the 133Cs27+ beam of 10 ms was obtained, as shown in Fig. 4.27c. Figure. 4.27c shows that a beam of sufficiently long and flat pulses can be extracted from the EBIS by the slow extraction method, although it is not a perfect flat-top shape.
The primary performance tests of the RAON EBIS charge breeder were conducted through various experiments using the Rb and Cs test ion sources. Using two stable Rb isotopes and additional K ions emitted from the Rb source, the charge breeding effect on several elements was confirmed. And, the effective current density for the Rb ions was calculated to predict the necessary parameters for the Rb ions or the ions with similar mass in the EBIS experiment. Moreover, while scanning and measuring the highly charged 85Rb and,87Rb ions, the setup for the beam diagnostics capable of distinguishing these two types of isotopes was completed. In the EBIS efficiency measurement experiment using the Cs ions, it was shown that the efficiency of the EBIS breeding the ions achieved 67.52%. The performance of higher efficiency could be satisfied through later operation parameter optimization. In addition, the charge breeding effect of the EBIS according to the breeding time and electron beam current was confirmed from the experimental results under various conditions. Also, the effective current density for the 133Cs ions was also derived for future EBIS operation to use it. Finally, through a pulse stretching experiment, the possibility of adjusting the length of the beam extracted from the EBIS using the slow extraction method was confirmed, as a pulse length of∼10 ms of Cs27+ was obtained. In both the Rb and Cs cases, the most abundant ions extracted from the EBIS coincided with A/q < 6 and the energy per nucleon of 10 keV/u, which satisfies the required conditions for the RAON facility. Based on the results using the test ion source, the performance of the EBIS is confirmed by continuing the experiment with the ISOL beamline and RFQ-CB.
4.4 Experiments with Stable Ions Transported from ISOL Beamline (Cs,
and the beam transportation test was conducted for their stable transmission.
4.4.1 Producing and Transporting 133Csq+ using 133Cs+ Ions
First, the Cs beam from the ISOL beamline was used to compare it with the experiment using the test ion source. The ion beam energy in the ISOL beamline is 20 keV and is transmitted as a DC beam to the RFQ-CB. Cs ions are cooled in the RFQ-CB to reduce energy spread and emittance using the He gases and are extracted in bunches containing∼108particles. The bunched beam enters the EQT line of the beam transportation line of the EBIS through the EBIS branch point and is transmitted to the EBIS. As shown in Fig. 4.28, the beam transported this way has a length of about 70µs (FWHM) and 1.14×108 particles.
- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
- 1 0 0
0
1 0 0 2 0 0 3 0 0 4 0 0
Beam Current [nA]
T i m e [µs ]
Figure 4.28: Cs beam injected from ISOL beamline measured at switchyard line.
To breed this beam, the SC magnet of 6T and an electron beam of 1 A were used as in the test ion beam experiment. Figure 4.29 shows the results of measuring the charge-bred ions with 40 ms breeding time by the dipole magnet scan with a slit width of 4 mm. As shown in Fig. 4.29 not only the injected Cs ions but also the ionized residual gases ions were measured. Compared with Fig. 4.13, the result of using the test ion source, the peak of A/q of 2 among the residual gases is significantly high, which is expected
4He2+to be caused by the He gas used in the RFQ-CB. The pressure of the He gas in the cooling section is used up to 1 Pa in the RFQ-CB. So it seems that the He gas flows into the EBIS breeding section and is ionized, or the He ions ionized in the RFQ-CB are transmitted and bred with the Cs ions.
Figure 4.30 is an enlarged view of the part of charge-bred Cs ions in the results of Fig. 4.29. As illus- trated in Fig. 4.19, the A/q distribution in which the targeted Cs27+has the highest peak was confirmed.
In addition, the amount of each ion was calculated from this result to obtain the charge distribution results in Fig. 4.31.
As shown in Fig. 4.31, Cs27+accounts for the highest amount, and Cs26+ similarly amounts. Also, compared to the result of the test ion beam, Fig. 4.20, the overall charge distribution was broader, and
0 1 2 3 4 5 6 7 8
0
2 0 4 0 6 0 8 0 1 0 0
C s q +
Charge / Pulse [pC]
A / q R e s i d u a l G a s e s
Figure 4.29: A/q spectrum for charge-bred residual gases and Cs ions measured at EBIS diagnostics line with ISOL beamline.
