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Oscillation of emittance and Twiss parameters in the negative ion (H−) beam from the J-PARC 2MHz RF ion source is measured by application of a double-slit emittance monitor located at the RFQ entrance. The emittance monitor is equipped with a newly-developed 60 MS/s data acquisition system, so that beam current oscillations at a few MHz can be observed with enough time resolution. From the measurement, it is shown that the beam phase space consists of: (1) a Direct Current component in the beam core; (2) a 2 MHz oscillating component which takes place in the diverging halo; (3) a doubled RF frequency (4 MHz) oscillation which faintly exists in the beam halo. The major component is the 2 MHz component, which resultantly decides the beam emittance oscillation frequency. A typical value of the beam emittance in the present experiment is 0.33 π mm-mrad, while the amplitude of the 2 MHz oscillation is around 0.04 π mm-mrad. The results indicate that the high-frequency oscillation component occupying about ten-percent of the beam from the RF source travels a few meters passing through a magnetic lens focusing system. [ABSTRACT FROM AUTHOR]

The Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven Hˉ ion source has been stably operated without any serious problems for about four years. The J-PARC LINAC successfully accelerated the 60 mA beam required for the J-PARC, when a 72 mA beam was extracted from the source. The high intensity beam with transverse emittances suitable for the RFQ is produced with several unique measures, such as, slight water molecules addition into hydrogen plasma, low temperature (about 70 ºC) operation of 45º-tapered plasma electrode with a 16-mm thickness accompanying with precise cesium density control, impurity (argon and/or nitrogen) elimination in the hydrogen plasma along with filter-field optimization, continuous-wave igniter plasma driven with a 50-W 30-MHz RF, and so on. In order to circumvent the beam intensity bottlenecks of the source, the extraction and acceleration voltages (VE and VA) higher than the design values of 10 kV and 40 kV were examined. A 100 mA beam, whose 93.2 mA has particle distributions in horizontal and vertical phase planes similar to those used for a common RFQ design, was stably operated with a duty factor of 5 % (1 ms x 50 Hz) by using the VE and VA of 12.4 kV and 49.6 kV, respectively. This beam intensity breakthrough will realize the next generation benchmark Hˉ ion source for high intensity and high energy Hˉ LINACs. The important information on the space-charge limited bottlenecks in the extraction and acceleration gaps will derive the optimal electrode shapes for the source operated with a beam intensity higher than 100 mA. [ABSTRACT FROM AUTHOR]

The beam brightness of the Japan Proton Accelerator Research Complex cesiated RF-driven H- ion source has been increased by using several unique measures, such as a low plasma electrode temperature operation, slight H2Os feeding in hydrogen plasma and so on. The source enables high energy linear accelerators (LINACs) to eject a beam with 60 mA peak intensity. In order to use the source stably for the LINACs operated with a duty factor higher than the present value of 1 msec×25 Hz, the conditions to minimize the extraction electrode current (IEE), whose main component was the electron current co-extracted with a 66 mA beam, were investigated. The 66 mA H- ion beam with a low IEE of 39 mA, which was about one-fourth of that in the ordinary operation, were stably operated by optimizing rod-filter-field (RFF), cesium (Cs) density and axial magnetic field correction (AMFC). Especially, the AMFC of only -30 Gauss had the largest IEE reduction by about one-third. The corresponding 95 % beam transverse normalized rms emittances were degraded by about 24 % due to the Cs density and RFF higher than those in the ordinary operation. [ABSTRACT FROM AUTHOR]

A cesiated RF-driven negative hydrogen ion source was started to operate in September, 2014 in response to the need for upgrading J-PARC's linac beam current. The ion source mainly comprises a stainless-steel plasma chamber, a beam extractor, and a large vacuum chamber with two turbo molecular pumps of 1500 L/s for differential pumping. The ion source has been successfully providing the required beam current to the accelerator without any significant issues other than a single-incident antenna failure occurred in October, 2014. Continuous operation for approximately 1,350 h was achieved with a beam current and duty factor of 45 mA and 1.25 % (0.5 msec and 25 Hz), respectively. [ABSTRACT FROM AUTHOR]

