Initial Experimental Results on Ion Cyclotron Resonance Heating Selectively Mixed Low Z Ions to Enhance Production Efficiency of Multicharged Ions on Electron Cyclotron Resonance Ion Source

According to the accessibility conditions of wave propagation in the magnetized plasma of an electron cyclotron resonance (ECR) ion source (ECRIS), it is speculated that the essential factor that determines the limitations in the multiply charged ion currents is the left-hand polarization wave cutoff (L-cutoff). It is necessary to overcome this limitation, we proposed the introduction of ion cyclotron resonance (ICR) or lower hybrid resonance (LHR) by lower frequency waves. We conduct heating low-mass ions selectively in enhanced producing multiply charged ion by mixing low-mass element gas, which has been conventionally performed in ECRIS, or relaxation of the potential well based on the existence of resonant electron particles by ECR. This paper will describe the initial experimental results of ICR application by introducing low-frequency RF electromagnetic waves in the ECRIS.


Introduction
Electron Cyclotron Resonance (ECR) Ion Source (ECRIS) has made great strides in related fields for many years since the group of founder Geller et al. [1] Its application is not limited to the field of accelerator science, but it is also applied to various fields, such as heavy ion radiotherapy [2], ion engines in aerospace field [3], bio-nano fields [4], and semiconductor fields.Microwaves are introduced in the transvers electric (TE) mode along the magnetic field using the waveguide itself as an antenna from the upstream mirror end.It has remained largely unchanged to today.[5] This ECRIS makes it possible to experimentally understand phenomena in existing plasma parameters by making use of the advantage of being able to measure the distribution.According to knowledge obtained from considerations based on observed facts of plasma parameters, current ECRIS has a density limit generated by the left-hand cutoff density limitation.It was suggested that this might cause the multiply charged ion current limit basically with frequent occurrence of instabilities.The conventional solution to this problem is to increase the resonance frequency of microwaves with frequency tuning technique and increase the magnetic field strength as a means of increasing the cutoff density of electromagnetic (EM) wave propagation.[6] Therefore, from the viewpoint of heating by EM waves, there are two ways to break through the limit of multiply charged ion current.In the highfrequency EM waves, one method is to induce resonance by mode conversion from EM waves with various cutoff densities, etc., to electrostatic waves with no cutoff densities.Results have already been obtained in upper hybrid resonance (UHR) experiments [7] and dual-ECR experiments, i.e., independently supplying microwave powers from both upstream and downstream of the ECR zone [8].On the other hand, in high-density regions, resonance phenomena using low-frequency EM waves (usually called Radio Frequency: RF) are known as lower hybrid resonance (LHR) and ion cyclotron resonance (ICR).In the case of ion heating, it can be considered that increasing multicharged ion current is attained by increasing the ion cooling effect [9] by selectively heating low-Z ions mixed.
According to our previous works on formation mechanism of multicharged ions in ECRIS based on experimental facts of the electron density ne and electron temperature Te parameters, the dominant collision process in the atomic and molecular processes in the ECRIS plasma is the ion-ion collision.[10,11] direct action on ions by EM waves is considered to be very effective, and it is also effective in experiments as shown below in this report.In addition, in the case of ECRIS, the small size of the equipment makes it impossible to use a method that induces ICR with wave propagation from an antenna that feeds EM waves.Then we decided to take directly applying an EM field with internal loop antenna inside the vacuum chamber.

Theoretical Background 2.1. Dispersion relations, resonances and cutoffs
The dispersion relation of EM waves in a homogeneously magnetized plasma with a z-axis magnetic field in a cartesian coordinate system is given by the dielectric tensor in the cold plasma approximation including the ion contribution as follows. where |  | ) j =e (for electron) i(for ion) ci (pi) and ce(pe) represent the ion and electron cyclotron (plasma) frequencies respectively qi represents charge of electron (j=e) i.e. qe=e (elementary charge) and j charge state ion species and i is the imaginary unit. ⊥  ×  ⫽   and   are the dielectric tensor elements.Here we use an expression similar to that of Lieberman.[12]  0 is the permittivity in vacuum and   ̿̿̿ is the relative permittivity tensor.
When the wave vector k (magnitude |k|= k) of the EM wave propagates in the direction forming an angle of  with respect to the magnetic field B the dispersion relation is given by the following relation.
where N (=k/k0=ck/ k0 is the wavenumber in vacuum) is the refractive index.In addition k⊥ and k⫽ represent the magnitude of the k in directions perpendicular and parallel to the magnetic field respectively and c and  represent the speed of light in vacuum and the angular frequency of the EM wave respectively.
The following relational expression is derived from Eq.( 2).
Assuming that  = 0 from the numerator on the right side of Eq. ( 3) dispersion relations of right-hand circularly polarized wave (R-wave) and left-hand one (L-wave) are derived as follows.
Electron cyclotron resonance (ECR) and ion cyclotron resonance (ICR) exist in the former and latter respectively.From  2 = 0   and Xe/Ar mixing (b) v.s.pulse period in 50:50 duty ratio of pulsed microwaves.
Considering the density dependence of EM waves at specific frequencies from the low-density region to the high-density region in the plasma we encounter the cutoff density limits of O-cutoff R-cutoff and Lcutoff respectively.[12][13][14] When the microwave frequency is 2.45GHz the O-cutoff limit density is about 7.5×10 16 m -3 and the L-cutoff limit density formed near the mirror center in the ECRIS(Osaka Univ.) at the same frequency is about 2.5-3.5×10 17m -3 in good agreement with the measurement results.[6]

