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NNBI Facility

Scientific objectives

The CRAFT NNBI facility is constructed to:

•     Developing all technologies needed for CWNegative NBI system

•     Building 1st stage CFETR NNBI

•     Master the method and technology for full scaleNNBI construction and testing

System Description andmain parameters

The CRAFT NNBI facility is mainly consisting ofthe host injector (located underground, including beam source and beam line),the power supply system, the diagnostic system, the control system, and thesupporting system (e.g., cryogenic system, cooling water system). The CRAFT layout is shown in Fig. 1and the host injector of NNBIsystem is shown in Fig. 2. The primary target of CRAFT NNBI system is toachieve a H0 beam power of 2 MW at the beam energy of 200~400 keVfor the beam duration of 100 s.

Beam source

The beam source used to generate high densityplasma with large area, generate negative ions and extract them to formnegative ion beam, then the high-current negative ions are accelerated andlaunched from the beam source. The beam source consists plasma generator andnegative ion accelerator. The plasma generator consists four RF drivers and onelarge area plasma expansion chamber. The negative ion accelerator consists fourgrids, which are plasma grid, extraction grid, acceleration grid and groundgrid. The negative ion is accelerated by two stage to achieved 400 keV beamenergy. The beam source (2.6 m high, 1.3 m wide and 1.1 m deep) is immersed inthe vacuum condition for better high voltage holding.

Beam line

The beam line mainly consists of vacuum vessel,vacuum valves, and the inner components, such as neutralizer, ERID(Electrostatic residual ion dump), cryopump panels and calorimeter. When ionbeam transmitted into the beam line, about 55% of them are converted tohigh-energy neutrals in the neutralizer. The residual negative and positiveions are electrically deflected onto the lateral dumps. Finally, the powerfulneutral beam is deposited and measured on the beam dump (i.e., calorimeter).The beam source vacuum vessel (5 m × 5 m × 5 m) is connected to the beamlinevacuum vessel (4 m × 4 m × 12 m) with a gate valve. Two sets of cryo-absorptionpumps are installed on the lateral inner walls of the beamline vacuum vessel.

Power supply system and transmission line

The power supply system consists ofacceleration grid power supply, extraction grid power supply, bias powersupply, bias plate power supply, plasma grid filter power supply, core snubberbias power supply starter filament and are power supplies, residual ion dumppower supply, RF power supply. The main parameters are shown in Table 1.

Other systems

The control system is designed with distributed controltechnology based on CODAC (Control,Data Access and Communication) framework, whichprovides beam pulse setting up, slow and fast plant control, supervision, dataacquisition and storage, interlock and protection. The diagnostic system iscomposed of general ion source and beam diagnostic system. The function ofgeneral diagnostic is to monitor the vacuum pressure, gas inflow, thetemperature of inner components and cooling water, water pressure, and so on,it keeps the NNBI system running safely. For ion source diagnostic, it is tomeasure the parameters of plasma, such as Cs, H-, Ne and Te, andassess the uniformity. The beam diagnostic system, it is to measure the beamdivergence, beam species and assess the beam uniformity. The other systems,such as cryogenic system and cooling water system are installed surrounding theNNBI injector.

Key Technologies

Thefollowing key technologies is expected to be solved by constructing the NNBIFacility:

•     Negativeions generation

•     Negativebeam acceleration

•     HVPS andtransmission

•     Highspeed cryopump