The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

PRODUCTION OF HIGH-CURRENT PROTON BEAMS WITH
A MICROWAVE ION SOURCE FOR ACCELERATORS
M. Tanaka*, S. Hara, T. Hae, T.Iga, K. Saitou, K. Amemiya, K. Hiramoto, S. Kakiuchi**Hitachi, Ltd., Power and Industrial Systems R&D Laboratory, Hitachi, 319-1221, Japan**Hitachi, Ltd., Nuclear Systems Division, Hitachi, 317-8511, JapanAbstract
A microwave hydrogen ion source was developed toimprove reliability, and to increase operation time ofproton accelerator application systems. The ion source
needs no filament in the discharge chamber, which leads
to reliability improvement and less maintenance time. The
developed source produced a maximum hydrogen ion
beam current of 70 mA (high current density of 360
mA/cm2, beam energy of 30 keV) with a 5 mm diameter
extraction aperture and 1.2 kW microwave power. The
proton ratio was increased with an increase in rf power
and reached around 90 % at 1 kW. Measured 90 % beam
normalized emittance was 0.4 π mm mrad. Rise times ofrf power and beam current to 90 % of final values wereabout 30 and 35 µs respectively at a pulse operation modewith 400 µs pulse width and 100 Hz repetition rate. The
dynamic range of beam currents was enlarged (3-63 mA)in the pulse mode with a modified rf waveform to assist
ignition of microwave discharge. These performanceparameters will be desirable for pulse operation
accelerator applications like proton therapy systems. Long
time operation stability (150 h) was confirmed at a beam
current of 51 mA; change in the current was 2%.

1. INTRODUCTION
Recently, proton accelerator systems have beendeveloped for many applications such as proton beamcancer therapy, neutron sources by atomic reaction at
proton beam target, RI production systems, and etc. We
have developed a high-current hydrogen microwave ion
source to increase beam output and reliability for theaccelerator application systems.
We Hitachi have long studied on microwave ion
sources for industrial applications. Around 27 years ago,
Sakudo started development of the microwave ion sources[1] and accomplished a commercial source to produce
heavy ion beams for ion implantation in semiconductor
fabrication processes. Amemiya developed a cylindricalmicrowave ion source for MeV acceleration of mA classhigh-charge-state heavy ions with RFQ in a high-energy
implanter [2]. Tokiguchi and Seki have worked on oxygen
microwave ion sources to fabricate SOI wafers [3].
We have also worked on development of high-current
low-emittance bucket type ion sources for nuclear fusion
devices and accelerators [4, 5].
We developed a new hydrogen microwave ion sources
based on the above technologies to improve performanceof the proton accelerator application systems.

Although T. Teilors[6], J. Sherman[7], R.Gobin[8] hadalready developed hydrogen microwave ion sources,
pulse generation of ion beams were not investigatedprecisely. We especially worked on production ofhydrogen ion beams with short pulse width (severalhundred µs) and high beam current of several tens mA for
pulse operation accelerator systems like medical
accelerators applied to cancer therapy. In this paper,
outline of the developed microwave ion source andexperimental results on pulse hydrogen ion beam
generation are described.

2. EXPERIMENTAL
Outline of the microwave ion source is shown in Fig. 1.
Microwaves (2.45 GHz) generated by a magnetron wereintroduced into a cylindrical discharge chamber through awave guide and rectangular-circular mode converter. Thechamber was made of stainless steel and had innerdiameter of 90 mm. A high density hydrogen plasma wasproduced in the discharge chamber, to which around 1
Sccm hydrogen gas was supplied. Magnetic field ofaround 0.08-0.09 Tesla, parallel to the ion beam axis, wasapplied to the discharge chamber using two coils tocouple the microwave with the plasma. For extracting ion
beams from the chamber, a three grid extraction system
was employed as shown in Fig.1: plasma grid (PG),
deceleration grid (DG) and grounded grid (GG).
Deceleration voltage (-2kV) was applied to DG in orderto suppress back stream electrons from beam plasma
generated downstream of the grids. PG has an extraction
Rectangular-circularmode converter
Microwave

