JAERI-Conf 95-021

PB32
ION TRAPPING EFFECT OBSERVED IN NAR
Masayuki NAKAJIMA, Koji YAMADA, and Teruo HOSOKAW A
NTT LSI Laboratories, Atsugi, Kanagawa, Japan

Abstract
The effect of trapped ions is shown to cause a serious
problem in NTT normal-conducting accelerating ring (NAR),
which adopted extremely low injection energy (15MeV). Although
the large beam size at injection reduces the effect of ions, the ionsseem to affect the beam more seriously than expected from thecalculation. In this paper, ion trapping effect in NAR are examinedboth theoretically and experimentally.
1. Introduction
To reduce the size and cost of the injector LINAC, weadopted low injection energy (15 MeV) for NAR1l 2l, which isused for SR applications at final energy (800 MeV). To remove
ions, twelve sets of button type electrodes are placed in the ringof 52.8 m circumference and are always used. However, a coupleof phenomena which seem to be caused by trapped ions areobserved. Generally speaking, lower energy electrons are more
liable to be affected from trapped ions but the evaluation is not sosimple because the energy dependent electron beam size also has
an important effect.
At 15 MeV, the large beam size at injection preserves
over a long period because of the long radiation damping time.
Therefore, the size of trapped ions is also large and the ion density
is low. The larger beam size result in smaller effect on electron
beam. On the contrary, the experiment shows that ion trapping
problems are serious though the electron beam size is large atinjection. Short life time and lack of appropriate monitors makethe analysis more difficult.
According to the calculation of beam potential, thepossibilities that the clearing electrodes are not placed properly
and that ions are accumulated in the bending sections where thepotential is deep are pointed out.
To avoid the effect of ions, we examined to adopt thepartial fill operation which is a well known method to avoid ion
trapping. The calculation and preliminary experiment showed that
this method is not effective at injection whereas it is effective instorage.
In this paper, ion trapping effect observed in NARis
described. Outline of NAR and calculated result of the beam
potential are presented in section 2 and the effect of trapped ionsis numerically examined in section 3 and 4. Experimental resultis shown in section 5. The effect of partial fill is tested in section
6. Ion trapping effect in NAR is compared to the effect in SuperALIS in section 7.

various conditions at injection. Therefore, two possible cases wereexamined. In case 1, f. = lOxl0-6 (1tmrad), f. = lxlQ-6 (1tmrad), f.
= 5xl0·3 are assumedxwhile f. is replaced tiy 2.5xl0·6 in case 2~Horizontal emittance is largexbecause electrons are injected bybumping closed orbit in horizontal plane.
Beam potential is computed against these two cases
assuming elliptical (rectangular) vacuum chamber and beam3l (Fig.
1). The inner surface of vacuum ducts including the bending
sections have nearly the same size except the section where RFcavity is located and RF knock out electrodes are placed. Thenthe size of vacuum duct is approximated to be constant rectangularshape of 120 x 58 mm2• Beam current is assumed to be 100 rnA.
To remove ions, button type electrodes are places at bothends of long straight sections (nearby QD) and in the middle ofshort straight sections (nearby QFc). The positions of theseelectrodes are also shown in Fig. 1.
As can be seen from Fig. 1, the beam potential in thebending sections is a few voltages deeper than that in the straightsections in case 1. This difference may produce the neutralization
pocket. However, as the result strongly depends on the beamemittances which have not been confirmed experimentally, theexistence of potential well cannot be proved from this calculation.
Just a possibility is presented.
3. Neutralization Factor
Neutralization factor (h) is defined as n. In where n. isthe number of trapped ions and ne is the number Of electrons. The10BeamSizeI

cryt:>
0

2. Beam Potential
Lattice functions, beam size, and beam potential arecalculated and shown in Fig. 1 for one fourth section of NAR.
Lattice functions are re-calculated assuming the conditions
used in injection experiment.
Beam size at injection is computed from the following
formula,Ox= Jf.x~x + (O,TJ) 2O"y(1)
10.0

5.0

BM=JE-y~ywhere f. and f. are the horizontal and vertical emittances, and f.
is the energy spread. Whereas these emittances and energy spread
during storage can be calculated theoretically, they depend on-338-

I (m)