In order to compare brightnesses of beams produced by different ion sources, a transverse emittance evaluation procedure with consistency and small ambiguity for different background noises is required. The procedure to evaluate emittances of beams produced by the Japan Proton Accelerator Research Complex (J-PARC) cesiated RF-driven H" ion source is presented in this paper. The ambiguity in emittance evaluations is eliminated by defining uniquely 100% and 95% beams with a reasonably corrected beam-signal base-level. Two 95%-beam transverse normalized root mean square emittances of beams, which are produced with almost the same 2-MHz RF power and cesiation condition but with different background noise, are estimated as almost the same values by this procedure. [ABSTRACT FROM AUTHOR]

Ignition and steady state phases of Radio Frequency (RF) plasma in J-PARC ion source has been investigated by numerical modeling. The model takes into account the transport of plasma particles (electrons, protons, hydrogen molecular ions and cesium ions) in electromagnetic (EM) field with collision processes. Inductively coupled and capacitive EM fields are simultaneously solved in the model with plasma transport. Applying KEK parallel computation system A (64 cores, 56 nodes with 256 GB memory per node), behavior of high density plasma up to 1019 - 1020 m-3 in the steady state is calculated. In the simulation, it has been clarified that inductively coupled electric field in azimuthal direction and magnetic field in axial direction play a key role to maintain high density plasma which oscillates with frequency up to doubled value of applied RF frequency. The spatial distribution plot of plasma density and EM field at each phase may lead to understandings on how RF plasma is kept stable inside the source chamber. [ABSTRACT FROM AUTHOR]

Operation of a cesiated RF-driven negative hydrogen ion source was initiated in September 2014 in response to the requirements of beam current upgrade in J-PARC linac. Delivery of the required beam current from the ion source to the J-PARC accelerators has been successfully performed. In 2016-2017 campaign, continuous operation of the ion source for 1,845 hours (RUN#75 from April to July 2017) was achieved with beam current, RF pulse width and repetition of 45 mA, 0.8 ms and 25 Hz, respectively. We present the operation status of the ion source and a high current H- beam with 70 mA extracted from the ion source for further high-power upgrade in J-PARC accelerators. [ABSTRACT FROM AUTHOR]

Balmer alpha line intensity in J-PARC Radio Frequency (RF) negative hydrogen ion source has been measured by photometry measurement. The line intensity shows several interesting time characteristics in different phases; (1) 2 MHz (RF frequency) oscillation just after plasma ignition and (2) 4 MHz (doubled RF frequency) and 2MHz coupled oscillation in the steady-state. From the comparison between numerical analysis, it has been explained that electron acceleration in inductively coupled electromagnetic field takes place with 4 MHz frequency which results in the 4 MHz line intensity oscillation. From the understandings of the background physics, we can conclude that this fast photometry measurement is a good diagnosis tool to understand whether RF plasma is in E-mode or H-mode in general RF ion sources. [ABSTRACT FROM AUTHOR]

In order to satisfy the Japan Proton Accelerator Research Complex (J-PARC) second stage requirements of an H ion beam of 60mA within normalized emittances of 1.5mm·mrad both horizontally and vertically, a flat top beam duty factor of 1.25% (500 s25Hz) and a life-time of longer than 1month, the J-PARC cesiated RF-driven H ion source was developed by using an internal-antenna developed at the Spallation Neutron Source (SNS). The transverse emittances of the source were measured with various conditions to find out the optimum operation conditions minimizing the horizontal and vertical rms normalized emittances. The transverse emittances were most effectively reduced by operating the source with the plasma electrode temperature lower than 70C. The optimum value of the cesium (Cs) density around the beam hole of the plasma electrode seems to be proportional to the plasma electrode temperature. The fine control of the Cs density is indispensable, since the emittances seem to increase proportionally to the excessiveness of the Cs density. Furthermore, the source should be operated with the Cs density beyond a threshold value, since the plasma meniscus shape and the ellipse parameters of the transverse emittances seem to be changed step-function-likely on the threshold Cs value. [ABSTRACT FROM AUTHOR]

The Japan Proton Accelerator Research Complex (J-PARC) was extensively damaged by the March 2011 earthquake. The ion source suffered some damage, such as dew condensation on its surface due to flooding in the accelerator tunnel, vacuum leakage due to viewing port breakage, and vacuum pump failure due to the strong quake. The restoration work was accomplished on schedule, and the linac beam operation was restarted on December 9, 2011. The ion source supplies the beam to the accelerator without serious earthquake after-effects. The availability of the ion source is one of the important issues because it is directly related to the operation time of the J-PARC. To evaluate the lifetime of the present ion source, we began two months of beam operation. The results of four recent beam runs show that the ion source is capable of continuous operation for approximately 1,200 h with the beam current of 17 mA. To enhance the ion source availability, we attempted to decrease the maintenance time to replace consumable elements. By unitizing the replacement elements and keeping them under the vacuum condition until just before installation, the maintenance was completed within 24 h without any beam performance degradation of the ion source. [ABSTRACT FROM AUTHOR]

