Lesson learned in Linac Commissioning Here I introduce 3 kinds of beam loss generated by following issues 1.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 σ x (mm) σ y (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) courtesy 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 MeV DTL 191 MeV DTL 191 MeV ACS 324 MHz972 MHz Design beam envelope Equi-partitioning condition

Intra Beam Stripping (IBSt) Beam size in ACS (Simulation) W/m W/m W/m 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 mA Pulse width : 500 us Repetition : 25 Hz Beam 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 Simulation T = 0.7 : W/m T = 1.0 : W/m T = 1.3 : 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 Current IS RFQ 50 MeV DTL 191 MeV SDTL 400 MeV ACS DB2 RCS DB1 Current transformer waveform (w/o chopping) Macro Pulse (100 us) Current transformer waveform (w/ chopping, duty 100%) 100 us Beam from RFQ Chopper RF Beam after chopper Dark current Macro 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. 60us50us 1mA Beam : 50 mA / 100 us Chopper Measured CT

Ion Source Dark Current Beam from RFQ Chopper RF Beam after chopper Dark current Macro pulse Lost in 3 GeV RCS 60us50us Beam from RFQ Chopper RF Beam after chopper Dark current Macro 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 gate 0.5V Beam Loss at Macro Pulse End around BM1 H-H- p BLMP21C BLMP21B BLMP21B : For low energy H - (ex. no acceleration by ACS) BLMP21C : For H + detection Top view of BM1 Beam after RFQ Chopper RF (duty 100 %) ss 10 us IS RFQ 50 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 LEBT RFQ RF chopper upstream ss Chopper RF (full chop mode) chopper downstream 10 us Present timing (chopping duty = 100%) beam at transient RFQ RF 1 st Arc: Chop 無し： マクロパルスより後ろに有意なロス 全 Chop ： マクロパルスと、マクロパルス前 100us にロス。 21C の信号が高いので、陽子起因のロスが多いように見える。 Scraper Section: 全 Chop: マクロパルスより後ろに有意なロス ← RFQ RF のタイミングをずらすと、それにしたがってずれる ← 3us 以上ずらせば、完全に消える。 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 No significant beam loss is observed after +3 us shift. Beam Loss Comp. of Different RFQ RF Timing

Lesson Learned in Linac Commissioning After 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 1 st 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 Ion Source 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 ε x /ε z = 0.7