Discharge
BN plate chamber

Insulator(FRP)

Solenoidcoil

Plasma grid(30kV)
Deceleration grid(-2kV)

Insulation column(FRP)

Grounded grid(0V)

Ion beam___________________________________________________________*

masanobu-a_tanaka@pis.Hitachi.co.jp

Figure 1: Outline of the microwave ion source.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

aperture of 5 mm diameter. Ion beams are accelerated
typically by 30kV between PG and GG. Structure ofextraction apertures of the grids and gaps between grids
are optimised to reduce beam divergence and emittanceusing beam trajectory simulation [9, 10].
Total extracted ion beam current was measured with a
Faraday cup (diameter: 34 mm) set 400 mm down stream
from GG. Emittance measurement was conducted with thefirst slit set 190 mm downstream from GG and a Faradaycup equipped with the second slit set 400 mm
downstream. Mass spectra of the extracted ion beams
were measured by a magnetic mass separator and a
Faraday cup.

3. EXPERIMENTAL RESULTS
3.1 Dependence of beam current on microwave
power
Dependence of pulsed beam current on forward
microwave rf power is measured as shown in Fig. 2. Pulsewidth was 400 µs and repetition rate was 100 Hz. Beamcurrent increased approximately proportional to rf powerin the range of 600-1200W and reached to 62 mA at 1200
W. Lowest limit power and beam current to maintainpulsed discharge stably was about 380 W and 14 mA. The
highest beam current was increased to maximum value of
70 mA (360 mA/cm2) by optimizing magnetic field by thesolenoid coils. Beam current of more than 100 mA can beobtained by increase in extraction aperture diameter tomore than 6 mm. Proton content in the beam was morethan 90 % at the rf power higher than 1 kW.

3.2 Characteristics of beam rise in pulse mode
It is desirable to reduce rise time of the pulsed beam
current for injecting beam into pulse operation
accelerators with as little beam loss as possible. Typical
output beam pulse width of linacs for proton therapy
synchrotrons is 50 µs and injection beam pulse width forthe linacs is several hundreds µs.

50

250

40200

30150

100

20

1.22 kW260W

rf power

200400600

800

1000

1200

100 µ sFigure 3: Pulse waveform of beam current
Pulse waveform of beam current obtained for themaximum current beam at a pulse operation mode with
400 µs pulse width and 100 Hz repetition rate is shown inFig.3 with rf power waveform. Rise times of the rf power
and the beam current to 90 % of final values were 30 and35 µs respectively. Total rise time was about 60 µs. We
also investigated time dependence of ion beam contents
by the magnetic mass separator. It is shown in Fig.4. Mostof ion contents were H2+ just after the start of beam rise
and ratio of H+ increases rapidly to around 70 % in 100 µs.
The time to reach 90% H+ ratio was around 200 µs.
Discharge process is considered to be as follows
according to these results. Ignition of the discharge isgenerated by ionization of hydrogen molecules andenergetic electrons in the discharge plasma cause
dissociation of molecules. This promotes production of
H+. Time constant of this process is around 200 µs, whichis considered to decide rise time of the H+ ratio. Beampulse after the 200 µs rise time can be appropriately used
for the pulsed beam acceleration of the linacs.

3.3 Emittance and current density distribution
Emmitance of the pulsed beam was measured as shownin Fig. 5. 90% normalized emittance was 0.4 π mm mrad

H8001400

Forward microwave power P fwd (W)Figure 2: Dependence of beam current on rf power

+

60

40

H
2050

1000Beam current

Ratio of H+ , H 2+ and H3+ (%)

300(mA/cm 2)

60

20 mA

P fwd=1200 W
P VC1=1.5 mPa
V acc=30 kV
V dec=-2 kV
I ccoil1=202 A
I cocoil2o =175 A

100

Ion beam current density J350

Ion beam current I f (mA)

7070 mA

H+2+300100

200

300

400time (micro second)

Figure 4: Time dependence of ion contents

500

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Divergence angle W (mrad)