In order to satisfy the Japan Proton Accelerator Research Complex (J-PARC) 2nd stage requirements of an H- ion beam current of 60mA within normalized emittances of 1.5πmm·mrad both horizontally and vertically, a flat top beam duty factor of 1.25% (500μs×25Hz) and a life-time of more than 50days, a cesiated RF-driven H- ion source using a internal-antenna developed at the Spallation Neutron Source (SNS) was developed. As similar as the SNS ion source, the 60kW pulsed 2MHz-RF and 200W CW 30MHz-RF systems are used in order to produce pulsed high-temperature 2MHz-RF plasma and CW low-temperature 30MHz-RF plasma. Each matching network for each system is composed of two vacuum variable condensers (VVCs). In order to supply pulsed 60kW-2MHz-RF power from the power supply (PS) on the ground level, a one-turn isolation transformer using FINEMET cores is installed between the PS and the J-PARC ion source. By comprehending the matching networks with the LTspice IV simulations and high- and low- power experiments and setting the parameters properly, the pulsed 2MHz-RF power up to 46 kW is successfully input to the hydrogen plasma without any misfire and with almost no reflected power. [ABSTRACT FROM AUTHOR]

In order to satisfy the Japan Proton Accelerator Research Complex (J-PARC) 2nd stage requirements of an H- ion beam current of 60mA within normalized emittances of 1.5πmm·mrad both horizontally and vertically, a flat top beam duty factor of 1.25% (500μs×25Hz) and a life-time of more than 50days, a cesiated RF-driven H- ion source using a internal-antenna developed at the Spallation Neutron Source (SNS) was developed. The horizontal and vertical emittances were measured by using two double (W) slit emittance monitors for several different conditions. Although the measured normalized rms emittances are more than 1.4-times larger than those of a J-PARC H- ion source driven with two tungsten (W) filaments probably due to the RF field around extraction gap, the J-PARC 2nd stage requirement of the 60mA beam within 1.5πmm·mrad was satisfied if a beam more than 66mA were extracted. [ABSTRACT FROM AUTHOR]

A cesium-free negative hydrogen ion source driven with a lanthanum hexaboride (LaB6) filament is being operated without any serious trouble for approximately four years in J-PARC. Although the ion source is capable of producing an H- ion current of more than 30 mA, the current is routinely restricted to approximately 16 mA at present for the stable operation of the RFQ linac which has serious discharge problem from September 2008. The beam run is performed during 1 month cycle, which consisted of a 4-5 weeks beam operation and a few days down-period interval. At the recent beam run, approximately 700 h continuous operation was achieved. At every runs, the beam interruption time due to the ion source failure is a few hours, which correspond to the ion source availability of more than 99%. The R&D work is being performed in parallel with the operation in order to enhance the further beam current. As a result, the H- ion current of 61 mA with normalized rms emittance of 0.26 πmm.mrad was obtained by adding a cesium seeding system to a J-PARC test ion source which has the almost same structure with the present J-PARC ion source. [ABSTRACT FROM AUTHOR]

In order to satisfy the Japan Proton Accelerator Research Complex (J-PARC) second stage requirements of an H- ion beam current of 50 mA and a life-time of 2000 hours, the development of a 13.56 MHz-rf-driven H- ion source was started by using an antenna developed at the Spallation Neutron Source (SNS). As the first step, the impedance of an antenna with hydrogen plasma, which is one of the most important parameters, was measured. For an rf-power of 0 kW, 1.25 kW or 7.45 kW, it was measured as j84.19+5.107 Ω, j80.22+11.82 Ω or j58.24+21.16 Ω, respectively. Its real-part had a rather large value and rather large rf-power dependence unexpectedly. By using the measured impedance, the circuit to produce rf-plasma in the plasma chamber on a -50 kV potential with a 13.56 MHz-rf-source on the ground potential was designed by the circuit simulation code LTSpice IV. [ABSTRACT FROM AUTHOR]