4 5 6 7
0
2 0 4 0 6 0
1 8 + 1 9 + 2 0 + 2 1 + 2 2 + 2 3 + 2 4 + 2 5 + 2 6 + 2 7 +
2 8 +
2 9 + 3 0 + 3 1 +
Charge / Pulse [pC]
A / q
3 2 +
Figure 4.30: A/q spectrum for charge-bred Cs ions measured at EBIS diagnostics line with ISOL beam- line.
the ratio of Cs26+ increased, while the proportion of Cs28+ decreased. There was a difference from the CBSIM calculation results using the previously calculated effective current density of 285 A/cm2. The reason for this effect seems to be the influence of the He ions mentioned above. As the He ions, including the existing residual gas in the EBIS, occupy the charge capacity, it is expected that their space charge effect changes the injection condition of the Cs ions and the overlapping condition with the electron beam. Therefore, the subsequent experiment is conducted by introducing the plan to reduce the influence of the He gas and ions.
The efficiency of the EBIS was calculated using the charge distribution result. The number of ions
1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2
05
1 0 1 5 2 0 2 5
0 .0 0E+ 00 2 .5 0E+ 06 5 .0 0E+ 06 7 .5 0E+ 06 1 .0 0E+ 07 1 .2 5E+ 07 Number of Particles (Experiment)
Relative Abundance [%]
C h a r g e S t a t e
E x p e r i m e n t C B S I M
Figure 4.31: Experimental and calculated charge distributions of Cs ions measured at EBIS diagnostics line with ISOL beamline.
injecting the EBIS was 1.14×108, and the number of the highly charged Cs ions obtained from the result was 5.00×107. So, it confirms the efficiency of 43.9%. This value is significantly lower than the result of the test ion beam, and one of the reasons can be seen as the influence of He ions. Although The total amount of ions entered in the EBIS is 1.14×108, some of these contain He ions, making the value appear low in the efficiency calculation. In addition, as mentioned above, as the injection conditions of the Cs ions change, and as He ions account for a significant portion of the charge capacity of the EBIS, it can make it difficult for Cs ions to exist in the breeding region stably. Another reason can be found in the experimental setup. Unlike the test ion beam experiment, in this experiment, the Einzel lens on the switchyard line was not operated normally due to the electrical breakdown issue. For this reason, the efficiency has decreased as the beam optics conditions have not been optimized.
The Cs ions extracted from the EBIS were transmitted to the ISOL beamline through the EQT line instead of the diagnostic line. The highly charged Cs ions go out through the EBIS branch point and are transmitted to the A/q separator of the ISOL beamline. And the ions with a targeted A/q value are separated through the dipole magnet and transmitted to the rear end. To confirm the transmission efficiency from the EBIS to the A/q separator, the charge distribution of Cs ions was measured by scanning using the dipole magnet, like measurement by the EBIS. Its results are illustrated in Fig. 4.32.
Compared with the previous result in Fig. 4.31, it can show that the charge distribution moves to the lower charge state, and the maximum peak becomes Cs25+, which is decreased by two states. Its reason is the increased vacuum pressure caused by the He gas used for the RFQ-CB. Based on the EBIS branch point closest to RFQ-CB, the degree of vacuum around it increases by about 10 times, and the Cs ions are recombined more, resulting in a change in the charge distribution. Furthermore, when calculating the total amount for ions with a charge state of 20+ or more from Fig. 4.32, the amount was calculated to be about 1.90×107. That is, the transmission efficiency from the EBIS to the A/q separator was 38%.
1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2
05
1 0 1 5 2 0 2 5
0 .0 0E+ 00 9 .4 9E+ 05 1 .9 0E+ 06 2 .8 5E+ 06 3 .8 0E+ 06 4 .7 4E+ 06 Number of Particles
Relative Abundance [%]
C h a r g e S t a t e s
Figure 4.32: Charge distribution of Cs ions measured at A/q separator on ISOL beamline.
Even if the charge-bred ions less than 20+ were not measured, the transmission efficiency was very low.
Injected Total Bred Number EBIS Total Bred Number Transmission Number in EBIS Efficiency in A/q Separator Efficiency
1.14×108 5.00×107 43.9% 1.90×107 38%
Table 4.1: Efficiencies of charge breeding and transportation of charge-bred Cs ions.
