Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 51 (2018)

Residual Gas Eﬀect in LEBT on the Transverse Emittance ofMultiply-Charged Heavy Ion Beams Extracted from ECRIS†
T. Nagatomo,∗1 V. Tzoganis,∗1,∗2 J. P. Mira,∗1,∗3 T. Nakagawa,∗1 and O. Kamigaito∗1CMOS
digital camera

TMP (220 L/s)

ICF203

RIKEN 18-GHz
SC-ECRIS

Pin-holes
plate

MCP
imaging
screen45 degreetilted mirror

BeamshieldU.H.V.
Stepper
Moter

Einzel Lens

TMP (1100 L/s)

50 mm

050100 mm

Pepperpotemittance meter PPEMGV

FaradaycupAnalyzingMagnet

Beamchopper

Solenoidlens

TMP (300 L/s)

Steerer
Gas injection
Port0GV
5001000 mm

TMP (450 L/s)

Fig. 1. Schematic of the RIKEN 18-GHz SC-ECRIS, LEBT,and PPEM.
†∗1∗2∗3Condensed from the article in the Proc. of 17th Int. Conf. onIon Sources (ICIS2017), Geneva, Switzerland, October 2017,in press
RIKEN Nishina Center
Cockcroft Institute, Daresbury, Warrington
iThemba LABS, Somerset West, South Africa

Initial (no gas injection)

Ar-gas injected in the LEBTLEBT Pressure = 1.4 x 10-3 Pa
202010101000-20

X-Y 40Ar9+-20-100x (mm)10X-Y 40Ar9+
Ar Gas Injection-20200-10-10-10-20y (mm)20y (mm)-10-20010x (mm)60404040x’ (mrad)

60

60

200200-20-20-40-40
X-X’ 40Ar9+-20-100x (mm)1020-100

102001020x (mm)200-20-40

X-X’ 40Ar9+
Ar Gas Injection

-60

-60

X-Y 40Ar9+
Ar Gas Injection-20

20x’ (mrad)y (mm)LEBT Pressure = 1.2 x 10-5 Pax’ (mrad)

When operating the electron cyclotron resonance ion
source (ECRIS) with high-intensity beam extraction, weoften experience the beam current changing in accordance with the pressure of the low energy beam transport (LEBT). After the LEBT is vented, the beam intensity is sometimes higher than usual, before we evacuate itto the reachable vacuum degree. Space-charge compensation is the one of the possible mechanism that cause
the phenomenon as discussed by Toivanen et al.,1) and it
should be an advantage when high-intensity ion beams
are extracted from the ECRIS. Thus, we investigatedthe eﬀects of residual gas in the LEBT on the transverse
phase space distribution of the 40 Ar beam, e.g., correlation between the beam proﬁle and emittance, by using
the pepper-pot emittance meter (PPEM).2)
Figure 1 shows the RIKEN 18-GHz superconducting
ECRIS (SC-ECRIS) with the following LEBT. Initially,
without the gas injection into the LEBT, the SC-ECRIS
was tuned for the Ar11+ beam current to be maximum,
typically ∼ 70 μA, with a 10-kV extraction voltage anda 600-W microwave. The total beam current extracted
from the SC-ECRIS was estimated as 0.9 mA from theelectric current of the extraction voltage source. ThePPEM, by which the four-dimensional transverse phasespace distribution was obtained, was positioned before
the solenoid lens to avoid the beam-optics complications caused by the helical motion of the beam traveling through the lens. In order to control the residual
gas pressure from 10−5 to 10−3 Pa, we intentionally injected the neutral Ar gas through the variable leak valve

X-X’ 40Ar9+
Ar Gas Injection-60-20-100x (mm)1020-20-10x (mm)Fig. 2. Obtained beam proﬁle (upper) and horizontal emittance (lower) of 40 Ar9+ beam by using PPEM.

attached at the gas injection port of the magnet chamberin Fig. 1.
Figure 2 shows the beam proﬁle (x-y plot) and thehorizontal emittance (x-x plot) of 40 Ar9+ beam at theLEBT pressure of 1.2 × 10−5 Pa (no gas injection) and1.4 × 10−3 Pa (Ar-gas injection). From Fig. 2, the beam
intensity of the central area of the hollow triangle-shaped
beam proﬁle appears to increase with Ar-gas injection.
However, by selecting a minor component in the emittance plot, a similarly shaped beam proﬁle is clearly
separated from the main component. From the systematic studies on the Ar beams from 7+ to 12+, the parasite component is concluded as the Ar beam with othercharge that captures an electron before the analyzing
magnet. For example, the Ar10+ beam, which is extracted as Ar11+ from the ECRIS, captures an electron
during the travel to the magnet, and is bend by themagnet with the charge of 10+, can mix in the Ar9+
beam because of their similar Bρ’s. In addition, fromthe obtained M/Q spectra, the valleys between the Ar
peaks become shallower as the LEBT pressure exceeds
10−4 Pa by the Ar-injection. Thus, we found that theother charge exchanges and the multiple scatterings withthe residual gas are no longer negligible at the LEBT
pressure of 10−4 Pa. In the case of ∼10−5 Pa, we detected no signiﬁcant signs of these eﬀects nor the spacecharge compensation.
References

1) V. Toivanen et al., Nucl. Instrum. Methods Phys. Res. B
268, 1508 (2010).
2) V. Tzoganis et al., Proc. 7st Int. Particle Accelerator Conf. (IPAC’16), paper MOPMR048 (Busan, Korea,2016), p. 361.

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