Outlines

• Introduction
• Ion Optics of Sweep Plates
• Design of the Sweep Plates
  a. Primary Sweep Plates b. Secondary Sweep Plates
• Trajectory Control
• Plasma/UV Loading Considerations
• Conclusions

• A complex sweep system is needed to get the probing ions
  through the small MST ports
• The MST-HIBP sweep system consists of primary sweep plates and secondary sweep plates
• Highly three dimensional ion trajectories require secondary sweep system
  which is new to RPI HIBPs.
• Plasma guns? Magnets suppression structure to reduce the plasma
  loading effect.

• Straight Plates
• Flared Plates
• Combination of straight and flared plates
• Drift space
  

Tansfer matrix method is used in ray tracing

• Trajectory in a drift space
  
• Trajectory in straight plates region
  
• Trajectory in flared plates region
  

where x0 and dx/dz|0 are position and slope at the entrance to a region, z is horizontal position in a region, f = qV/W, represents deflection strength of the plates, q is ion charge in unit of e, W is ion beam energy in unit of eV, V is the sweep plate voltage in unit of volt, and k=2tan(B) is the slope of the plates.

Fringe Filed Consideration

• Fringe field was neglected in previous HIBP designs
  
• Analytical solution of the electric field can be found by conformal mapping. But it's hard to put it into the computer code.
• A simplified linear model is suggested. The ion trajectory in the front and back fringe regions are decided by the following equations respectively.
  

Equations (1) - (5) are used to determine the sweep plates sizes and to
determine applied voltages needed.

Requirements of the deflection:

For a 200keV primary ion beam, +-20 degrees in poloidal and +-5 degrees in toroidal directions respectively, with maximum 20 kV power supplies.

Crossover sweep design is adopted

The effective sweep center is moved to the entrance port

Back stream design to get a compact structure

• The slope of the flared part is decided by the requirement that the primary ion beam tangentially clears the plate.
• The position of the straight part is decided by the same critarion.
• How to decide the position of the 1st plates? Decided by how much space we want to leave.

Primary toroidal sweep plates

Same design process as that of the poloidal plates

A large angular range is present on the secondary beamline

Angular range that can be accomodated by the energy analyzer is limited

Characteristics of the electrostatic energy analyzer

• Poloidal direction --> 2nd order focus property
• Toroidal direction --> no focus
  
• We want secondary ion beam injects into the energy analyzer with angular range +-3 degrees in poloidal direction and as small as possible in toroidal direction.
• Energy analyzer: sitting about 1.4m from the plasma
• Poloidal direction: 1 pair of plates, +-17 degrees -->+-3 degrees, with 10kV power supplies driving
• Toroidal direction: 2pairs of plates, +-5 degrees --> 0degree, with 4kV power supplies driving

Advantages

1. Analyzer is sitting far away from the machine, reduce plasma/UV loading effect

2. Only one pair of poloidal sweep plates is needed

Trajectory calculations through the plasma show that the secondary
ion beams having toloidal angular range much larger than +-5 degrees
degrees that can be handled.
Edge diagnostics and core diagnostics need different secondary
beamline orientations.

Determination of voltages on the secondary sweep plates

Transfer matrice can be used to decide the voltage applied on the secondary sweep plates.

• For the poloidal (radial) plates

where Vin = dx/dz|0 is initial slope of the trajectory; k = 2tan(B)

• For the toroidal plates

In the above, Vin = dx/dz|0 is the initial incident trajectory slope; k = 2tan(B). Therefore, we can decide the voltage on the straight plate (top) is Vs1 = Wb.f1, and voltage on the flared plate (top) is Vs2 = Wb.f2.

Solutions of Laplace equation in the sweep regions are used to trace ion beams

• Simplified linear model has been used only for the design purpose
• Field from solving Laplace equation is used in real world.

Active trajectory control

• Multiple sample volumes within a single shot requires simultaneously changing voltages on all the sweep plates
• CHS implementation. Magnetic field is well known there
  
• MST magnetic field partially known.
• Our initila operation: running multiple trajectories for a few plasma conditions until a database is built up with sample volume locations and sweep voltages
• Accuracy will be improved by iterating experimental results and trajectory simulations

Two sources of electric charges in the sweep region

• Plasma diffusion through the vacuum ports
• UV induced photo-electrons

Voltage will be short out if the power supplies' current limit (10mA ~ 20mA) are exceeded

Loading tests showed that the current loading is plasma dominated

• Magnet suppression structures have been designed for both 2" entrance and 4.5" exit ports
• Magnetic strength ~ 1500G at the entrance, and ~200G at the exit port
• A metal screen grid will be helpful to keep plasma away from the sweep plates. Debye length ~7430(KT/n)^1/2m ~0.1mm at the edge. Fine mesh screen is required. HIBP requires high transparancy (>90%)

Primary and secondary sweep plates have been designed

• 4 pairs primary sweep plates and 3 pairs secondary sweep plates
• Entrance angular range +-20 degrees in poloidal, and +-5 degrees in toroidal directions

Crossover sweep design

• The effective sweep center is moved close to the plasma

A simplified linear model is introduced to treat the fringe fields

• Just for the design purpose
• Field from solving Laplace equation is used after design phase

Build databases for the sweep information during the initial operation

Plasma suppression structures could be

• Magnetic field structure;
• Metal screen grid

For more information about the sweep plates design, please refer to the following technique report.

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