60

40

V =30 kVacc
Vdec =-2 kV
Ifcup =57 -62 mA=1.5-2.2 mPa
Pvc1
c1

90% normalized emittance= 0.42 π mm mrad

200Rf power forignition
Pfwd1=960 W-20-40-60-10

Beam current
after rf powerdecrease
If2=3.3 mA

Initial beamcurrent
If1=55 mA

Measured position: 190 mm downstream
from the ion source exit
-505

P VC1=1.4 mPa
V acc =30 kV
V dec=-2 kV
I ccoil1=180 A
I coil2=191 A

Rf power after decrease forlower beam current
Pfwd2=320 W

Figurer 6: Modified rf waveform forstable production off lower current beam
10

Vertical position Y (mm)Figure 5: Measured emittance diagram
for the beam current of around 60 mA. This emittance canbe acceptable for most of general linacs. Vertical beam
current density distribution was calculated according tothe measurement. The distribution can be fitted to a
Gausian distribution with a small beam tail of less than
5%. 90% diameter of the beam was evaluated to be about
11 mm by integration of the current density distribution.

3.4 Extension of beam current dynamic range bymodified rf pulse waveform
It is difficult to maintain stable pulsed discharge if
microwave rf power is decreased to lower than the limit
value for decreasing beam current as described in Sec. 3.1.
We modified the rf waveform to assist ignition for stable
discharge at the lower rf power as shown in Fig.6.
At the start of the discharge, microwave rf power was set
960 W, which is high enough for ignition. Rf power wasdecreased to 320 W to decrease beam current after around
100 µs from the discharge start. As a result, discharge canbe maintained stably with the low rf power, which islower than the lowest limit power described in Sec.3.1.
Beam current can be changed by this method over thewide dynamic range from 3.3 mA to 60 mA without any
changes in ion source operation parameters except for
microwave power. These performance will be desirablefor expanding beam current dynamic range of pulse
operation accelerator applications like proton therapy
systems.

3.5 Evaluation of long time operation stability
A 150 h running test of the ion source was conducted toevaluate long time operation stability. Ion beams of 51
mA (260 mA/cm2) were produced with 1kW rf power,
400µs pulse width and 100Hz rate in the test. Beamcurrent can be maintained to be 51 mA with a change of ±2% without feed back control. No damage of ion source
parts was found after the test. Only a maintenance part isa magnetron outside vacuum chamber.

4. CONCLUSION
A microwave ion source was developed for proton
accelerator applications systems. It is confirmed that theion source has enough ability to supply high-current
proton beams stably to the systems at both DC and pulse
mode operation. Maximum beam current obtained was 70mA (360 mA/cm2) with proton ratio beyond 90%, 400µs
pulse width and 100Hz repetition rate. Normalized 90%
emittance was 0.4 π mm mrad (rms emittance:0.1 π mmmrad) at beam current of 60 mA. Rise time of the pulsed
beam current and proton ratio were 60 µs and 200 µs,
respectively, which can be acceptable for pulse operation
accelerator systems like proton therapy systems. 150 h
operation stability was also confirmed with no damage ofany parts and long time maintenance free operation can be
expected according to this result and magnetron life time.
We have proceeded to the next research step in which
we are developing a lens system for focusing ion beams
for injection to RFQ.

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[5] M. Tanaka, et al.,Proc. of 2000 meeting of theAtomic Energy Society of Japan, Aomori, Japan,
2000, p.171(in Japanese)[6] T. Teilor and Jhon S.C. Wills, Nucl. Instrum.
Methods Ph ys. Res., A309(1991)37.
[7] J. Sherman, et al. Rev. Sci. Instrum.,69(1998)1003.
[8] R. Gobin, et al., Rev. Sci. Instrum.,69(1998)1009.
[9] M. Tanaka, et al., Nucl. Instrum. Methods
Ph ys. Res., A449(2000) 22.
[10] Y. Ose, et al.,Proc. of the 10th Symp. on Ion
Sources and Ion-Assisted Tech., Tokyo, 1986, p.207.