Intense highly charged ion beam production and operation with a superconducting electron cyclotron resonance ion source

The superconducting electron cyclotron resonance ion source with advanced design in Lanzhou (SECRAL) is a superconducting-magnet-based electron cyclotron resonance ion source (ECRIS) for the production of intense highly charged heavy ion beams. It is one of the best performing ECRISs worldwide and the first superconducting ECRIS built with an innovative magnet to generate a high strength minimum-B field for operation with heating microwaves up to 24–28 GHz. Since its commissioning in 2005, SECRAL has so far produced a good number of continuous wave intensity records of highly charged ion beams, in which recently the beam intensities of ${{}^{40}\mathrm{Ar}}^{12+}$ and ${{}^{129}\mathrm{Xe}}^{26+}$ have, for the first time, exceeded 1 emA produced by an ion source. Routine operations commenced in 2007 with the Heavy Ion accelerator Research Facility in Lanzhou (HIRFL), China. Up to June 2017, SECRAL has been providing more than 28,000 hours of highly charged heavy ion beams to the accelerator demonstrating its great capability and reliability. The great achievement of SECRAL is accumulation of numerous technical advancements, such as an innovative magnetic system and an efficient double-frequency ($24+18\mathrm{GHz}$) heating with improved plasma stability. This article reviews the development of SECRAL and production of intense highly charged ion beams by SECRAL focusing on its unique magnet design, source commissioning, performance studies and enhancements, beam quality and long-term operation. SECRAL development and its performance studies representatively reflect the achievements and status of the present ECR ion source, as well as the ECRIS impacts on HIRFL.

Figure 1

Schematic view of the ECR ion source structure with minimum-B magnet configuration.

Figure 2

Layout of the SECRAL ion source and its beam transport line.

Figure 3

Conventional components of the SECRAL source: (a) schematic view of the injection tank; (b) photo of the injection tank end.

Figure 4

The SECRAL source with 24 GHz microwave coupling.

Figure 5

Magnet coil configuration of the highly charged ECRIS. Classical ECRIS magnet coil configuration of Sext-In-Sol. The SECRAL superconducting magnet coil configuration of Sol-In-Sext.

Figure 6

View of sextupole-coil end in which the adjacent end electric-current flows oppositely. Interaction between the end currents and the axial fields results in strong radially inward force F1 and outward force F2.

Figure 7

SECRAL magnet structure and superconducting coil configuration.

Figure 8

Fabricated SECRAL magnet assembly and its detailed coil structure.

Figure 9

SECRAL axial magnetic field distribution. (a) Designed axial mirror magnetic field (${\mathrm{B}}_z$) distribution on the axis for SECRAL 24–28 GHz operation. (b) 3D field distribution at the x-z plane in the plasma chamber.

Figure 10

SECRAL double waveguides coupling system both at 18 GHz.

Figure 11

SECRAL microwave coupling system with $24+18\mathrm{GHz}$ double frequency heating.

Figure 12

CSD spectrum with the source optimized for ${{}^{129}\mathrm{Xe}}^{20+}$ and ${{}^{209}\mathrm{Bi}}^{50+}$. (a) Xenon CSD spectrum with the source optimized for ${{}^{129}\mathrm{Xe}}^{20+}$ at 2.9 kW of 18 GHz microwave power and 25 kV extraction voltage. (b) Bismuth CSD spectrum with the source optimized for ${{}^{209}\mathrm{Bi}}^{50+}$ at 2.6 kW of 18 GHz microwave power and 25 kV extraction voltage.

Figure 13

Xenon beam intensity study with different chamber and microwave power. (a) ${{}^{129}\mathrm{Xe}}^{27+}$ beam intensity evolution as a function of microwave power and comparison among stainless steel (SS) chamber at 18 GHz, aluminum (Al) chamber at 18 GHz, and aluminum chamber at $18+14.5\mathrm{GHz}$. (b) Xenon beam intensity enhancement with aluminum chamber at $18+14.5\mathrm{GHz}$ double frequency heating.

Figure 14

Bismuth beam currents at the total microwave power 5–6 kW with a new oven and compared to those at the power less than 4 kW.

Figure 15

CSD spectra with the source tuned for ${{}^{209}\mathrm{Bi}}^{31+}$, ${{}^{209}\mathrm{Bi}}^{50+}$ and ${{}^{238}\mathrm{U}}^{33+}$. (a) Bismuth CSD spectrum with the source optimized for ${{}^{209}\mathrm{Bi}}^{31+}$ at total microwave power 5.1 kW of $24+18\mathrm{GHz}$. (b) Bismuth CSD spectrum with the source optimized for ${{}^{209}\mathrm{Bi}}^{50+}$ at total microwave power 4.3 kW of $24+18\mathrm{GHz}$. (c) Uranium CSD spectrum with the source optimized for ${{}^{238}\mathrm{U}}^{33+}$ at total microwave power 3.7 kW of $24+18\mathrm{GHz}$.

