JAERI-Conf 95-021

PB01
LONGITUDINAL EMIITANCE MEASUREMENT
FOR 433 MHZ PROTON LINAC
Hideki DEWA. Takashi SUGIMURA, Masaki KANOO, Masanori IKEGAMI, Valeri KAPIN,Hiromu TONGUU, Toshiyuki SHIRAI, Hiromi OKAMOfO, Yoshihisa IWASHITA.
Hi.rokazu FUJITA, Shigeru KAKIGI. Akira NODA and Makoto INOUE.
Institute for Chemical Research, Kyoto University, Uji Kyoto 611, Japan
Abstract
We are developing a new device to measure thelongitudinal emittance of the proton beam accelerated by theAlvarez linac. For the longitudinal emittance measurement,
the simultaneous measurement of the phase and energy isrequired. The protons scattered by a gold target placed on thebeam line are deflected by an RF electric field and detected bya position sensitive detector (PSD). We can reconstruct thephase and energy of the protons on the target by theposition and energy information from the PSD.

1. Introduction
The information of the longitudinal emittance may be
useful for the understanding of the operation condition of thelinear accelerator. But it has not yet been measured for thehigh frequency proton linac. There are two difficulties in thismeasurement. At frrst, the simultaneous measurement of thephase and kinetic energy of the beam requires very careful
treatment. Secondly, the phase measurement requires a hightime resolution.
The longitudinal emittance measurements for the H"
beam using the laser neutralization technique were studied at
Los Alamos laboratory 1,2). They neutralize the H" beam by
the short pulsed laser for the time slice of the beam, andseparate them by the dipole magnet. Then the energy
distributions of the neutralized beam are measured. But thistechnique cannot be used for the proton linac.
The measurements of the phase distribution for theproton beam were studied by some laboratories 3,4). Theydeveloped such devices as detect the secondary electrons
emitted from the biased thin wire target and deflected by an
RF electric field. Only a part of the beam in the short time
region sliced by this RF shutter are detected with the Faradaycup. The time distribution of the secondary electronsobtained from this monitor is well considered to have thesame longitudinal structure as that of the original beam. This
device has a good phase resolution, but cannot measure thekinetic energy of the proton beam.
We have been developing the new longitudinalemittance monitor which detects the phase and energy of theproton simultaneously since autumn in 1994 5). A focusing
system using permanent magnets has been studied in order todecrease the RF voltage of the deflector and increase thephase resolution. The mechanical design of this newmeasurement system and the fabricated devices are described.

The ?MeV proton beam from the Alvarez linac isscattered by the narrow Au strip target. Some protons
scattered to the 90 degree go through the thin slit and thePMQs. The PMQ1 and PMQ2 focus the protons to the PSD.
Then the particles are deflected horizontally by the RFdeflector whose RF frequency is the same as that of theAlvarez linac. The deflection angle mainly depends on theRF phase when the protons go through the RF gap. ThePMQ3 enhances the deflection angle. Then the position andthe energy of the protons are measured by the PSD. The yieldof this measurement is estimated to be about 23 counts/s atthe peak beam current of 1 rnA with the pulse width and therepetition of 60 J.Ls and 180 Hz, respectively. The signals
from the PSD are amplified and AID converted in the circuitsystem. The raw data are accumulated in the personalcomputer. The longitudinal emittance can be reconstructedby this position and energy distribution data.

Fig. 1 Schematic view of the longitudinal emittancemonitor.
3. Simulation of the system

2. Measurement System
The longitudinal emittance monitor mainly consists offive parts: Au target, RF deflector, permanent-magnetic
quadrupole-lenses (PMQ), position sensitive detector (PSD),
and the· circuit system. The schematic view of the monitor isshown in Fig. 1.

Simulations of this measurement have been performed
to determine the parameters of the system. The optimization
factors are the RF voltage, the deflection angle and theposition resolution on the PSD. The calculated positions on
the PSD as a function of the original position in the bunch
applying the decided device parameters are shown in Fig. 2.

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The initial longitudinal distribution of the proton beam isassumed to be uniform in a rectangular phase space. The
phase spread is ± 90 degree and the energy spread is ± 100
keV. The position X on the PSD depends on the longitudinalposition ZO and the kinetic energy. Because the deflection
voltage is sinusoidal. the X is not proportional to the ZO.
The spread range on the PSD is about 12.6 mm.
The position data which shows the initial phase value
have errors. because of the energy spread and the fmite sizeof the target and sliL This spread limits the phasemeasurement resolution. We estimated the error less than 4.5
degree assuming the energy resolution of the PSD as 40 keV.

X'12mmshown in the photo 1. and the specifications of the cavity
are shown in Table 1. The inner conductor is tapered in order
to increase the shunt impedance. The resonant frequency istuned by a cll34 mm plug tuner with 30 mm moving range.
The folders of the PMQs are fixed before and after the voltage
gap. The unloaded Q and the shunt impedance are evaluated as
12680 and 4.69 MO. respectively by the 3-0 fieldcalculation with the MAFIA code. The measured resonant
frequency of the fabricated cavity was 420 MHz which islower than design value. Then the electrode gap was enlarged
from 4.0 mm to 4.65 mm for tuning. The Q value wasmeasured to be 7010 after the resonant frequency was tuned
to 433.0 MHz. The design value of the electric field is 116
keV/em. The high electric field is desirable for the better
resolution of the phase measuremenL But the high electricfield needs a high power source. We estimate the necessary
power as 1.1 kW by the measured Q value and the calculated'ZJQ. We tried the high power test and more than 1.1 kW peak
power could be supplied.