This is, for the same reason as mentioned above, because the Einzel lens of the switchyard line could not be used normally. So it was not possible to optimize the beam optics for the transmission of the highly charged ions.
After the experiment using the Cs ions transported from the TIS, the effect of the He gas on the EBIS experiment while preparing for the Sn beam extracted from TIS was tested. The charge breeding was performed as before using the Cs test ion beam of the EBIS for both cases with and without the He gas in the RFQ-CB. The He gas pressure at the RFQ-CB used the experimental condition of 0.5 Pa (5×10−3 mbar). The vacuum data in the presence or absence of the He gas is shown in Table 4.2, and there are
Branch Point EQT Line Switchyard Line Without He Gas 1.32×10−8mbar 2.02×10−8mbar <5.00×10−9mbar With He Gas (0.5 Pa) 1.32×10−7mbar 1.51×10−7mbar 9.00×10−9mbar
Table 4.2: Change of vacuum pressure by He gas in RFQ-CB.
increases of 10 times in the EBIS branching point and 7.5 times in the EQT line. The measurement of the highly charged ions was scanned using the A/q separator, and Fig. 4.33 shows the results. As illustrated in Fig. 4.33, when there is the He gas in the RFQ-CB, the charge distribution is lowered by two states, as in Fig. 4.32. In addition, the distribution was broader than when there was no He gas. If it
2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e
W i t h o u t H e G a s
H e P r e s s u r e = 0 . 5 P a
Figure 4.33: Charge distributions of Cs ions with and without He gas measured at A/q separator on ISOL beamline.
is possible to reduce the leakage of the He gas in the RFQ-CB, this problem can be reduced, resulting in improved results in the ISOL operation.
This experiment confirmed the effect of charge breeding, and the Cs ions with the high charge state in the EBIS satisfy the requirements of A/q < 6 and energy per nucleon of 10 keV/u. However, the EBIS efficiency in this experiment did not reach the efficiency of the test ion beam test, and the transmission efficiency was low in the beam transmission. Therefore, for the normal operation of the ISOL beamline, commissioning is carried out by supplementing the He gas problem with the RFQ-CB and the electrical setup for the beam optics.
4.4.2 Producing and Transporting 120Snq+ using 120Sn+ Ions
Before the experiment using the Sn beam, a structure for the differential pumping was installed after the RFQ-CB to reduce the influence of He gas. Before installing the structure, the inner diameter was 50 mm, but the internal diameter was 20 mm to reduce the leakage of the He gas by the aperture. Table 4.3
Branch Point EQT Line Switchyard Line Without He Gas 2.40×10−8mbar 2.10×10−8mbar <5.00×10−9mbar With He Gas (0.5 Pa) 3.40×10−8mbar 7.25×10−8mbar 5.00×10−9mbar Table 4.3: Change of vacuum pressure by He gas in RFQ-CB after installing aperture.
shows the vacuum pressure in each area depending on the presence or absence of the He gas after the installation of the structure. The vacuum in all lines is doubled or less by the He gas, so the effect of the He is reduced compared to before.
After that, the charge breeding test of EBIS was continued using the 120Sn, a stable isotope of 132Sn
to be used later. 120Sn ions are cooled and bunched in the RFQ-CB and sent to the EBIS, and the results of measuring the beam reaching the switchyard line with the Faraday cup are illustrated in Fig. 4.34.
The amount of Sn ions entering the EBIS is 1.11×108, and the bunch width is about 30µs (FWHM).
- 5 0 0 5 0 1 0 0 1 5 0 2 0 0
- 1 0 0
0
1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0
Beam Current [nA]
T i m e [µs ]
Figure 4.34: Sn beam injected from ISOL beamline measured at switchyard line.
Charge breeding was performed with an electron beam of 1 A and a breeding time of 40 ms targeting to make a charge state of 24+ the highest abundance.
The charge-bred Sn ions were extracted from the EBIS by adjusting the EBIS HV platform so that the energy per charge was 50 keV/. And, the dipole magnet scan was performed on the diagnostic line to measure the A/q spectrum. As illustrated in Fig. 4.35, the target charge state of 24+ could be obtained with the highest peak, satisfying the energy per nucleon of 10 keV/u for 120Sn24+ions.