Ion Cyclotron Resonance (ICR) Heating
Propagation of L-wave ion cyclotron waves along magnetic lines of force is necessary to induce ICR, but ECRIS is small and it is considered difficult to induce ICR based on EM wave propagation.Therefore, we consider direct excitation of various ions of ECRIS by a coiled antenna from the strong magnetic field side near the ICR region.Considering the magnetic field strength of the 2.45GHz ECRIS, when using the available induction heating (IH) power source for Fe evaporation source (frequency 50-25 kHz), it has an ICR zone for Ar ions.Therefore, it is possible to verify the principle by conducting experiments to increase the efficiency of multiply charged ion generation by selectively heating Ar as light Z ions with respect to Xe multiply charged ions.An initial experiment was conducted to verify whether the introduction of the ICR antenna has a fatal effect on the ECR plasma and the production of multiply charged ions.In addition, when the ion heating is performed with the ICR zone in the region near the ECR zone, it becomes possible relax the potential well near the ECR region by the ECR.

Experimental Apparatus
The magnetic field configuration of the ECRIS (Osaka University) consists of a pair of mirror coils A and B and an auxiliary coil C for adjustment near the resonance point formed in the center of the mirror magnetic field.It has a configuration in which octupole magnetic fields are superimposed by permanent magnets.The operating pressure is approximately 10 -4~-3 Pa.The 2.45GHz microwaves for ECR discharge are independently introduced from two magnetron sources with a maximum power of 1.3kW.The microwave fed from the Ti rod antenna from the side port near the extraction electrode.The magnetron is CW and capable of pulse operation according to the external pulse waveform.The ICR antenna is inserted from the upstream side of the mirror magnetic field along to the axis by using a feedthrough.The position of the ICR antenna is variable by about 100mm in the z-axis direction.The ICR antenna used in the initial experiment was insulated by covering a Mo wire with 1mm in diameter with bell-shaped ceramic beads with an inner diameter of 2 mm and an outer diameter of 5 mm.The number of turns was set to 2 1 4 turns so that the impedance would match, considering the matching with the IH power supply.Figure 1 shows the installation of the ICR antenna in ECRIS.In the case of IH power supply, the ICR resonance region of Ar + ions at about 40 kHz and movable position range of the ICR antenna are shown in the figure below.

Typical CSD's of pure Xe and Xe mixed Ar in CW and pulse operation
Prior to conducting ICR experiments by introducing low-frequency RF electromagnetic waves in ECRIS, experiments were conducted to confirm the high yield of multiply charged ions by introducing light gases using CW microwaves, and to introduce pulsed microwaves.The efficiency of multiply charged ion production was confirmed with good reproducibility in both the Ar (light gas) introduction experiment during Xe multiply charged ion production and the He (light gas) introduction experiment during Ar multiply charged ion production.Therefore, we conducted a pulsed microwave introduction experiment during the Xe/Ar mixing experiment, and investigated the dependence of the multiply charged Xe ion current on the pulse period.Different pulse period dependences of multiply charged Xe ion yield were observed for pure Xe and mixed Xe/Ar.In particular, at specific magnetic field strengths (Coil A, B 150A, CoilC 2A), the effect of increasing multiply charged Xe ions was obtained at long frequencies with pure Xe, whereas those with Xe/Ar mixture at near the ICR frequency of Ar was obtained.(See Fig. 2) From this, it became clear that there is something selective effect, and we embarked actual ICR experiments as shown below.

Typical CSD with and without low frequency RF in Xe mixed Ar
Figure 3 shows the mass/charge distribution (CSD) of a typical extracted ion current when low frequency RF is introduced in the case of Xe/Ar mixture.The incident microwave power is 100 W, and the low-frequency RF power is shown in blue when it is not introduced and in red when it is introduced.When low-frequency RF power is introduced, the RF current flowing through the ICR antenna is about 28-30A.When the IH power supply was used, the frequency varied with the RF power, in this case the RF frequency was 31.9kHz and the input power to the IH power supply was 160W.Particularly, Xe 7+ enhancement has been observed with RF introduction.It can be seen from Fig. 4(b) that the frequency changes from 40 kHz to 31.8 kHz when the incident power is increased by 120 W or more due to the characteristics of the IH power supply.Since the ICR antenna is placed between the ICR resonance points corresponding to Ar (see Fig. 4(c)), the Xe 7+ ion current increases in response to the increase in incident power (Fig. 4(a)), and increase in the average Xe charge state can be also seen from Fig. 4(b).

Preparation and advanced further ICR experiments on ECRIS
Based on the results of this initial experiment, we increased the number of turns of the ICR antenna like this experiment to 6 turns, which is more effective, and installed it in ECRIS.The insertion effect of this ICR antenna and the effect of low frequency EM wave introduction has been confirming without fatal influence on ECR plasma generation.In the summer of 2022, a Grant-in-Aid for efficient multi-charged ion generation by low-frequency resonance on this ICR experiment and LHR to ECRIS was adopted, and we are currently preparing to conduct more effective experiments.

Figure 3 .
Figure 3.Typical charge state distribution of Figure 4. Low frequency RF power dependence of extracted multiply charged ion current.Xe 7+ ion current (a) and average Xe q+ charge state (b).Blue is without RF and red is with RF.Ar + resonance point position and ICR antenna position (c) at frequencies of 40.1 kHz (red) and 31.8 kHz (blue), respectively.