Lesson learned in Linac Commissioning

Here I introduce 3 kinds of beam loss generated by following issues1. Intra beam stripping (IBSt) in ACS

2. Dark current of an ion source

3. Beam accelerated by transient RFQ RF

T. Maruta KEK/J-PARC

ACS Beam Loss

σx (mm) σy (mm)

• We have observed continuous beam loss in ACS, and residual radiation is higher than our expectation.

• One reason is the property of current transformers.

• We have been investigating the source of beam loss particles for countermeasure.• We measured the contribution of the Intra Beam Stripping (IBSt)

3 times RF frequency jump at SDTL to ACS. longitudinal focusing becomes higher in ACS⇒ ⇒ Beam size at ACS is narrower than SDTL to suffer the equi-partitioning condition.

IBSt is inversely proportional to the beam size.

50 MeVDTL

191 MeVDTL

191 MeVACS

324 MHz 972 MHz

Design beam envelopeEqui-partitioningcondition

Intra Beam Stripping (IBSt)

Beam size in ACS (Simulation)

0.010 W/m

0.020 W/m

0.026 W/m

0.032 W/m

IBSt (Simulation)

We prepared 4 kinds of optics w/ different T ratio; T= 1.3, 1.0 (default), 0.7 and 0.3.

Peak current: 30 mAPulse width : 500 usRepetition : 25 HzBeam duty : 56 %

• The identical optics is applied to DTL – SDTL (T = 1.0).• 3D matching in MEBT2.• Measure the beam loss in ACS and beam profile at the L3BT entrance.

ACS BLM Signal Comp. at T = 0.7, 1.0 and 1.3

SimulationT = 0.7 : 0.020 W/mT = 1.0 : 0.026 W/mT = 1.3 : 0.032 W/m

R (T=0.7/T=1.0) = 0.77

R (T=1.3/T=1.0) = 1.23

The ratios of BLM signal of each T-ratio is well consistent w/ the simulation after ACS08.IBSt could be dominant source of the ACS beam loss.

ACS BLM signal (signal saturation is corrected)

Ratio of ACS BLM signal

Beam Loss by Ion Source Dark CurrentIS RFQ

50 MeV DTL 191 MeV SDTL 400 MeV ACS

DB2

RCS

DB1

Current transformer waveform (w/o chopping)

MacroPulse(100 us)

Current transformer waveform (w/ chopping, duty 100%)

100 us

Beam fromRFQ

Chopper RF

Beam afterchopper

Dark currentMacro pulse

Lost in 3 GeV RCS

• Dark current exists before a macro pulse. Current is about 2 mA (4% of 50 mA).

• The dark current is partially scraped by the chopper, but RF width is not sufficient.

• Un-scraped dark current causes a beam loss in 3-GeV RCS.

60us 50us

1mA 1mA

Beam : 50 mA / 100 us

Chopper

MeasuredCT

Ion Source Dark Current

Beam fromRFQ

Chopper RF

Beam afterchopper

Dark currentMacro pulse

Lost in 3 GeV RCS

60us 50us

Beam fromRFQ

Chopper RF

Beam afterchopper

Dark currentMacro pulse

120us 10us

100 us

1mA

The chopper RF timing is optimized, and then check the current transformer again.

• Extend former RF width to 120 us

• The chopper RF width of a former macro pulse is extended to fully cover the dark current.

• After the timing change, we again measure the CT waveform, and the dark current does not detected.

• The beam loss in RCS is drastically reduced.

L3BT_BLM55

L3BT_BLM57

Chopping Duty 100%

Beam gate0.5V

Beam Loss at Macro Pulse End around BM1Beam after RFQ

Chopper RF(duty 100 %)

ss

10 us

IS RFQ50 MeV

DTL 191 MeV SDTL 400 MeV ACS

DB2

RCS

DB1

Small amount of beam exists after macro-pulse.• This beam cannot be scraped

by chopper. beam property must be ⇒

different from the main part.

Insert carbon plates on beam line to intentionally loss

0.5V

10 us

Timing around Macro Pulse End

Beam @ LEBT

RFQ RF

Beam @ chopperupstream

ssChopper RF(full chop mode)

Beam @chopperdownstream

10 us

Present timing (chopping duty = 100%)

beam at transientRFQ RF

Lost in L3BT scraper

Extinction level of this region looks worse than the beam in beam gate

beam gate

ss

10 us

Proporsed timing (delay the RFQ RF end)

Accelerated by nominal RF

Lost in L3BT scraper

beam gate

After some studies, we found that the loss comes from the beam accelerated by transient RFQ RF

L3BT_BLM55

L3BT_BLM57

Beam Monitor Timing

Chopped (duty 100%), RFQ timing shift : 0 us

L3BT_BLM55

L3BT_BLM57

Beam Monitor Timing

Chopped (duty 100%), RFQ timing shift : 3us

Beam gate Beam gate

No significant beam loss is observed after +3 us shift.

Beam Loss Comp. of Different RFQ RF Timing

Lesson Learned in Linac CommissioningAfter a replacement of linac elements (ex. front-end replacement) and beam power upgrade, new beam loss may appears. We have to pay attention to beam loss distribution in the 1st commissioning.

Time structure of beam loss is a good hint of source.(We normally monitor the integrated BLM signals)

It is difficult to find the beam loss caused by timing in linac single commissioning. Communication with downstream accelerator is important.

Gate Timing Relating to Dark Current

Beam from RFQ

Arcing

Modulation

It is expected that the beam is extracted only when the modulation is on

RFQ RF

800 us

650 us

IonSource

Design(expectation)

Reality

Rise-up of macro pulse is determined by modulation

50 〜 500 us

Dark Current

Small fraction of un-modulated beam is inside the RFQ acceptance, and then accelerated to 400 MeV

Hoffman Stability Chart at εx/εz = 0.7