4 5 6
0
2 0 4 0 6 0 8 0 1 0 0
Charge / Pulse [pC]
A / q
2 0 + 2 1 +
2 2 + 2 3 + 2 4 + 2 5 +
2 6 +
2 7 +
2 8 + 2 9 +
Figure 4.35: A/q spectrum for charge-bred of Sn ions measured at EBIS diagnostics line.
The number of ions corresponding to each charge state was analyzed from the A/q spectrum to
obtain charge distribution data. The calculation was carried out using the CBSIM to compare this with the calculation results. The current density of the electron beam used in the CBSIM is 245 A/cm2 to obtain the calculation result consistent with the experimental result for 120Sn. It is also different from 166.4 A/cm2 obtained through the calculation using the magnetic field. Therefore, 245 A/cm2 can be used as the effective current density for 120Sn, which can predict the effect of charge breeding depending on the electron beam current and the breeding time. In addition, Fig. 4.36 shows that the charge distribution in the experiment and the calculation are similar.
1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0
05
1 0 1 5 2 0 2 5
0 .0 0E+ 00 4 .9 6E+ 06 9 .9 2E+ 06 1 .4 9E+ 07 1 .9 8E+ 07 2 .4 8E+ 07 Number of Particles (Experiment)
Relative Abundance [%]
C h a r g e S t a t e
E x p e r i m e n t C B S I M
Figure 4.36: Experimental and calculated charge distribution of Sn ions measured at EBIS diagnostics line.
The highly charged ions with the highest relative abundance of 120Sn24+ were transmitted to the ISOL beamline, and the charge distribution of these ions was measured through the A/q separator. The
1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0
05
1 0 1 5 2 0 2 5
0 .0 0E + 00 4 .4 6E + 06 8 .9 2E + 06 1 .3 4E + 07 1 .7 8E + 07 2 .2 3E + 07 Particle Number
Relative Abundance [%]
C h a r g e S t a t e
Figure 4.37: Charge distributions of Sn ions measured at A/q separator on ISOL beamline.
results are illustrated in Fig. 4.37, and the charge state of 23+ accounts for the most significant amount.
Although the aperture was installed after the RFQ-CB to reduce the influence of the He gas, it is believed that the charge distribution shifted toward the low charge state without completely eliminating the He effect. However, in the case of the Cs, the 2 states were lowered, but in this Sn experiment, the influence of He gas seems to have decreased as only the 1 state was lowered.
By calculating the amount of all ions in Figs. 4.36 and 4.37, the efficiency of the EBIS and transmis- sion efficiency from the EBIS to the ISOL beamline are calculated in Table 4.4. Table 4.4 shows that the
Injected Total Bred Number EBIS Total Bred Number Transmission Number in EBIS Efficiency in A/q Separator Efficiency
1.11×108 9.92×107 89.1% 8.92×107 89.9%
Table 4.4: Efficiencies of charge breeding and transportation of charge-bred Sn ions.
charge breeding efficiency of EBIS was 89.1%. It was almost doubled compared to the Cs experiment, and the transmission efficiency from EBIS to the ISOL beamline of 89.9% was increased by about 2.4 times. These results are because the beam optics could be optimized for the ion beams injecting to and extracting from the EBIS by fixing the electrical problem related to the Einzel lens in the switchyard line in the previous experiment.
Through the Sn experiment, the charge breeding results were obtained to satisfy A/q < 6 and the energy per nucleon of 10 keV/u. The charge breeding was performed so that 120Sn24+was the most sig- nificant amount in the EBIS. And, the above conditions were satisfied when the ion beam was extracted with 50 keV/q. Furthermore, the optimization conditions were identified by changing the electrical con- figuration obtaining the EBIS efficiency of nearly 90%. Also, the beam optics were optimized so that the transmission efficiency was also close to 90% in the transmission from the EBIS to the ISOL beam- line. However, even though there was still the phenomenon in which the charge state was decreased by the He gas while transmitting, the effect was significantly reduced compared to the Cs experiment. By obtaining the effective current density for the Sn beam, it is possible to predict the operation parameters for poducing the target charge state when using the RI beam, 132Sn.
