Improvements on the stability and operation of a magnetron ${\mathrm{H}}^{-}$ ion source

The magnetron ${\mathrm{H}}^{-}$ ion sources developed in the 1970s currently in operation at Fermilab provide beam to the rest of the accelerator complex. A series of modifications to these sources have been tested in a dedicated off-line test stand with the aim of improving different operational issues. The solenoid type gas valve was tested as an alternative to the piezoelectric gas valve in order to avoid its temperature dependence. A new cesium oven was designed and tested in order to avoid glass pieces that were present with the previous oven, improve thermal insulation and fine-tune its temperature. A current-regulated arc modulator was developed to run the ion source at a constant arc current, providing very stable beam outputs during operations. In order to reduce beam noise, the addition of small amounts of ${\mathrm{N}}_2$ gas was explored, as well as testing different cathode shapes with increasing plasma volume. This paper summarizes the studies and modifications done in the source over the past three years with the aim of improving its stability, reliability and overall performance.

Figure 1

Schematic of Fermilab’s magnetron ${\mathrm{H}}^{-}$ source.

Figure 2

Linac beam pulse (top) at a $5\mu \mathrm{s}/\mathrm{div}$ scale with visible 50 to 100 ns microstructures at a shorter scale of $200\mathrm{ns}/\mathrm{div}$ (bottom). Vertical scales are $500\mathrm{mV}/\mathrm{div}$.

Figure 3

Schematic of the ${\mathrm{H}}^{-}$ production cycle showing the gas pulse and arc discharge.

Figure 4

(a) Piezoelectric valve case. (b) Piezoelectric valve shown with the Viton disk glued on it.

Figure 5

(a) Picture of the Series 9 gas valve. (b) Schematic of the gas valve and its different parts.

Figure 6

Datalog plot of the beam current, arc discharge current, vacuum pressure and local temperatures over a five-day period using the piezoelectric gas valve in the source.

Figure 7

Datalog plot of the beam current, arc discharge current, vacuum pressure and local temperatures over a five-day period using the solenoid gas valve in the source.

Figure 8

Beam current and cathode temperature vs arc pulse width using the solenoid gas valve in the ${\mathrm{H}}^{-}$ source.

Figure 9

(a) Cesium oven made of copper (left) and vial with 5 g of elemental cesium (right). (b) New cesium oven made of stainless steel with $2\mathrm{\Omega }/\mathrm{m}$ heating wire around it.

Figure 10

Temperature response of the different oven configurations as a function of current.

Figure 11

Circuit diagram of the current-regulated arc modulator.

Figure 12

Prototype of current-regulated arc modulator chassis showing the 700 V transformer, dc power supplies, high-voltage card, trigger/feedback card and the MOSFET.

Figure 13

Performance of the current-regulated modulator over three days running in the ion source. The arc current remains constant whereas the arc voltage and vacuum pressure wander.

Figure 14

Experimental setup for gas mixing into the ion source.

Figure 15

Scope picture of the beam current signal from the toroid downstream of the source. Beam extracted at $-35\mathrm{kV}$. Horizontal and vertical scales are $20\mu \mathrm{s}/\mathrm{div}$ and $10\mathrm{mV}/\mathrm{div}$, respectively.

Figure 16

Noise ratio as a function of ${\mathrm{N}}_2/{\mathrm{H}}_2$ ratio.

Figure 17

Noise ratio as a function of arc current with and without ${\mathrm{N}}_2$ in the gas mixture.

Figure 18

Cathodes used in the ion source: (a) Standard dimpled cathode; (b) hollow cathode; (c) thin cathode; (d) drilled through cathode.

Figure 19

Noise ratio as a function of arc current for different cathodes. Extraction voltage is $-35\mathrm{kV}$.

Figure 20

Noise ratio as a function of pressure for different cathodes. Arc current is 15 A and extraction voltage is $-35\mathrm{kV}$.

Figure 21

Diagram of the magnetron discharge stability as a function of magnetic field B and gas density n. The diagram was by V. Dudnikov.