.;~~ ..
·~-------~~-~------~030mm

Fig. 2The simulated positions of the protons on the PSD for therectangular longitudinal phase space distribution. ZO means
the longitudinal position at the target and X is the positionon the PSD.
4. Target
The target is located at the 20 em downstream from theAlvarez linac. The narrow Au strip target is evaporated on
the thin carbon foil whose thickness is 20 J.lg/cm2 • The
deposited area is 0.7 mm x 3.0 mm and the Au thickness isRF cavity for deflecting the protons. ThePhoto 1
rectangular one seen in the center is an 30 X 14 mm2electrode for deflection gap.

0.1 J.lm. The target should be set on the straight line passing
centers of the PMQs. The alignment error should be less than
0.1 mm to obtain enough measurement accuracy. The angle
between the accelerated beam direction and the normal line
to the target plane is 20 degree. In this way. the area of theAu viewed from the PSD are reduced to nearly 0.2 mm x 3.0
mm. and .the scattering cross section becomes large. The
mean energy loss in the target is 9.8 keV that is not smallenough but compromised value with the yield.
The proton beam is scattered by the Au on the target
and is also scattered by the carbon film. The protons
scattered by the contamination material in the target can beseparated by the kinetic energy of the scattered protons.
because the kinetic energy of the proton scattered by Au is
6.93 MeV. and one by carbon is 5.91 MeV.

5. Cavity
The RF cavity produces the electric field which deflects
the proton scattered by the target. The deflection cavity is-249-

Table 1
Specifications of the RF cavity.
433.0
frequency [MHz]7010
unloadedQ
4.65
electrode gap [mm)30.0electrode length [mm]14.0electrode height [mm]200
outer conductor diameter [mm]100
inner conductor maximum diameter [mm]14
inner conductor minimum diameter [mrn]210outer conductor height [mm]106
inner conductor height [mm]Aluminum with Cu-plated
outer conductor material
Copperinner conductor material
Coppershorting plate material
54.0
design gap voltage [keV]
1100
necessary peak power [W]JAERI-Conf 95-021

6.PMQ
The permanent magnetic quadruples (PMQ) are set tothe holders in the RF cavity. The PMQ is assembled from theeight trapezoidal magnet pieces made of Nd-Fe-B (NEOMAX41H). The PMQs installed in the holder is shown in photo 2.
The specifications of the PMQs are given in Table 2. ThePMQ1 and PMQ3 defocus the protons, and the PMQ2 focus
them horizontally. The PMQ3 enhances the deflection
angle. The 0.1 mm X 3.0 mm slit is located on the front
cover of the PMQl. The slit is made of the four pieces of 0.2
mm tantalum plates.

Photo 2 The PMQ folders. Right one has PMQ1 and PMQ2,and left one has PMQ3. The folders are made of copper-plated
stainless . The tantalum slit is attached on the PMQl.

and AID converted to 13 bits. The timing of the event in the60 J.LS of the RF macro pulse is also measured. We can know
the time dependence of the longitudinal emittance in the RFmacro pulse. The outputs of the three AID converters go topersonal computer and the raw data are memorized. The datataking program is KODAQ 6) which is a flexible dataacquisition system based on NEC PC-9801 distributed byInstitute for Nuclear Study (INS), Univ. of Tokyo. The
histograms of the three ADC-outputs and two-dimensional
scattered plots for the ADC-outputs of the position andenergy signals can be seen on the graphic display by using
this program.

Fig. 3

Table 2
Specifications of the PMQs.

The circuit diagram for the data taking system.
8. Acknowledgments

The authors would like to express theiracknowledgment to Dr. Sugai of INS for manufacturing theexcellent Au target. The authors would also like to express
their acknowledgment to Mr. Ornata of INS for helping themto use the convenient data taking program "KODAQ".

PMQ-No.
12
3
Bore diameter [mm]1010
12Numbers of the Segments
8<-<-Height of the segment [mm]9.18 9.18 8.18
Lower base of the segment [mm] 11.75<-- <-Upper base of the segment [mm]4.14 4.14 4.94
Field gradient [kG/em]-19.5 +19.4 -14.8
Length [mm]43.0 43.0 45.0
Material
Nd-Fe-B <-- <--References

7. PSD and circuit system
The PSD (IPP 1508-500) is a Si charged particle
detector with PN junction, whose front surface is P-typesemiconductor and back side is N-type semiconductor. There
are two contacts on the surface side edges, and the resistance
between them is about 10 kQ. The bias voltage is+ 80 V tothe backside N-type semiconductor. The one connector on
the P-type side is grounded, and another connector is theoutput of the signal proportional to the position and theenergy. The connector for theN-type side is the output of thesignal proportional to the energy. Therefore the position isgiven by the ratio of the two signals. The area of the PSD is
8x15 mm2, and the position resolution is 0.15 mm. Thisdetector is set at the 75 mm from the exit of the PMQ3. For
decreasing the X-ray from the Alvarez linac, we set the Pb
shield around the detector. Two magnets are also used for
sweeping out the electron cloud coming from the deflection
cavity.
The circuit diagram for the signals from the PSD isshown in Fig. 3. The two signals from the PSD are amplified

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Higgins, 0. R. Sander, and D. P. Sandval, "Noninterceptive
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Shida, "A Data Acquisition System based on a PersonalComputer", IEEE Trans. on Nucl. Sci, Vol. NS-39, No. 2,
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