QD QF

Fig. 1 Lattice functions, beam size, and beam potential

JAERI-Conf 95-021

neutralization factor at injection is roughly estimated with ion
clearing electrodes4>. Assuming that the residual gas molecules
are CO, the ionization time ('t. . ) which is the average timerequired for an electron to ionize0 amolecule is about one second.
If there is no mechanism to clear ions, neutralization factor
increases in proportional to elapsed time from injection and reaches
as large as 100 % in one second. On the contrary, if a molecule
ionized and trapped by the electron beam drifts along the electron
beam with thermal velocity (a few hundreds meters per second)and is cleared at the electrodes which are placed in four to fivemeters intervals, an ionized molecule remains trapped for 10 msec.
Therefore, the neutralization factor approaches to the equilibrium
value of 1 % (= 10 msec I 1 sec) in about 10 msec. If H molecules
are assumed, the neutralization factor is estimated sm;;ller becauseof the larger ionized cross section and the faster thermal velocity.
As ions run faster in bending sections than in straight sections,the neutralization factor should be smaller.
In the following section, the effect of ions is estimatedassuming the 1 % neutralization factor.
4. Numerical Evaluation of Ion Trapping Effect
In this section, the effect of trapped ions is numericallyevaluated in terms of tune shift, pressure rise, and two beam
instability.
The vertical tune shift due to trapped ions is expressed

where r is the classical electron radius, yis the Lorentz factor, R
is the mean radius of the ring , v are the betatron tunes. Assumingthat the beam size is cr =5 mmxiind cr = 2 mm (this assumption isclose to case 2 in sectlon 2) and thatythe ions have the same sizeas that of electrons, tune shift at injection is estimated to be 0.0068
at 100 rnA and 1 %neutralization factor. Tune shift is small
compared to electron energy because the beam size is large atinjection. As the electric fields produced by ions consist of large
non-linear components, the same amount of tune spread isexpected. Judging from this calculation, neither tune shift nor tune
spread seems to be fatal as long as ions are removed efficiently at
the clearing electrodes.
We tried to measure this tune shift experimentally but
could not perform yet due to the difficulties of the precise tune
measurement. The strength of RF knock out has to be controlled
as small as possible so that it does not influence trapped ions. Thetune have to be measured in a few tens milli-seconds because of
the short life time at injection.
Pressure rise due to ions is estimated from5>(3)

and we can derive 3.4xl011
1 I m 3 at 15 MeV assuming
the same size and theneutralization factor. As
compared to the residualgas density, 3.5x10 13 at
1x10·9 Torr, the pressure
rise is very small.
In some condition,
the electron oscillation andion oscillation become
coupled and the amplitudes
of both oscillation growrapidly 5>6l. The thresholdneutralization factors arecalculated according to ref.
5. Supposing v Y = 1.20, I =100 rnA, cr =5 mm, and cr= 2 mm, xthe instability
occurs when h is larger than
0.60 in case of H 2molecules. The threshold is larger if heavier molecules areassumed. Therefore, this kinds of instability cannot happen witha few % neutralization whichever molecules are assumed.
5. Injection Experiment
In normal operation of NAR, an electron pulse from
LINAC is injected in acceleration. To examine the injection
condition, the experiment described in this section is performed
with keeping the magnetization at constant injection level.
Current decrease after one pulse injection from LINAC
is observed with DCCT at 15 MeV static level (Fig. 2). The
difference with and without ion clearing electrodes can clearly beseen. However, considering the expected life time from vacuum
pressure is longer than 10 seconds, the observed life time is stillshort even when the ion clearing electrodes are used.
The rising time of DCCT is about 100 msec which is too
slow to observe the transition current just after injection. Then,the RF pick up signal of the electron beam is observed withspectrum analyzer which is set to measure RF acceleration
frequency (125 MHz) with zero span mode (Fig. 3). The signal isthought to be in proportional to (the square of) the current. As thevertical axis in Fig. 3 is in logarithms scale, the current graph is
expected to be straight as long as the beam current decreases withconstant life. On the contrary, the slope gets steeper about 30
msec (without ion clearing) and 50 msec (with ion clearing) afterinjection.
This life time change is thought to be due to ion trapping
effect from the following reasons. One is that the effect gets weaker
if the ion clearing electrodes are used. Another reason is that thetiming when the electron beam begin to be affected is consistent
with the intervals required to produce ions of a few %neutralization. Estimated from the tune shift calculation in section
2, it is possible that the life shortens due to the tune shift inducedby trapped ions of a few % neutralization.
Ion production rate at injection may be faster than therate calculated in section 3 because much electrons which cannot
be captured in RF bucket or have large emittances exist and ionize
molecules, though these electrons are lost after a while and do ·not contribute to the beam current.
Even when the ion clearing electrodes are used, Fig. 3 (a)shows the similar shape as that of Fig. 3 (b) (without clearing).
This indicates that ions cannot be removed well even if the clearing
electrodes are used.
Therefore, theassumption that ions candrift along the electron
orbit freely and clear at
the electrodes does notseem valid. As expectedfrom the calculation ofcase l in section 2, ionsmight be accumulated atneutralization pockets
where beam potential isdeep.
These interpretations arepresumptions and are
not proved yet.
However, no otherinterpretation ispresented thus far.
6. Partial Fill Operation

Fig. 2Current decrease after injection
Upper line : ion clearing on
Lower line : ion clearing off
Horizontal: 100 msec I div: 20 rnA I div
Vertical

