SIMAC Lab

Part 1. Introduction to SIMAC

A. Adjusting the RF driver.

The purpose of this section of the lab is to ensure the user can use the SIMAC program competently; can make adjustments to linac beam parameters, and can read back the effect of the adjustment on the other linac parameters. The example adjusts the RF driver power to the klystron amplifier. This will have an effect on the RF power into the accelerating waveguide, but not on the beam current. It will also affect other parameters as seen below

  1. Launch SIMAC. Set the beam to ON, and choose E = 15 MV and PRF = 120 Hz. Open the Klystron and Accelerator windows.
  2. Adjust the RF drive from 67 to 100 Watts. Observe the klystron operating point on the Klystron Characteristic curve, in the Klystron Window.
  3. In the main window, record the behaviour of the RF Out, Gun I, Target I and dose rate. Record these in the spreadsheet (worksheet Part 1).
  4. In the Accelerator window, observe the behaviour of the RF, Gun I, Tar I, Acc E. Record these in the spreadsheet (worksheet Part 1).
  5. Leaving the RF drive at 100 W, adjust the Bmag I to maximize the dose rate. Record the same parameters as in steps (a) – (b) in the spreadsheet.

Note: this is for illustration purposes only. On a real linac, Bmag I should never be adjusted.

Discussion topics:

  1. For part 2 (a) and (b) which parameters were affected by adjusting the RF drive?
  2. Which were not?
  3. Explain this behaviour.
  4. Why does changing the bending magnet current in part 3 affect dose rate?
  5. What clinical affect does changing the bending magnet current have? Why should Bmag I never be changed on a working linac?

B. Adjusting the modulator.

The purpose of this section of the lab is to get further insight into linac beam parameter adjustments by adjusting the voltage of the klystron voltage pulse. This also affects the operating point on the klystron saturation curve, but differently than by adjusting the RF driver.

  1. Launch SIMAC. Set the beam to ON, and choose E = 15 MV and PRF = 120 Hz. Open the Klystron and Accelerator windows.
  2. Adjust the klystron voltage between 118 and 138 kV. Observe the klystron operating point on the Klystron Characteristic curve, in the Klystron Window.
  3. In the main window, record the behaviour of the RF Out, Gun I, Target I and dose rate. Record these in the spreadsheet (Klystron and Accelerator page).
  4. In the Accelerator window, Observe the behaviour of the RF, Gun I, Tar I, Acc E. Record these in the spreadsheet (worksheet Part1).
  5. Set the Klystron V at 122 kV, adjust Bmag Ibetween 140 and 160 A. Record the same parameters as in steps (a) – (c) in the spreadsheet (worksheet Part1). Note: this is for illustration purposes only. On a real linac, Bmag should never be adjusted.

Discussion topics:

  1. For part 2 (a) and (b) which parameters were affected by adjusting the klystron voltage?
  2. Which were not?
  3. Explain this behaviour.

Part 2. Beam loading and beam finding

A. Beam loading

The purpose of this section of the labs is to investigate beam loading. One method to adjust beam energy is to change the beam current. The beam loading theory says that for a fixed amount of microwave power, the electron beam energy decreases as more beam current is injected into the linac. This exercise will explore the effects of increased beam current on other linac beam parameters, and how the clinical beam is also affected.

  1. Launch SIMAC. Set the beam to ON, and choose E = 15 MV and PRF = 120 Hz.Open the Klystron and Accelerator windows.
  2. Adjust the Gun V between 7 and 13 V. Observe the effect on the beam load line by watching the linac operating point on the load line curve in the Accelerator window.
  1. In the main window, record the behaviour of the RF Out, Gun I, Target I and dose rate. Record these in the spreadsheet (worksheet Part 2).
  2. In the Accelerator window, observe the behaviour of the RF, Gun I, Tar I, energy. Record these in the spreadsheet (worksheet Part 2).
  1. Set the Gun Vto 13 V. Adjust Bmag I to maximize the dose rate. Record the same parameters as in steps (a) – (b) in the spreadsheet (worksheet Part 2).
  2. Set the Gun V to 7 V. Adjust Bmag I to maximize the dose rate. Record the same parameters as in steps (a) – (b) in the spreadsheet (worksheet Part 2).

Note: Steps 3 and 4 are for illustration purposes only. On a real linac, Bmag should never be adjusted.

Discussion topics:

  1. For part 2 (a) and (b) which parameters were affected by adjusting the Gun V?
  2. Which were not?
  3. Explain this behaviour.
  4. For Gun V settings of 7 and 13 V, are the peaked dose rates the same or different? Why is this?

