BEPCⅡ束流引入的真空测量误差

Beam-Induced Errors in Vacuum Measurements of Beijing Electron Positron Collider

当BEPCⅡ储存环的束流突然丢失时,真空计和离子泵的压力测量值都先有一个瞬时的减小,之后才逐渐降低到本底压力。这表明束流运行时压力测量值中存在着束流引入的正误差,误差量正是测量值减去实际值之差。丢束之后束流引入的误差立即消失,真空计测量值反映的是实际压力。对于某个真空计,利用突然丢束后压力测量值随时间变化的数据进行非线性拟合,得到一条负指数衰减的抽空曲线,根据它外推可以得到丢束时刻的实际压力。对于若干次突然丢束,用这种方法观察丢束流强不同时该真空计误差量的变化,发现误差量与流强基本呈正比例关系。通过直线拟合求得比例系数,也就得到了在任意流强下根据该真空计压力测量值来计算实际压力的公式。

We addressed the systematic errors originated from the synchronized photoelectron beams, which desorbs quite a few gaseous molecules adsorbed on the vacuum chamber walls,in mesuring the base pressure of the large storage ring of Beijing electron positron collider （BEPCⅡ）. Since the realistic beam-induced partial pressure cannot be measured directly, it can only be evaluated by data fitting of the time evolution of the pressures, recorded by a specific iongauge in the time when the beam suddenly aborts. From this negative exponential decay pmnping-down curve,can the measurement error-the beam-induced partial pressure-be extrapolated. Interesting finding is that the partial pressure is basically proportional to the beam current, as expected. And a linear data-fitting gives the proportion coefficient of the equation, which we derived to evaluate the realistic base pressure all the time when the beam with varied intensity is on.