Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 50 (2017)

Four-dimensional transverse phase space distribution changes withrespect to degree of vacuum of LEBT†T. Nagatomo,∗1 V. Tzoganis,∗1,∗2,∗3 M. Kase,∗1 T. Nakagawa,∗1 and O. Kamigaito∗1†∗1∗2∗3

Condensed from the article in the Proc. of 22nd Int. Workshop on ECR Ion Sources (ECRIS2016), Busan, Korea, August 2016, paper WEPP35, in press.
RIKEN Nishina Center
Cockcroft Institute, Daresbury, Warrington
Department of Physics, University of Liverpool- 149 -Beam Current (μA)

100
40Ar8+40Ar9+80

40Ar11+6040200

εx/εx01.00.80.6
0.40.2

40Ar8+ ( ε = 91 mm*mrad)x040Ar9+ ( ε = 59 mm*mrad)x040Ar11+ ( ε = 41 mm*mrad)x00
1.0

εy/εy00.8
0.6
0.4
0.2

40Ar8+ ( ε = 98 mm*mrad)
y0
40Ar9+ ( ε = 64 mm*mrad)
y0
40Ar11+ ( ε = 45 mm*mrad)y00
1.0

ε4D/ε4D0

The four-dimensional transverse emittance 4D is afundamental quantity, which is the degree of beam
quality, and is invariant under linear symplectic transformations, such as beam optics components including
solenoid lenses and skew quadrupole lenses. The emittance indicates the area of distribution in the phasespace; thus, the smaller 4D is, the better the beamtransport eﬃciency is. Because 4D is not decreased
during the beam transport unless special techniquesof beam cooling are used, it is essential to provide
a beam having an emittance as small as possible atthe extraction area of the ion source. Especially, atthe beam-extraction area, the total current of the extracted ions reaches the order of milli-amperes; thus,
the importance of the space-charge eﬀect is considered
to be serious at the present intensity.
The space-charge eﬀect is induced by the repulsive
forces between the positively charged ions inside of
the beam and causes degradation of the beam quality,
namely the emittance growth. The possibility to compensate for the space-charge eﬀect by injecting a neutral gas into the low-energy beam transport (LEBT)
following the ion source was discussed by Toivanen
et al.1) The electrons produced through collisions between the neutral gas and the beam are considered
to moderate the electric repulsive forces. Based onthat report, we intend to clarify how the 4D of multiply charged argon beams evolves with respect to theamount of krypton (Kr) gas injected in the LEBT ofthe RIKEN AVF cyclotron. The LEBT is connectedto the 18-GHz superconducting ECR ion source (SCECRIS) and the RIKEN AVF cyclotron. We havemeasured 4D by the pepper-pot emittance meter2) installed immediately behind a solenoidal lens following
the analyzing magnet.
Figure 1 shows the change of emittances ofAr8+,9+,11+ -ion beams with respect to the gas pressureof LEBT, which represents the amount of injected Krgas. The Kr gas was injected from a port attached ata vacuum chamber of the analyzing magnet, and theLEBT pressure was measured by a cold cathode gauge,
which was also attached to the camber of the analyzing
magnet. In Fig. 1, the emittances of x,y and 4D werenormalized by the values obtained under the conditionof no Kr-gas injection, which were x0 , y0 , and 4D0 ,respectively. In Fig. 1, all the obtained emittances de-0.8
0.6
0.40.2010-72240Ar8+ ( ε4D0 = 6600 mm *mrad )40Ar9+ ( ε224D0 = 2800 mm *mrad )40Ar11+ ( ε224D0 = 1400 mm *mrad )

10-6

10-5

LEBT Pressure (mbar)

Fig. 1. Obtained beam currents and rms emittances x ,y , and 4D of 40 Ar8+,9+,11+ with respect to the LEBT
residual gas pressure.

crease as the amount of Kr gas increases. Especially, it
is noteworthy that the emittances decrease in spite of
the increases of the observed beam currents in the Ar8+and Ar9+ cases. Further, it is interesting that the 4Dof all charge states seem to decrease similarly to eachother, despite the fact that their projections x andy decrease diﬀerently for each charge state. In orderto clarify the emittance-reduction mechanism, we areplanning to conduct further experiments that wouldarrow us to distinguish between the space-charge compensation eﬀect in the LEBT and the gas mixing eﬀectof Kr gas penetrating the ECR plasma.
References
1) V. Toivanen et al., Nucl. Instr. Meth. B 268, 1508
(2010).
2) V. Tzoganis et al.: Proc. 7st Int. Particle Accelerator Conf. (IPAC’16), Busan, Korea, paper MOPMR048(2016), p. 361