Ⅱ-8. Accelerator

RIKEN Accel. Prog. Rep. 47 (2014)

Magnetic field clamp in direct plasma injection scheme †M. Okamura,＊1,＊2 T. Kanesue,＊1 T. Yamamoto,＊3 Y. Fuwa＊2＊4
Longitudinal Axis (Beam Direction)

Z [mm]RFQ Vanes

A new set of vanes for the radio frequency
quadrupole (RFQ) accelerator was commissioned using thehighly charged iron beam in Brookhaven National
Laboratory (BNL). To supply high intensity heavy ion
beams from a laser ion source (LIS) to the RFQ, the direct
plasma injection scheme (DPIS)[1,2] with a confinementsolenoid was adopted. By in troducing the solenoid field,
the plasma expanding angle can be controlled and thecapability of LIS drastically enhanced[3]. In an LIS, thepeak value of beam current is inversely proportional to the
cube of the plasma drift distance. In order to stretch the ion
beam pulse length, a longer plasma drift length is required,
which simultaneously reduces the current amplitude. The
solenoid can compensate this reduction effortlessly.
however, the solenoid field causes another difficulty. The
fringe field of the solenoid overwraps the ion extraction
area where exist a static extraction electric field andfocusing RF field. Generally, when we apply a magnetic
field on a high-gradient electric field, discharges may beinduced. To accelerate Fe14+ in the DPIS set up, the nozzle
emits ions that have a static voltage gap of 33.3 kV overlaid
by the fringe of the RFQ field of ±20.5 kV at 100 MHz. A
magnetic field of few hundreds Gauss is present in the same
space simultaneously. To investigate the fields, OPERA2Dand 3D [4] were used.
Figure 1 shows an example of the electric static
field simulation by OPERA2D. The nozzle is filled by thelaser plasma, and ions are extracted by the field gradient
between the vanes and nozzle. The plasma and the extracted
ion beams move along the z axis from bottom to top in thefigure. The plasma sheath is formed at the top of the nozzle.
Another high electric field is induced towards the end wallof the RF cavity. When we apply a solenoid field, magnetic
flux of the same direction as the electric field is induced.
When discharges occur, electrons emitted from the vanesurface are accelerated by the electric field towards thenozzle and guided by the magnetic field. This easily triggers
further discharges. To prevent this, the copper-made end
wall flange, which is a part of the cavity, was replaced by aplated iron flange. Another iron-made disk called barrier
flange was also installed. The magnetic field was reduced toa few gauss, as shown in the simulation in Fig. 2. This
modification enabled us to apply sufficient extraction
voltage on the nozzle.
†Condensed from the article in REVIEW OF SCIENTIFICINSTRUMENTS 85 02B907 (2014)＊1
Collider Accelerator Department, BNL＊2
Riken-BNL Research Center＊3
Waseda University＊4
Kyoto University-30000.0

180.0-35000.0-15000.0-20000.0-25000.0-30000.0

170.0
160.0
150.0
140.0 -25000.0-20000.0

End wall of RF cavity-35000.0-30000.0-15000.0-10000.0-5000.0
0.0
5000.010000.020000.025000.0
130.0 15000.0
30000.0

120.0
110.0
25000.0

100.0

Nozzle

90.0
0.0

10.0

30.0

50.0

70.0

90.0

110.0

130.0

150.0
R [mm]

Figure 1 Static electric field simulation at the beam
extraction region.
15000.020000.0
10000.030000.0
5000.0Figure 20.0

Magnetic field simulation with the field clamps.

Using the modified magnetic structure, we observed theaccelerated iron beams at the downstream of the RFQ. The
pulse width was about 1.5 µs on using a 1.0 m length
solenoid filed at 105 G. The RF power was adjusted tomaximize the current, which was about 3 mA. The result
was obtained without beam analysis behind the RFQ, andthe currents include all accelerated particles. The test was
not intended to obtain maximum beam current; however,
we could confirm that all the devices are working asintended.
References

1) M. Okamura, T. Takeuchi, R. A. Jameson, S. Kondrashev, H.
Kashiwagi, K. Sakakibara, T. Kanesue, J. Tamura, and T.
Hattori. Rev. Sci. Instrum. 79, 02B314 (2008)
2) T. Kanesue, J. Tamura, M. Okamura, Rev. Sci. Instrum. 79,
02B311 (2008)3) M. Okamura, A. Adeyemi, T. Kanesue, J. Tamura, K. Kondo,and R. Dabrowski. Rev. Sci. Instrum. 81, 02A510 (2010)
4) OPERA-TOSCA, www.cobham.com- 160 -