Radiation-protection experience at Belle/ summary of beam abort system

22 September, 2003 T.Tsuboyama (KEK)

Purpose of Beam abort system (1)Protection of Belle/SVD (silicon vertex detector).

• Belle SVD (1999) was made with VA1 (1.4 um), which can survive up to 1 Mrad dose.

• Even in the first year of KEKB operation, when beam current was much lower than now, SVD sometimes suffered a few krad dose in one beam loss.

• When large background enters the silicon sensor and preamplifier, the huge charge flowing into the readout chip causes serious damage to the front end amplifiers and can degrades its performance at once. – Fatal single event latch up, fortunately, has not happened.

• When KEKB beam is lost close to Belle, KLM (IFR for Babar) suffers large charge and its readout system tripped often.

Purpose of Beam abort system (2)Protection of Movable masks

• Movable masks are installed to reduce the background to Belle.• Serious damages to masks happened when beam got out of control.

– Some masks can effectively reduce Belle background when they are adjusted close (a few mm?) to the beam orbit. When beam becomes unstable, the high-current beam can hit such a mask directly.

– The electro-magnetic shower leaves huge heat in the mask head and damages the mask at once.

– Once a movable mask is damaged, the operation condition of KEKB for enough beam life and quiet background condition becomes narrow.

• Replacement of movable masks requires a few months preparation and one to two days KEKB shutdown.

Beam abort statistics

• Rate: 5-7 per day• The rate depends on

accelerator conditions• Abort sources

– RF --- Klystron and Cavity

– Beam loss --- Ionization chamber and Silicon PIN diodes in the tunnel

– Belle abort: PIN diodes close to the SVD readout chip

– Earth quake

RF

Beam loss

Manual

Abort request from RF: Beam phase abort

• The phase between the accelerating RF field and beam bucket (beam phase) is used in the feed back loop of the KEKB RF system.

• When the normal accelerator condition is lost, (for example when more than two RF stations trip) the deviation of the beam phase in another RF system becomes larger than normal level and the beam will be out of control in a short time.

• Even 0.5 % of beam-energy deviation, the beam can hit movable masks and can cause damages to masks and SVD.

Beam phase abort (cont.)

• Since 2001, beam-abort is requested when the beam phase becomes out of control. RF voltage of SC cavity is also used as a beam-abort source.

• The beam can be discarded before the unstable beam damages movable masks and the Belle detector.

• Since then, the frequency of beam loss with high dose to Belle and movable mask accidents decreased significantly.

Beam abort due to beam loss• There are cases BPA can not predict fatal beam instability.

• Total of 109 5 m-length ionization chambers in installed the KEKB tunnel.

• PIN diode monitors are installed around movable masks.

• PIN diode monitors are also installed on the interaction-point beryllium chamber of Belle in order to protect Belle/SVD.

• These monitors request aborts when the background increases.– There are some occasions when background enters to Belle while the KEKB

operation is not influenced.

– Dust trapping

– Vacuum leak around IR

– Bad setting of movable masks

– Bad injection condition

– Relatively low but persistent background. (decided by software)

Belle/SVD radiation monitor

• On each probe, 3 sensors are integrated.

• RADFET (dose integration)

• PIN diode (high gain)

• PIN diode (Low gain)

• RTD

• 16 hybrids were installed– 8 close to the IP chamber

– 8 close to the readout chip

PIN diode readout

• There are two independent sensors and circuits.– In normal operation, the radiation level is ~10 mrad/sec.

– We would like to abort beam immediately when background becomes extremely high, 100 rad/sec for example.

– In order to meet the requirement, two diodes are used in each probe and read out independently.

– The software-abort request is sent if the radiation dose in 1 minute exceeds 10 rad.

• Bias voltage is not applied to PIN diodes.– Pro: Small temperature dependence of the offset level.

– Con: The natural depletion depth becomes shallower according to integrated dose, gain correction after irradiation is necessary

Diagnostics of beam loss

• The beam current, beam phase, vertical and horizontal orbits of beam are recorded with oscilloscopes and fast data loggers at every beam abort. The KEKB operators diagnose the reason of the beam loss.

Typical beam phase abort

The beam loss starts before the beam phase shifted.

Yellow ---beam phaseCyan --- RF powerGreen ----beam currentMagenta --- abort request

Another example

• First a beam loss happened long before than this figure, then the beam phase starts oscillation. The beam is aborted when the phase exceeded the limit.

• The loss monitor outputs started increase just before the beam abort.

Vacuum “burst” causes beam-phase change• In this case, the vacuum increased suddenly in a vacuum chamber.

The beam lost its energy. The feed back system shifted to the phase to recover the accelerating energy. Since the recovery is not successful, the beam could not be held. The beam phase changed significantly before the beam was lost.

Vacuum level

Summary• After the installation of “beam-phase abort (BPA)”, the

rate of accidents to movable mask and Belle due to lost beam reduced significantly. The rate of BPA is 0.5 to 1 times per day.

• Abort due to beam-loss monitors in the tunnel, close to the movable masks and close to Belle/SVD helps to abort the beam before whole beam is lost in the ring and Belle.

• The RF monitor data, beam monitor data and loss monitors are recorded by fast data loggers and oscilloscopes at each beam abort. Analysis of the logged data is indispensable for understanding the reason of beam instability and for determining how to reduce damages to the KEKB and Belle.