4.4.3 Producing and Transporting 23Naq+using 23Na+Ions
In addition to Sn, one of the other candidates to be used as a RI beam is Na, so the commissioning is performed using the stable Na ions. This experiment confirms the optimal parameters of the beam optics for the Na ions, and the properties of the charge distribution are analyzed to derive the effective current density. Na ions are extracted from the TIS using salt and transmitted to the RFQ-CB. The transmitted beam is cooled and bunched in the RFQ-CB as in the previous experiment. However, since the Na ions have a very low mass compared to the Cs and Sn experiments, they are bunched with a relatively small amount. At the switchyard line, the number of Na ions is 3.87×107, and their beam length is about 30 µs, similar to the Sn experiment, as shown in Fig. 4.38.
- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 - 5 0
0
5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
Beam Current [nA]
T i m e [µs ]
Figure 4.38: Na beam injected from ISOL beamline measured at switchyard line.
This ion beam is used to multiply their charge state in the EBIS, where the Na ions have a smaller atomic weight than in the previous experiments, an element with fewer electrons. Therefore, if the breeding time is set to 40 ms as in the previous experiment, most electrons are removed, and ions with an A/q value of close to 2 increase. In particular,23Na11+and23Na10+have A/q values of 2.09 and 2.3, respectively, which overlapped with the data of the residual gases and thus cannot be distinguished. If ions with A/q very close to the ionized residual gases are targeted, even if the ions are separated from them through the A/q separator, they are not perfectly separated and mixed with them and transmitted.
For this reason, in the Na experiment, the breeding time is 14 ms so that the most abundant charge state is 7+, A/q=3.286, distinguished from the residual gases, and the charge distribution can be analyzed.
2 3 4 5 6
05
1 0 1 5 2 0
5 + 6 +
7 + 8 +
Charge / Pulse [pC]
A / q
9 +
Figure 4.39: A/q spectrum for charge-bred Na ions measured at EBIS diagnostics line.
Figure 4.39 shows the A/q spectrum measured by the dipole magnet scan in the EBIS as a result of
this experiment. It can be seen that 23Na7+ has the highest peak, and the charge state from 5+ to 9+
can be distinguished from the residual gas. The ions with a charge state of 10+, A/q of 2.3, can not be distinguished from two ions among the residual gas ions, 16O7+ (A/q=2.286) and 14N6+(A/q=2.333).
Therefore, if the breeding time is increased further, it becomes difficult to analyze the charge distribution accurately.
The number of ions for each charge state classifiable is in Fig. 4.40. And, the effective current density was calculated as 166.4 A/cm2using the CBSIM by matching its result with the experimental result. The gun coil magnetic field in this experiment was 0.16 T making a magnetic field at the cathode
4 5 6 7 8 9 1 0
0
1 0 2 0 3 0 4 0
0 .0 0E+ 00 3 .0 0E+ 06 6 .0 1E+ 06 9 .0 1E+ 06 1 .2 0E+ 07 Number of Particles (Experiment)
Relative Abundance [%]
C h a r g e S t a t e
E x p e r i m e n t C B S I M
Figure 4.40: Experimental and calculated charge distributions of Na ions measured at EBIS diagnostics line.
0.215 T, lower than in previous experiments. So, the current density calculated through the magnetic field is 202 A/cm2. The effective current density for the Na ions was lower because the light ions can quickly obtain electrons from the residual gases and ions. Considering that the amount of ions with a charge state of 4+ and 10+ is hard to measure, it can be seen that the charge distribution is a little broader than the calculation result. This tendency is similar to the previous experiments because the electron and ion beams do not overlap perfectly in the breeding region.
The charge multiplied ion beam was transmitted to the A/q separator to measure the charge distri- bution and transmission efficiency to the ISOL beam line. Figure 4.41 shows the charge distribution measured using the A/q magnet. Compared with Fig. 4.40, the charge distribution were almost the same without change, and 23Na7+ occupied the most significant amount and transmitted to the ISOL beam- line. Unlike the Sn beam experiment, the reason why the distribution has hardly changed is expected to be related to the atomic weight of the ion beam used. In the case of Sn ions, the atomic weight is higher than that of Na ions, so the bred charge state is relatively more heightened. Therefore, in the case of the Sn beam, the charge exchange with the background gas occurs more efficiently during transporting to the ISOL beamline. It results that the movement to the low charge state can occur more quickly. How-