be displaced by the beam space charge field and acquire a
transverse velocity. For a Gaussian density distribution, a beam of
2x10^4^, beta max of 16m and an emittance of 120 π mm mrad, the peak
space-charge electric field will be 10^5^V/m which will impart a
transverse velocity 33% of that due to the applied field. This will
result in significant profile broadening unless a stabilizing magnetic
field is applied parallel to the electric field.

For a 1-kG field the Larmor radius will be under 0.1 mm for electrons
created with 10 eV or less. To prevent the magnet from causing a
distortion of the circulating beam orbit, half-length magnets of the
same field strength but opposite polarity will be located before and
after the detector. The range of 180 mm will be covered using 64
channels, each of which will be 2.8 mm wide, much larger than the Larmor
radius. The two detectors will require two detector magnets and four
corrector magnets. All six magnets will be connected in series to the
primary magnet bus. In addition one corrector magnet from the detector
plane will have a trim coil. Therefore the system will require control
of three magnet power supplies.

![](media/image1.pct){width="5.99375in" height="2.8979166666666667in"}

Figure 1. Electrical schematic showing the biasing voltages inside the
transducer head and the details of the preamp mounted at the beam line.

Detector calibration will be done with an Electrogen electron generator
array mounted across the beamline from the anode-MCP assembly. The EGA
is a MCP which has been doped to be a uniform, cold electron source when
biased. In normal measurement mode the bias voltage will be zero and the
output electrode will function as the sweep-field electrode. In
measurement mode there will be a bias voltage placed across the MCP and
the voltage on the electron suppression grid will be lowered by 300-400V
to allow the flow of electrons from the output of the MCP.

This uniform flow of electrons will be measured and the gains of the 64
channels adjusted in software to compensate for gain nonuniformities. In
this manner we will be able to compensate for the gradually lowering of
the gain during normal MCP aging.

![](media/image2.pct){width="5.995138888888889in"
height="4.795833333333333in"}

Figure 2. Schematic showing the control, voltage, and signal lines
between the acceleartor tunnel and the instrument racks.

**IPM System requirement**

**Number of installed units:**

Ring: 2

**Minimum Requirements:**

Operating mode: Turn on the supplies, take a data set over \~1 second of
machine time, turn off the voltages, and then read the data from the
digitizer memory.

Peak beam current range: 15mA to 60A in the ring

Bandwidth: 5 MHz

Profile measurement resolution: ±2.8mm

Profile measurement accuracy: ±2.5% of nominal beam width

Aperture radius: 100mm

Update rate: Variable according to measurement requirements.

**Display modes:** 1. Average profile for any one turn. The average
profiles from consecutive turns can be displayed in a mountain-range
plot.

2\. Mountain-range plot showing the profile vs. position in the bunch
for any one turn.

3\. Mountain-range plot showing the profile at one position in the bunch
vs. turn.

4\. Display options 1-3 will be available from a single machine cycle or
can be the average of several machine cycles. This gives measurement
capability of the early turns in the machine cycle without the necessity
of a gas leak system.

**Discussion:**

The microchannel plates cannot be left biased continuously. The normal
operating mode will be to bias the detector, take ten profiles/turn
continuously over as many machine cycles as the onboard ADC memory will
support, and turn off the supplies. The data will then be read from ADC
memory and processed into one or more of the display modes listed above.
For machine studies this process can be repeated as rapidly as the
processing speed allows. For normal machine operation this process will
be set up to run at 5-10 minute intervals.

**IPM System Specifications**

**Number of installed units:**

Ring: 2

**Transducer specifications:**

Collector anode: 64 channels, spaced 2.8mm center-center

Clear aperture: 200mm diameter

Sweep electric field strength: 75kV/m, 15kV across 20cm aperture.

Focussing magnetic field strength: 0.1T minimum

Focussing magnetic field accuracy: Maximum field line runout ≤10% of
channel spacing.

Beam trajectory correction: Each three-magnet assembly will correct the
beam trajectory deviation to ≤1.0mrad.

**Amplifier specifications:**

Microchannel plate: The first stage of signal amplification is a
microchannel plate. Gain adjustment is by varying the bias voltage
across the plate. Nominal gain is 5x10^3^.

Amplifier: A chassis containing 64 amplifiers will be mounted on the
detector flange. Each amplifier will have a nominal gain of 1V out for
an input current of 300nA.

Bandwidth: 5 MHz.

Signal cables: Individually shielded, twisted pairs.