이온 빔 동작 순서

An automatic controller is normally used for startup or shutdown of an endHall source. The sequence most often used is described below. Knowledge of this sequence should be useful in programming a controller, or troubleshooting when operating problems arise. The currents and voltages used are defined in Fig. VI-2.

1. Pumpdown.

The vacuum chamber in which the gridless source is installed shoul be pumped to a low enough pressure for operation and maintained at that pressure long enough for adequate outgassing before starting. The considerations with a gridjess source are the same as for a gridded ion source, and the pumpdown description for a gridded ion source (Sec. II.B. 1) should be consulted if there are any doubts proper procedure.

2. Gas flow.

The correct gas flow should be established. Higher volume flows (usually measured in scem, or standard cubic centimeters per minute) are typically required for larger sources, lighter gases (lower molecular or atomic weights), and higher vacuum-chamber pumping rates. The flows for starting are often higher than those required for operation after starting. The performance is often described in terms of gas flow, but the mean gas pressure in the ion source is a more fundamental parameter for describing performance. The pressure in the surrounding vacuum chamber contributes to this mean pressure and can therefore affect the required gas flow.

3. Cathode heating current.

The cathode heating current, I, should be established. This heating current should be above the value required for steady-state operation. If an electromagnet is used, the magnet current, I should be increased at this time. Depending on the particular design and operating condition used, the magnet current should be increased to either a moderate value or to the full operating value.

4. Anode potential.

The anode potential, V should be increased from near zero to a value roughly approximating the expected operating value. The discharge should start during this increase.

5. Normal operation.

The anode potential should be adjusted until the anode current, I, is at the desired value. If an electromagnet was initially set to only a moderate current, the magnet current, L, should nov gas flow should be adjusted at constant anode current until the anode voltage, V, is also at the desired value. Finally, the cathode heating current, 1, should be decreased until the cathode emission equals the anode current. Done manually, several sequential adjustments would be required to simultaneously obtain the correct values for the anode voltage and the anode and emission currents. With a controller, these adjustments are performed automatically.

6. Shutdown.

The normal shutdown for a gridless ion source is to reduce the anode potential, Va, while keeping the cathode heating current, Ic, constant. This sequence avoids undesirable high-voltage excursions for the ion beam.