The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Basic Design of Beam Diagnostic Device for Injection Line of the JAERI Tandem Accelerator
Susumu HANASHIMA
Japan Atomic Energy Research Institute (JAERI)
2-4, Shirakatasirane, Tokai-mura, Ibaragi 319-1195, Japani. Typical beam current : 10nA to 1 11 A.

Abstract
Basic design of beam diagnostic device has been madefor the injection line of JAERI tandem accelerator. Thedevice can measure distribution of beam flux in the fourdimensional phase space. The measurement is made withscanning phase space by two deflectors and counting
incident particles shifted to the center of beam line by thedeflectors.

1 Introduction
Distribution of a particle beam on the phase space is oneof concerns in an accelerator system. (The phase spacemeans a space of showing position and direction of particles
here.) In the JAERI tandem accelerator, many kind of ion
species from light ions to heavy ions like gold are injectedto the tandem from one of 3 negative ion sources. We musttune these injection beams to transport them through thetandem. This task is done by manual tuning withobservation of beam profile monitors and Faraday cup
current readings in the present. But the following method ismore straightforward. In the method, we first measure aphase space distribution of the incident beam. After that,
we tune optical elements of the upstream to give thedistribution of the beam that the accelerator can accept.
This method needs an instrument to measure the distribution.
For the daily operation of the accelerator, the measuring
time must be short. Previously, Tajima and others havemade measurements of emittance of the incident beam forthe tandem [1). The purpose of the work was mainly
knowing emittance value and not intended to use on daily
accelerator operations. Its measurement time was very long.
So we have started development of a new instrument for themethod in the injection line of the tandem.

2 Design objectives

One of the typical characteristics were about 16mm ·mrad for the emittance and 15mm for width of the beam of
the injection line[1 ). The result is used to estimate thedensity of the beam.

3 Design Policy and Construction
We considered using collimetor which define position
and gradient of beam current in X and Y axis. The
measurement is made by scanning position in the phasespace which pass through the collimetor. Measuring speed
is very important in our case. So, mechanical scanning isavoided. and scanning by electric method is suitable.
Electrostatic scanning is selected because the energies ofparticles for deferent ion species are constant and low.
Electrostatic field is independent from mass of particles.
Because expected electrical current through the collimetor
is very small and integration of electrical current is time
consuming, we decide to measure the counting rate ofparticles through the collimetor.
The above policy leads a construction showing Fig.l.
From the up stream, the first electrostatic deflector, thesecond electrostatic deflector, the first aperture, the secondaperture and a particle counter are aligned. The twoapertures are set in the center of beam line and they form acollimeter. Combination of the two deflectors works as
beam shifter and deflector. It selects a part of the beam toshift to the center of the phase space, which pass throughthe collimetor. In this configuration, other devices for theaccelerator namely slits and or Faraday cups can be set
among the elements after the second deflector.

Beam

We set following design objectives, considering theinstrument will be set in the injection line of the tandem andused in daily accelerator operation.
a. Size: the instruments can be set in the space of theinjection line of the tandem.
b. Energy and charge state of Particle: 220Ke V andminus 1 nominal.
c. Dimensions: Enables 4 dimension measurements.
The instruments can measure distribution of beam in 4
dimensional phase space, that is X-X'-Y-Y' space.
d. Resolution of position: About 0.3 mm.
e. Resolution of direction: Less than 0.3mrad.
f. Scan range of position: Wider than ± 10mm.
g. Scan range of direction: Wider than ± 8mrad at thecenter of the beam line.
h. Measuring time: Less than 30seconds for coarse
measurement.

-479-l>Deflector1

III

Deflector2 Aperture1 Aperture2D t te ec orn

I DnD[}lControl processor
Fig.1 Construction of Beam Diagnostic Device:
The apertures and the particle detector are retracted from the beamcourse when a measurement is not made.

4 The Deflectors
We started the design of the deflectors from parallel

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

plate deflector. Characteristic of the deflector is as follows .
.Ll X' =(q • V ·l)/(2d ·E)(1).LlX = (q ·V·l2)/(4d ·E)(2)

Where .LlX' and .LlX show change of gradient and changeof position which caused by the deflector at its end point
respectively. E and q are the energy and the charge of theparticle respectively. V, 1 and d are the voltage, the lengthand the gap width of the deflector respectively. For fasterscanning, deflection voltage V needs to be lower. So, thelength, 1 is desired to be longer. We think about combining
two deflectors for different direction, X and Y into one
deflector. Figure 2 shows concept of a shroud typedeflector, which can deflect beam in either X and Y
The shroud deflector consists of thin wire
directions.
electrodes stretched between two insulator frames, forminga box shape. The wires are connected to the next wires withresisters. The amounts of deflection for X and Y directions
are added or subtracted and fed into the corner wires. In thebox shaped area, parallel electrostatic field is obtained withexception of disorders near the wires and the field is asuperposition of fields obtained with application for X or Y
deflections separately. In our design, parameters of 1 =600mm and d = 50mm are selected for the deflectors.
Maximum deflection voltage of 1KV is selected. It is in therange where a high-speed amplifier is available in amoderate cost. The deflector has another merit that aloading effect of the beam current is very small.

5 Collimeter
The design of the collimeter is restricted by resolution
for position and direction of measurements and countingrate of the particle detector. From the resolution, twoapertures of 0.2mm X 0.2mm with an interval of 1000mm
are enough. On the other hand, typical flux of the particle
through the collimeter for the incident beam of 1 /.1 A is
expected to be about 4 X 107counts/seconds from the workby Tajima etc.[1 ]. This counting rate is too large for aparticle detector described later. Sizes of apertures should
be selected for appropriate counting rate.

6 Detection of Particles
Particle through the collimeter hits an appropriate target
material and generates secondary electrons, which areamplified by a micro channel plate (MCP). To get higher
permissible counting rate, the secondary electrons must bescattered to the active area of the MCP. We expect
maximum counting rate of about 106counts/second with anappropriate optical design and usage of a low resistance andsmall-pore MCP. This counting rate means that we canobtain 103counts in 1milli-seconds. The collimeter will be
optimized to limit the counting rate.

7 Scan Algorithm and Control Processor
This device scans a beam profile in the 4 dimensional
phase space. The space is too large for the device to scancorner to corner. So, scanning algorithm is very important.

Fig.2 Shroud Type Deflector
We are considering two steps scan, where a coarse scan ismade in the first step and secondly a fine scan is made in
the region where beam current is detected in the coarse scan.
It is another problem to display a result of the scan with 4parameters. But in many cases, representations of two
density maps in position and direction of axis, integrated forthe other axis are enough.
A control processor is needed to establish complicatedscanning algorithm. The deflection voltages of deflectors
are controlled by the processor through digital to analog
converters(DAC's). The processor also has a counter and atimer to measure flux in the channel. And the processor
must be fast to enable a fast measurement. We hope theprocessor can output new setting values for the deflectors
every several microseconds.

8 Conclusion
The basic design work of beam diagnostic device for theinjection line of the JAERI tandem accelerator has been
The device has been designed for daily
discussed.
accelerator operation. But, the distribution measured by thedevice in the four~dimensional phase space may give us
new information about ion sources.

References[1] S. Tajima et al., "-!7 /7 b:fJDJ*~.AMJ::'-,b.O)X.2'Y -!7 /'A tltiJJE(Emittance Measurement of Incident
Beam of JAERI Tandem Accelerator)", JAERI-memo
02-104-480-