Figure 16

Compact mode convertor from ${\mathrm{TE}}_{01}$ to ${\mathrm{HE}}_{11}$.

Figure 17

Dependence of beam intensity on total microwave power at $24+18\mathrm{GHz}$ coupled at different microwave coupling schemes. (a) ${{}^{129}\mathrm{Xe}}^{27+}$ beam; (b) ${{}^{40}\mathrm{Ar}}^{12+}$ and ${{}^{40}\mathrm{Ar}}^{14+}$ beam.

Figure 18

CSD spectra at total microwave power 8.5–8.7 kW of $24+18\mathrm{GHz}$ double frequency heating with the $\varnothing 20\mathrm{mm}$ ${\mathrm{TE}}_{01}$ circular waveguide. (a) The SECRAL source optimized for ${{}^{40}\mathrm{Ar}}^{12+}$. (b) The SECRAL source optimized for ${{}^{129}\mathrm{Xe}}^{30+}$.

Figure 19

Dependences of the best beam intensities of ${{}^{129}\mathrm{Xe}}^{27+}$ on the total incident microwave power density at $24+18\mathrm{GHz}$ and $18+14.5\mathrm{GHz}$.

Figure 20

${{}^{40}\mathrm{Ar}}^{14+}$ beam intensity dependence on microwave power of the second supplemental heating frequency 18 GHz when the main frequency 24 GHz power is fixed.

Figure 21

Comparison of the axial magnetic field distribution between SECRAL and VENUS. (The blue line VENUS1 and the dark line SECRAL are the magnetic field distribution for optimizing high intensity ${{}^{209}\mathrm{Bi}}^{30+}$ beam respectively. The red line VENUS2 is the new field distribution for VENUS after modifications.)

Figure 22

${{}^{209}\mathrm{Bi}}^{31+}$ (a) and ${{}^{129}\mathrm{Xe}}^{30+}$ (b) beam intensity evolution with different technical developments from the year 2006 to 2015.

Figure 23

SECRAL beam emittance study. (a) Xenon beam emittance measured at low microwave power of 24 and 18 GHz. (b) Dependences of normalized rms emittance on 18 GHz microwave power measured at SECRAL. (c) Dependences of normalized rms emittance on the extraction peak field at 18 GHz SECRAL.

Figure 24

Dependences of normalized rms emittance on the beam ${{}^{209}\mathrm{Bi}}^{31+}$ intensity and the 24 GHz microwave power at SECRAL.

Figure 25

Beam emittance phase-space patterns and beam images for the beam ${{}^{209}\mathrm{Bi}}^{31+}$ at different conditions for SECRAL operating 24 GHz.

Figure 26

Suppression effect of beam instabilities by two-frequency heating ($28+24\mathrm{GHz}$) and three-frequency heating ($28+24+18\mathrm{GHz}$).

Figure 27

SECRAL beam stability study. (a) Beam intensity instability of ${{}^{129}\mathrm{Xe}}^{27+}$ at $435\mathrm{e}\mu \mathrm{A}$ when SECRAL was tested with single frequency 24 GHz heating at 3.5 kW power. (b) ${{}^{129}\mathrm{Xe}}^{30+}$ beam intensity stability at $235\mathrm{e}\mu \mathrm{A}$ with $24+18\mathrm{GHz}$. (c) ${{}^{209}\mathrm{Bi}}^{31+}$ beam intensity stability at $330\mathrm{e}\mu \mathrm{A}$ testing continuously during 2.5 hours.

Figure 28

SECRAL operation statistic data for the HIRFL accelerator. (a) Beam delivering time by SECRAL from May 2007 to December 2016 in comparison to LECR3. (b) Typical beams provided by SECRAL during operation for the HIRFL accelerator.

Figure 29

Enhancement of SFC cyclotron beam energy and beam intensity due to SECRAL operation. (a) SFC beam energy improvement by SECRAL compared with the normal conducting ECR ion sources LECR0&1 and LECR2&3. (b) Enhancement of SFC beam intensity and energy by SECRAL compared with the normal conducting ECR ion sources LECR2&3.

Figure 30

CSR performance enhancement by SECRAL compared with the normal conducting ECR ion sources LECR2&3.

Figure 31

Contributions of SECRAL and the normal conducting ECRIS at IMP to physics research based on the HIRFL accelerator.

Figure 32

The SECRAL-II ECR ion source at beam commissioning.