It is well known
that partial fill operation
is an effective method toremove trapped ions.
This method is
examined as a measure
to avoid ion trapping
effect at injection.
Partial fill operation instorage is also-339-Fig. 3 RF pick-up signalof electron beam(a)- upper- ion clearing on(b) - lower - ion clearing off

JAERI-Conf 95-021

8. Conclusion
Ion Masses
CUrrent(A)

0.01

0.02
0.03
0.04
0.05
0.06
0.07

1 ••..... 44 1 •.••... 44 1 •..•••. 44 1 •...••• 44 1 . . . . . . . 44 1 ....••. 44xxxxxxxx

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In NAR, ion clearing electrodes are placed at twelve points
and are always used. Even if the clearing electrodes are used, thelife time at injection is very short due to the ion trapping effect .
The possibility that ions are trapped in the bending sections where
the beam potential is deep is presented. Partial fill operation modeis examined to remove trapped ions but is not shown to be effectiveat injection. Although ion trapping effect is not clearly analyzed,
the elucidation and countermeasure of this problem are expected
to improve NAR beam current.

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XXX:X X XX XXXXXXXXX XXXXXXXXXXXXX XX:X:XX XXXXXXX XXXXXXXX XX:X:XXX XXXXXXXXX XXXXXXX X XXXXXXXXXXXXXXX XXXXXX XXXXXXXXXXXXXX XXXXXXXXXXAcknowledgment

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1/21
5/17
13/9
17/5
19/3
21/1
Number of Consecutive Bunches I Number of Missing Bunches

The authors would like to express our appreciation to SR
operators who cooperated in acquiring the experimental data.

Fig. 4 Linear stability of ions of mass
=1, 2, 12, 16, 18, 20, 22, 28, 40,44(x indicates stability for the given ions)

References

investigated for the purpose of confirming the effect of thisoperation.
First of all, the effect of partial fill is numericallyestimated. Fig. 4 is a calculated result in storage (800 MeV). The
partial fill operation can be effective to remove ionized lightmolecules such as H2• At injection energy (15 MeV), ion motion
always remains stable. The difference is caused by the difference
in beam size. At 15 MeV, because of the large beam size, thefocusing force per bunch is too weak for ions to escape the electron
orbit while the ions are drifting between bunches.
The effect of partial fill operation is also examined
experimentally. To perform partial fill operation in our system
which adopt one pulse multi-tum injection, LINAC must beoperated in a burst mode. That is, the electrons extracted from anelectron gun are modulated by the radio frequency of NAR andpartially removed in a period of the harmonic number of NAR.
The partial fill operation functions well, however, the present
injected current is about 30 - 50 rnA, which is less than the normal
operation (over 100 rnA). RF pick up signal shows the bunch isfilled as expected even if electrons are accelerated.
The effect of partial fill operation to avoid ion trappingwith little current is examined by worsening vacuum pressure.
Some of vacuum pumps are turned off to enhance ion trapping
effect.
When more than three bunches omitted, beam profile
observed with CCD camera is always stable even if the vacuum
pressure is worsened. In full bunch operation, beam profile blinks
if the ·vacuum pressure is worsened. Inferred from the calculation
described above, this phenomenon seems to be due to two beam
instability caused by light molecules such as H2• Beam current
was about 30 mA in this experiment.
We also compared the full bunch operation mode and 5bunches omitted mode at 15 MeV injection. In both operation,
the current increased with ion clearing electrodes on. This showsthat ions are trapped without ion clearing electrodes.
Although this partial fill experiment is preliminary, partial
fill operation is not shown to be effective to remove ions at 15
MeV. This result is consistent with the calculation.

1. A. Shibayama, et al., Rev. Sci. Instr., Vol. 60, p. 1779 (1989).
2. M. Nakajima, et al., this conference.
3. G. Grobner, et al., CERN/ISR-TH-VA/75-27, (1975).
4. E. Bozoki, et al., BNL 43103 (1989).
5. Y. Baconnier, CERN 85-19, p. 267 (1985).
6. Y. Kamiya, et al., Proc. of the 5th Symp. on Accelerator Sci.
and Tech., p. 292 (1984).

7. Comparison to Super-ALIS
Maximum 200 mA beam current is performed in SuperALIS, which also used 15 MeV injection energy. One of thedifferences from NARis its circumference. That is, from eq. (2),tune shift is in proportional to the number of electrons stored aslong as the other conditions, such as the beam size or neutralization
factor, are the same. If the beam current is the same, the numberof electrons is proportional to ring circumference. Therefore, tune
shift in NAR of 52.8 m circumference is expected three times aslarge as that of Super-ALIS of 16.8m. The other deference is theplaces of ion clearing electrodes. Super-ALIS has clearing
electrodes along the orbit in the bending sections as well as button
type electrodes at the straight sections. In NAR, button typeelectrodes are located only in the straight sections.

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