TWIN Source

Overview

To address operational challenges related to large-size, multi-RF driver, inductively-coupled plasma source, a TWo-driver-based, Indigenously built Negative (TWIN) ion source R&D program has been established in IPR. The prime objective of the TWIN source program is to manufacture a large size, fusion grade, high-power ion source indigenously. In addition, the ion source will be used as the plasma load to test the functionality of IPR’s indigenously developed 1MHz solid-state RF generators before they are hooked to the ITER-DNB ion source. The TWIN source experiments are planned in two phases: (1) Plasma production phase, (2) Negative ion beam extraction phase.

Presently, the hydrogen plasma is produced in both the RF-drivers of the TWIN source simultaneously using an 180 kW, 1 MHz tetrode tube-based RF Generator. The TWIN source can be operated either by keeping the RF-drivers antenna coils and the impedance matching unit exposed to the atmospheric pressure (Air mode operation) or by keeping the coils immersed inside vacuum (vacuum mode operation). Air mode operation is envisaged for low-power, and vacuum mode is for high-power operation. TWIN source body is actively cooled and is designed for a 5-second continuous plasma shot with full 180kW power. So far, 75kW RF power has been coupled to the ion source. Plasma density > 5e17 m^3 is observed in the expansion chamber of the ion source volume, around 20 cm away from the RF drivers. TWIN source is also being used to test the indigenously developed 40 kW solid-state RF generators (SSRFG). Soon, a 200kW solid-state RF generator, to be used for ITER-DNB at INTF, will be connected.

TWIN Source

Experiments

Experiment 1.

In the Beam Extraction Phase, plasma operation will be optimized to achieve an extracted negative hydrogen-ion current density of approximately 35 mA/cm² with Caesium (Cs) vapour injection into the plasma. TWIN Source extraction grids (~1 x 0.5 m²) are manufactured indigenously using a similar vacuum brazing method for the cooling channels. The grid system is in the assembly stage.

Experiment 2.

The Data Acquisition and Control System (DACS) of the Twin Source (TS) experiment includes a control core program, a graphical user interface (GUI), data acquisition software, and front-end signal-conditioning electronics. The TS control architecture is broadly based on the ITER CODAC Core system. The control core program has been developed using Siemens Step 7 software, while SCADA functionalities are implemented. For data acquisition, a National Instruments PXIe platform equipped with NI digitizer cards has been adopted. LabVIEW Real-Time software has been utilized for real-time data acquisition applications. The TS system comprises approximately 200 control channels and 152 acquisition channels. Signals are transmitted via an in-house-developed fiber optic link, ensuring electrical isolation and enhanced noise immunity. Plasma operation in the TWIN source started in 2018. The source is still in the commissioning phase. Historical experimental learning progress with different source configurations is described in a tabular form.

Table: Historical learning progress with the TWIN source operation

CONFIGURATION	OPERATION	OPERATION	Year	EXPERIENCE/LEARNING
Initial Phase with 180kW Tube-based RF Generator with insulated antennas	Super-cusp magnet configuration in the RF drivers’ backplates	Applied power: 55kW in single driver, 35kW in two drivers	2018-20	Inefficient power coupling, weak plasma light, frequent RF coil breakdowns.
Operation with a 40kW solid-state RF generator with insulated antennas	Super-cusp magnet configuration in the RF drivers’ backplates	Applied power: 40kW in a single driver, 39kW in two drivers	2020-21	Inefficient power coupling, weak plasma light.
Operation with a 40kW solid-state RF generator with insulated antennas	Line-cusp magnet configuration in the RF drivers’ backplates	Applied power: 40kW in a single driver, 39kW in two drivers	2022-25	Efficient power coupling to plasma, Strong plasma light.
Operation with 180kW Tube-based RF Generator with insulated antennas. The operation started after a long break, after reconfiguring the RFG cooling water circuit, bypassing multiple damaged flow switches.	Line-cusp magnet configuration in the RF drivers’ backplates with different matching capacitor configurations	Applied power up to 75kW in two drivers configuration.	2022-25	Efficient power coupling moves from high pressure (1.5 Pa) to low pressure (0.3 Pa). Directional coupler inclusion in the RF circuit to study coupled and reflected power.
Operation with 180kW Tube-based RF Generator with bare antennas (DNB coil configuration) supported by Teflon comb spacers. Operation with a 40kW solid-state RF generator at the DNB HV deck, 40m away from the TWIN source. Bare antennas (DNB coil configuration) connected through a 40m long flexible coaxial RF Transmission line.	Line-cusp magnet configuration in the RF drivers’ backplates with different matching capacitor configurations	Applied power up to 25kW in two drivers configuration. Applied up to 16kW in two drivers configuration to avoid coil breakdown.	2025-26	Plasma was created in two drivers, but the inter-turn coil breakdown started at higher power. Intermittent plasma generation. Efficient Impedance matching effort is ongoing.