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

Energy Analysis of a H- Beam Using Detached Electrons
Katsuhiro SHINTO, Masayuki TAKAHASHI*, Masahiro YOSHIMOTO**,Akira TAKAGI and Yoshiharu MORI
Accelerator Laboratory, High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan*Miyazaki University
1-1 Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan**Doshisha University
1-3 Totani, Tatara, Kyotanabe, Kyoto 610-0321, JapanAbstract
An electron of a negative hydrogen ion (R) can be easily
detached by collisions with an external laser beam or residual
gas molecules in the beam line, because its electron affinity issmall. The detached electrons provide information about theoriginal R ions. The detached electrons should have the same
velocity as the original R ions. By measuring the energy ofdetached electrons, the energy of a H" beam can be estimated
The preliminary experimental results of an energy analysis ofa R beam using detached electrons are presented.

1. Introduction
In the KEK 12 GeV proton synchrotron (KEK-PS),
charge-exchange multi-tum injection with a negativehydrogen ion (R) beam has been used Good matching
between the longitudinal emittance of the injected beam andthe longitudinal acceptance of the synchrotron is required tomake beam acceleration efficient. For longitudinal matching,
the central value of the energy and the energy distribution of
the injected beam must be carefully measured For thispurpose, a magnetic energy analyzer at the injection beam line
has been used in normal operation. In this scheme, however,there is a problem that during the measurement, the beamcannot be injected into the synchrotron.
Recently, the measurement of a photodetached electronsby the laser injection has been reported By introducing aphoton corresponding to the binding energy between the atomand the additional electrons, the electron of the R ion is easily
detached To obtain information about the original R beam,
the detached electron or the atomic hydrogen should bemeasured If external laser light has a short time widthcompared with that of a bunched beam from the injector linac,
the energy distribution of the beam in each longitudinal
position of the bunches can be measured.
We have developed a compact and simple device for anenergy distribution measurement of a R beam using detachedelectrons. Before using a laser, the energy measurement ofdetached electrons by gas detachment is carried out.

2. Experiment

An R ion is an ion in which one electron attaches to a
hych-ogen atom and its electron affinity is 0.754 eV. The
electron can be detached by collisions with residual gas atoms
in the beam line. This detached electron has the same velocity
as the original R ion, if the energy transfer in the collision isnegligibly small. The energy of the detached electron issmaller than that of the H ion by 111838. Since the magnetic
field strength neecb:l to analyze this low-energy electron
distribution is so small, there is no disturbance to the H· beam.
The energy analyzing system using gas cell is shown in
Fig. 1. A preliminary experiment was made using the 750 keV
pre-injector beam line. The pre-injector ion source wasoperated in the pulsed mode; the pulse H beam had a width of
0.2 ms and a reputation of 20 Hz. Three acceleration energies(709.8 keV, 546.0 keV and 455.0 keV) were chosen. Thebeam intensity was kept at about 1 rnA during the experiment.
After a 30 degree bending magnet in the beam line, an analyzer
magnet was installed inside the vacuum chamber. A small gas
cell come installed in the beam pipe. Electrons coming fromupstream of the beam line could be eliminated with the 30degree bending magnet.
MCP(Micro-Channel Plate)
Supressor plate=II

H"BeamBII
Bending
MagnetGasCell)(1SlitAnalyzer
MagnetFaradayCupFig. I Schematic diagram of the energy analyzing system for electronsdetached from a H" beam using a gas cell.

Those electrons detached from H ions in the H 2 gas cellwere deflected by the analyzer magnet. The detached electrons
were measured by MCP (Micro-Channel Plate). A plate infront ofMCP was installed. By applying a negative voltage tothis plate, the energy distribution of the detached electrons wasmeasured A small Faraday cup was installed stream down of-519--

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

the chamber, and the intensity of R beam current wasmonitored during the experiment. Two turbo-molecularvacuum pumps were installed.

MCP signals when the hydrogen energies were 709.8 keV,
546.0 keV and 455.0 keV, respectively is shown in Fig. 3.
The differentiatedvalues of the dataofFig. 3 is shown in Fig.
4.

