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TRIUMF LOGO The TRIUMF Cyclotron 2005-2010

R. Baartman, R. Poirier, G. Dutto

Sept. 21, 2002


1  Design

The cyclotron was designed for 100 µA at 500 MeV, 400 µA at 450 MeV. The much larger current at slightly reduced energy is due to an activation limit.

2  Status


3  Details

3.1  Activation limit

Electromagnetic stripping of H-->H0 ramps up steeply from negligible at 450 MeV to 8% at 500 MeV. An Auxiliary Accelerating Cavity (AAC) was installed to get the beam through the dangerous region more quickly. This is operating routinely since 2000 and reduces the e-m stripping to 5%. If large amounts of current are to be directed to radioactive beam production, this should be done at 450 Mev.

3.2  Engineering limit

Between 30% and 50% of the injected beam is lost on the first few turns, of which about half is lost on the centre post. This can be as much as 100 Watts. Thermocouples showed high and strongly beam-dependent readings. High temperatures -> more RF sparking. The solution, implemented in the Jan. 2002 shutdown was to install a cooled beam absorber against the centre post. (Before and after). We now feel 400 µA cw operation is possible, but have insufficient beam-dump capacity to prove it.

3.3  Physics limit

Q: Since PSI can reach 2 mA, why can't we?
A: TRIUMF cyclotron has weak focusing at injection.

Vertical focusing is caused by azimuthal modulation of the magnetic field (cut it in sectors). This modulation cannot extend to small radius where "small" means less than the magnet gap. TRIUMF has both a large magnet gap and a small first turn. Therefore we must rely on electric focusing, which only occurs if the beam is crossing the dee gaps while the electric field is falling. The phase acceptance window is therefore from 0° to 40°. Moreover, phases where the slope is small (near 0°) are most weakly focused. Space charge defocuses the beam and as a result, the higher the local current, the more shifted to positive phases is the acceptance window. See illustration. This ultimately results in a hard upper limit to the beam current. We believe this limit to be around 500 µA . (Reminder: 500 µA average in 36° bunches means the average current in the bunch is 5 mA!. This is achieved by bunching.)

This requires some ISIS improvements as well as possibly additional cooled electrodes.

For setting up the cyclotron to high intensity, it is important to be able to dump the beam somewhere. This will result in more reliable high current ISAC operation. This will require an additional beam dump of 200 µA capability. If this is done at low energy (~100 MeV), it only requires a 20 kW dump. Possible locations are on BL2C, and a new BL5C.


4  What's needed?

To supply additional ISAC targets, as well as the current target stations, as well as maintaining at least 150 µA for muons and 50 µA for BL2C requires around 400 µA total beam.

This requires some ISIS improvements as well as possibly additional cooled electrodes.

For setting up the cyclotron to high intensity, it is important to be able to dump the beam somewhere. This will result in more reliable high current ISAC operation. This will require an additional beam dump of 200 µA capability. If this is done at low energy (~100 MeV), it only requires a 20 kW dump. Possible locations are on BL2C, and a new BL5C.


5  Cost (k$)

High Intensity Requirements

REFURBISH 4V AREA 250
ADDITIONAL BEAM DUMP CAPACITY 500
CYCLOTRON AREA 600
TOTAL 1350

Non-High-Intensity Requirements

UPGRADE DIAGNOSTICS 750
IMPROVED STABILITY 250
SAFETY (Monitor & Interlocks) 200
IMPROVED RELIABILITY 1100
TOTAL 2300

Details...