B. Finding Beam

In this section of the lab, we simulate the work done when wetting up a new linac energy (operating point) for the first time. An initial linac set point of 6 MV is chosen, and then this is changed to 10 MV by changing parameters that follow both the load line of the linac, and keeping the bending magnet pass through energy matched to the beam energy.

Launch SIMAC. Set the beam to ON, and choose E = 6 MV and PRF = 120 Hz. Open the Klystron and Accelerator windows.

  1. Tune any parameters required to maximize the dose rate for a 10 MV beam. Record your steps in the spreadsheet (worksheet Part 2).

Hint: In the accelerator window, watch the operating point on the load line, and as well the BMagE set energy. Try to keep the two energies the same.

  1. Produce a plot of beam energy vs bending magnet current.

Discussion topics:

  1. Is there a systematic method to beam energy setup, or do you use a random approach?
  2. Which parameters were most effective at adjusting beam to a new energy?
  3. If you were doing this on a real machine, are there any other considerations you may consider?
  4. Is it possible to harm the machine when adjusting its operating parameters? Describe the risks.
  5. What is the shape of the graph produced in part 2? What shape was expected?

Part 3. Beam steering

In these exercises, we will explore the effect of beam centering on beam symmetry, and of beam energy on the beam flatness.

A. Symmetry

  1. Launch SIMAC. Set the beam to ON, and choose E = 6 MV and PRF = 120 Hz. Set the radial and transverse jaw positions to 20 cm for 40 x 40 cm beam.
  2. Open the Treatment Head Window.
  3. Vary the Pos R current from -100 to 100mA in steps of 10 mA.
  4. In the treatment head window, observe the Radialand Transverse flatness and symmetry valuesand record these in the lab spreadsheet colums O and P. As well, observe the beam position and angle on the target on the plots in the treatment head window.
  5. Also in the treatment head window, observe the beam edge locations (IP and XP). Record the IP target edge and gun edge location in the lab spreadsheet colums Q and R.
  6. Load a fresh 15 MV parameter set.
  7. Vary the Pos R current from -100 to 100mA in steps of 10 mA.
  8. In the treatment head window, observe the Radial and Transverse flatness and symmetry valuesand record these in the lab spreadsheet colums O and P.
  9. Also in the treatment window, observe the beam edge locations (IP and XP). Record the IP target edge and gun edge location in the lab spreadsheet colums Q and R. AS well, observe the beam position and angle on the target on the plots in the treatment head window.
  10. When finished measuring effects of the Pos R steering coil, we will repeat this, but with a beam that has an alignment problem. After exploring the effects of beam steering on a well behaved beam, we will study the effects of beam steering on a beam that is poorly aligned. To do this, load the exercise file marked Beam Steering exercise, and calculate beam profiles. Then, load the linac status marked beam steering status, and update the beam profiles.
  11. Repeat steps 1 –5 and record the new values in the spreadsheet, placing the values in colums Q and R.

Questions:

The lab spreadsheet automatically produces plots of Beam symmetry and IP target edge beam edge as a function of Pos R current. Study these plots, and consider these questions:

  1. What is the relationship between steering coil current and R symmetry?
  2. For a given beam symmetry, the coil current is higher for the lower energy beam. Can you explain why?
  3. The beam edge profiles should have a step shape to them. Based on the theory in section E of Anderson (JACMP, V16, no. 3, 2015), do you think this step shape is real, or a limitation of the simulation method?
  4. Regardless of the shape of the beam edge plots, is there a systematic difference between the beam edge position with the beam offset as compared to the ones with the beam offset?
  5. Is it possible to achieve a perfect beam profile by steering alone for the beams explored in step 13? Why?

B.Flatness

  1. Launch SIMAC. Set the beam to ON, and choose E = 6 MV and PRF = 120 Hz. Open the Treatment Head Window.
  2. Load the parameters you obtained at the end of Part 2B (Finding Beam), and record these in the spreadsheet (worksheet Part 3).
  3. In steps of 10 A, decrease the Bmag I setting to 65 A. At each step, re-peak the beam using Kly V only. Record all parameters, including flatness and symmetry. Observe the shape of the beam profiles in the Treatment head window.
  4. Produce a plot of flatness vs. beam energy.
  5. Obtain a new parameter set for 6 MV, but with maximum dose rate. Record your steps in the spreadsheet, including flatness.

Questions:

  1. What is the form of flatness vs beam energy dependence?
  2. Why does the dose rate decrease as you are changing beam energy in step 4?
  3. Does beam flatness change as a function of dose rate? Why?

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