3. Experimental results and discussions
In a preliminary experiment, the output signal from
MCP was measured as a function of the R beam energy, bychanging of coil current of analyzer magnet from 0 A to 1 A.
The suppressor voltage for the low-energy electrons in front of
MCP was set to -150 V. When H2 gas was introduced into thecell (Pccll= I x 10 5 Torr), a sharp peak of the detached electrons
was observed, as shown in Fig. 2. The position of this peak
Retarding voltage (V)30r-----,------r-----,,-----~----~

\~

;•20.

,H- beam energy 709.8 keV

e withH---------······ ···'•:o2 gaswithout H 2 gas~

Hg. 3 Characteristics of the MCP signal by changing the suppressorvoltage.

l• •:······ ··:···· ·····:

10 ···········~~~~~~~~cl:~o~~~~:,:~~~c~:~~-~-~·~·~~

oL
0200iII•

400

600

800

1000

---~--~!14~5 0_k·~_l;

Coil current (rnA)

30,-----,------,-----,----- -.-----.
H beam energy 546.0 keV• with Hz gas------------i.--------- ~o,-w_it_ho_u_tH_,-;:g_as___,

20

••'•

Suppressor voltage (V)10Fig. 4 Differentiated value of the data in Fig. 3.The peak of thedifferential curve appeared at a suppressor voltege of 404.2 Y in the case0 cococccco.ccccc
0200I400I
600

800of 709.8 keY. The suppressor voltages were found to be 338.8 Y and265.8 Y, in the cases of H- beam energies of 546.0 keY and 455.0 keY,
respectively.

1000

Coil currenl (rnA)
60,-----.------.-----,----- -r-----.

.•:•'···:..:

s> 40'...

....

-~ti~

.....tl...

20:

Q@0200.

H beam energy 455.0 keVwith H 2 gas
0 without H 2 gas

The detached electrons energies measured by the retarding
method for various R beam energies. The linearity between
the energies of the detached::lectrons andthe H- ions seems tobe well within the errors. In this analysis, we neglected theenergy transferefftrts at collisions between R ions andH 2 gas
molecules. This may be supported because we have measured
very forward-direction electrons. In the future measurement
using laser beam, this situation will be improved.

e:!L........

J__ ,

::

c.cf<e-0 !~co <C<Cr'cccccocccceo~_..
ij

400

600!

:Ll."J • • • •_fLJ

800

1000

Coil current (rnA)

Fig. 2 Characteristics of the MCP signal by changing the coil current ofthe analyzer magnet.

depended on the energy of the R beam. An energy
measurement of the detached electron was achieved by aretarding method. The coil current of the analyzer magnet wasfixed to obtain a peak of the detached::lectrons as shown in Fig.
2. Then, the suppressor voltage was swept from 0 V to -500 Vto observe the energy distribution of detached electrons. The-520-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

SOOr------ -r-------r--- ----r-------,

H. beam energy (keV)
Fig. 5 Correlation between the H· beam energy and the detached electronenergy obtained by the retarding method. The inclination of the solid lineis the ratio of an electron to an H- ion. The plotted points are the dataobtained fi·om Fig. 4.

4. Conclusion .
A simple apparatus enabled the use of a detachedelectron to perfonn an energy analysis with an error of lessthan 5 % by this experiment. By this device, we could easily
observed detached electrons from a R beam by the residual gas
method Futthennore, simulations are planned to explain thegas detachment mechanism. An improvement in the energy
analysis with the retarding method is required to perform bettermeasurements.

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[2] R. E. Shafer, AlP conference proceedings 451, 191(1998).
[3] P. G. Harris, H. C. Bryant, A. H. Mohaghegghi, R. A.
Reeder, C. Y. Tang, J. B. Donahue, C. R. Quick, D. C.
Rislove, H. Sharifian, H. Tootoonchi and T. C. Altman,
Nuclear Instruments and Methods in Physics Research, A 2 9 2,
254 (1990).

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