The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

ENERGY ANALYSIS OF NEGATIVE HYDROGEN IONS USINGPHOTODETACHED ELECTRONS
Masayuki TAKAHASill, Akira TAKAGI and Yoshiharu MORIHigh Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, lbaraki 305-0801, JapanAn electron of a negative hydrogen ion (H) can beeasily photodetached by collisions with a laser beam ( A=514.4 nm, 2.4 eV), because its electron affinity is small(0.754 eV). The photodetached electrons have beenobserved by the laser beam method at the H beamenergies of 700, 650 and 600 keV, respectively. A
preliminary experimental result of an energy analysis ofa H beam using photodetached electrons is presented.

information about the original negative ion beam, thephotodetached electron or the atomic hydrogen (If)should be measured. If external laser light has a shorttime width compared with that of a bunched beam fromthe injector linac, the energy distribution of the beam ineach longitudinal position of the bunches can bemeasured.
We have developed a compact and simple arrangement
for measuring the energy and an energy-distribution
measurement of a H beam using photodetachedelectrons.

1 Introduction

2 Experimental Apparatus

In the KEK 12 GeV proton synchrotron (KEK-PS),
charge-exchange multiturn injection with a negativehydrogen-ion beam has been used. Good matching
between the longitudinal emittance of the injected beamand the longitudinal acceptance of the synchrotron isrequired to make efficient beam acceleration. For
longitudinal matching, the central value of the energy
and the energy distribution of the injected beam must be
carefully measured. For this purpose, a magnetic energyanalyzer at the injection beam line has been used innormal operation. However, during the measurement thebeam cannot be injected into the synchrotron.
Recently, measurement of photodetached electrons bylaser light has been reported[l]-[3]. By introducing
photons corresponding to the binding energy between theatoms and the additional electron, the electron of thenegative hydrogen ion is easily detached. To obtain

A negative hydrogen ion is an ion with one electron
attached to a hydrogen atom with an electron affinity of
0.754 eV. An electron can be easily detached from anegative hydrogen ion by a collision with an laser beam.
This photodetached electron has the same velocity as theoriginal negative hydrogen ion, if the energy transfer inthe collision is negligibly small. The photodetachedelectron's energy is much smaller than the energy of thenegative hydrogen ion by 111838. Since the magnetic
field strength needed to analyze this low-energy electron
distribution is so small, there is no significant
disturbance to the original negative ion beam.
An energy analysing system using a laser beam isshown in Fig. 1. A preliminary experiment was madeusing the 750 kV preinjector beam line. The preinjector
ion source was operated in a pulsed mode with theAbstractFCLaser Beam
H- BeamHO BeamAnalyzer
Magnet30° Bending Magnet
Interaction Region

SI it

Slit

Supressor Mesh
MicroChannel Plate (MCP)
Figure 1: Schematic diagram of the energy analysing system for electrons photodetached from a H beam using an Ar
laser beam.

-358-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

pulsed H" beam width of 200 /.L sec and a repetition rateof 20 Hz. Three acceleration energies of 700, 650, 600
keV were chosen at this experiment. The beam intensity
was kept at about 1 /.L A during the experiment. After a
30 o bending magnet, an analyzer magnet was installedinside the vacuum chamber. Electrons coming fromupstream of the beam line could be eliminated with the30° bending magnet. An Ar laser (A. =514.4 nm, 5W)
was used for photodetached. The laser power was 0.3W
(6W/cm2 ) measured at the FC. The Ar laser beamintercepts a H" beam at 0 o •This electrons photodetached from H" ions weredeflected by the analyzer magnet. The photodetachedelectrons were measured by a micro-channel plate(MCP). A suppressor mesh electrode system in front of
MCP was installed. By changing a negative voltage tothis mesh, the energy distribution of the photodetachedelectrons can be measured. A small Faraday cup wasinstalled down stream of the chamber to monitor theintensity of H. beam current during the experiment.
Figure 2 shows the energy analysing system placed in thevacuum chamber.

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Figure 2: Photograph of the energy analyzing system inthe vacuum chamber.

3 Experimental Results and Discussions
In this preliminary experiment, the output voltage from
the MCP was measured as a function of the H" beamenergy by changing coil current of the analyzer magnet
from 0 to 1000 rnA. The suppressor voltage for thelow-velocity electrons in front of the MCP was set to-100 V. When an Ar laser beam was injected in to the H"beam, an increase of the photodetached electrons wasobserved, as shown in Fig. 3. The position of this peakdepended on the energy of the H. beam. An energy
measurement of the photodetached electron was achievedby this retarding method. During this measurement, thecurrent of the analyzer magnet was fixed to obtain a peakof the photodetached electron, as shown in Fig. 3. The
suppressor voltage was swept from 0 to -500 V toFigure 3: Characteristics of the MCP signal by changing
the coil current of the analyzer magnet. The position ofthis peaks depended on the energy of the H" beam.
During these measurement, the retarding voltage was set
to -100 V to reject the low-velocity electron component.
observe the energy distribution of photodetachedelectrons. Figure 4 shows the example of MCP signals
with/without laser light at the H" beam energy of 700 keV.
Figure 5 shows the difference of MCP signals between
data with/without laser beam, when the H" beam energies
were set to 700, 650, and 600 keV, respectively, bychanging the preinjector accelerating voltage. As the caseof a H" beam energy of 700 keV, the MCP signal
decreased rapidly near a retarding voltage of -380 V asshown in figure 5(a). The same tendency appeared for theother two cases. Figure 6 shows the differentiated valuesof the data of Fig. 5. The peaks of the differential curves
appeared at a retarding voltage of 381 eV (FWHM 24eV) for 700 keV, 352 eV (FWHM 26 eV) for 650 keV,and 332 eV (FWHM 44 eV) for 600 keV, respectively.
Figure 7 shows the photodetached electron energies
measured by the retarding method versus H" beam-359-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001~>)

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the retarding voltage, when the ff beam energy was kept
to 700keV.

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Figure 7: Correlation between the ff beam energy andthe photodetached electron energy by retarding method.
The plotted points are the data obtained from Fig. 6.

200energies. The plotted points are the data obtained fromFig. 6. The linearity between the measured energies ofthe photodetached electrons and the given energies of ffions seems to be well with the errors of less than 8%. In
this analysis, the energy-transfer effects at collisions
between a ff ion and a photon is negligible in thisexperiment. This may be supported because we havemeasured very forward-direction electrons.

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A simple apparatus enabled the use of a photodetachedelectron to perform an energy analysis with an error ofless than 8% in this experiment. With this device, wecould easily observed photodetached electrons from a ff
beam by the laser light method. Farther improvement ofthe energy analysis by use of the retarding method isexpected to provide better measurements.

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6 References"o-400-500Retarding Voltage (V)Figure 5: The difference between data with/without laser
beam versus retarding voltage. ff beam energies were setto (a) 700 keV, (b) 650 keV and (c) 600 keV, respectively.

[1] P. G. Harris et al., Nucl. Instrum. Methods Phys. Res.
A 292, 254 (1990)[2] D. P. Sandoval, AlP conference proceedings 333, 445(1995)[3] R. E. Shafer, AlP conference proceedings 451, 191(1998)-360-